Index: head/en_US.ISO8859-1/books/developers-handbook/x86/chapter.xml =================================================================== --- head/en_US.ISO8859-1/books/developers-handbook/x86/chapter.xml (revision 53385) +++ head/en_US.ISO8859-1/books/developers-handbook/x86/chapter.xml (revision 53386) @@ -1,6310 +1,5562 @@ x86 Assembly Language Programming This chapter was written by &a.stanislav.email;. Synopsis Assembly language programming under &unix; is highly undocumented. It is generally assumed that no one would ever want to use it because various &unix; systems run on different microprocessors, so everything should be written in C for portability. In reality, C portability is quite a myth. Even C programs need to be modified when ported from one &unix; to another, regardless of what processor each runs on. Typically, such a program is full of conditional statements depending on the system it is compiled for. Even if we believe that all of &unix; software should be written in C, or some other high-level language, we still need assembly language programmers: Who else would write the section of C library that accesses the kernel? In this chapter I will attempt to show you how you can use assembly language writing &unix; programs, specifically under FreeBSD. This chapter does not explain the basics of assembly language. There are enough resources about that (for a complete online course in assembly language, see Randall Hyde's Art of Assembly Language; or if you prefer a printed book, take a look at Jeff Duntemann's Assembly Language Step-by-Step (ISBN: 0471375233). However, once the chapter is finished, any assembly language programmer will be able to write programs for FreeBSD quickly and efficiently. Copyright © 2000-2001 G. Adam Stanislav. All rights reserved. The Tools The Assembler The most important tool for assembly language programming is the assembler, the software that converts assembly language code into machine language. Two very different assemblers are available for FreeBSD. One is as1, which uses the traditional &unix; assembly language syntax. It comes with the system. The other is /usr/ports/devel/nasm. It uses the Intel syntax. Its main advantage is that it can assemble code for many operating systems. It needs to be installed separately, but is completely free. This chapter uses nasm syntax because most assembly language programmers coming to FreeBSD from other operating systems will find it easier to understand. And, because, quite frankly, that is what I am used to. The Linker The output of the assembler, like that of any compiler, needs to be linked to form an executable file. The standard ld1 linker comes with FreeBSD. It works with the code assembled with either assembler. System Calls Default Calling Convention By default, the FreeBSD kernel uses the C calling convention. Further, although the kernel is accessed using int 80h, it is assumed the program will call a function that issues int 80h, rather than issuing int 80h directly. This convention is very convenient, and quite superior to the µsoft; convention used by &ms-dos;. Why? Because the &unix; convention allows any program written in any language to access the kernel. An assembly language program can do that as well. For example, we could open a file: kernel: int 80h ; Call kernel ret open: push dword mode push dword flags push dword path mov eax, 5 call kernel add esp, byte 12 ret This is a very clean and portable way of coding. If you need to port the code to a &unix; system which uses a different interrupt, or a different way of passing parameters, all you need to change is the kernel procedure. But assembly language programmers like to shave off cycles. The above example requires a call/ret combination. We can eliminate it by pushing an extra dword: open: push dword mode push dword flags push dword path mov eax, 5 push eax ; Or any other dword int 80h add esp, byte 16 The 5 that we have placed in EAX identifies the kernel function, in this case open. Alternate Calling Convention FreeBSD is an extremely flexible system. It offers other ways of calling the kernel. For it to work, however, the system must have Linux emulation installed. Linux is a &unix; like system. However, its kernel uses the same system-call convention of passing parameters in registers &ms-dos; does. As with the &unix; convention, the function number is placed in EAX. The parameters, however, are not passed on the stack but in EBX, ECX, EDX, ESI, EDI, EBP: open: mov eax, 5 mov ebx, path mov ecx, flags mov edx, mode int 80h This convention has a great disadvantage over the &unix; way, at least as far as assembly language programming is concerned: Every time you make a kernel call you must push the registers, then pop them later. This makes your code bulkier and slower. Nevertheless, FreeBSD gives you a choice. If you do choose the Linux convention, you must let the system know about it. After your program is assembled and linked, you need to brand the executable: &prompt.user; brandelf -t Linux filename Which Convention Should You Use? If you are coding specifically for FreeBSD, you should always use the &unix; convention: It is faster, you can store global variables in registers, you do not have to brand the executable, and you do not impose the installation of the Linux emulation package on the target system. If you want to create portable code that can also run on Linux, you will probably still want to give the FreeBSD users as efficient a code as possible. I will show you how you can accomplish that after I have explained the basics. Call Numbers To tell the kernel which system service you are calling, place its number in EAX. Of course, you need to know what the number is. The <filename>syscalls</filename> File The numbers are listed in syscalls. locate syscalls finds this file in several different formats, all produced automatically from syscalls.master. You can find the master file for the default &unix; calling convention in /usr/src/sys/kern/syscalls.master. If you need to use the other convention implemented in the Linux emulation mode, read /usr/src/sys/i386/linux/syscalls.master. Not only do FreeBSD and Linux use different calling conventions, they sometimes use different numbers for the same functions. syscalls.master describes how the call is to be made: 0 STD NOHIDE { int nosys(void); } syscall nosys_args int 1 STD NOHIDE { void exit(int rval); } exit rexit_args void 2 STD POSIX { int fork(void); } 3 STD POSIX { ssize_t read(int fd, void *buf, size_t nbyte); } 4 STD POSIX { ssize_t write(int fd, const void *buf, size_t nbyte); } 5 STD POSIX { int open(char *path, int flags, int mode); } 6 STD POSIX { int close(int fd); } etc... It is the leftmost column that tells us the number to place in EAX. The rightmost column tells us what parameters to push. They are pushed from right to left. For example, to open a file, we need to push the mode first, then flags, then the address at which the path is stored. Return Values A system call would not be useful most of the time if it did not return some kind of a value: The file descriptor of an open file, the number of bytes read to a buffer, the system time, etc. Additionally, the system needs to inform us if an error occurs: A file does not exist, system resources are exhausted, we passed an invalid parameter, etc. Man Pages The traditional place to look for information about various system calls under &unix; systems are the manual pages. FreeBSD describes its system calls in section 2, sometimes in section 3. For example, open2 says:
If successful, open() returns a non-negative integer, termed a file descriptor. It returns -1 on failure, and sets errno to indicate the error.
The assembly language programmer new to &unix; and FreeBSD will immediately ask the puzzling question: Where is errno and how do I get to it? The information presented in the manual pages applies to C programs. The assembly language programmer needs additional information. Where Are the Return Values? Unfortunately, it depends... For most system calls it is in EAX, but not for all. A good rule of thumb, when working with a system call for the first time, is to look for the return value in EAX. If it is not there, you need further research. I am aware of one system call that returns the value in EDX: SYS_fork. All others I have worked with use EAX. But I have not worked with them all yet. If you cannot find the answer here or anywhere else, study libc source code and see how it interfaces with the kernel. Where Is <varname>errno</varname>? Actually, nowhere... errno is part of the C language, not the &unix; kernel. When accessing kernel services directly, the error code is returned in EAX, the same register the proper return value generally ends up in. This makes perfect sense. If there is no error, there is no error code. If there is an error, there is no return value. One register can contain either. Determining an Error Occurred When using the standard FreeBSD calling convention, the carry flag is cleared upon success, set upon failure. When using the Linux emulation mode, the signed value in EAX is non-negative upon success, and contains the return value. In case of an error, the value is negative, i.e., -errno. Creating Portable Code Portability is generally not one of the strengths of assembly language. Yet, writing assembly language programs for different platforms is possible, especially with nasm. I have written assembly language libraries that can be assembled for such different operating systems as &windows; and FreeBSD. It is all the more possible when you want your code to run on two platforms which, while different, are based on similar architectures. For example, FreeBSD is &unix;, Linux is &unix; like. I only mentioned three differences between them (from an assembly language programmer's perspective): The calling convention, the function numbers, and the way of returning values. Dealing with Function Numbers In many cases the function numbers are the same. However, even when they are not, the problem is easy to deal with: Instead of using numbers in your code, use constants which you have declared differently depending on the target architecture: %ifdef LINUX %define SYS_execve 11 %else %define SYS_execve 59 %endif Dealing with Conventions Both, the calling convention, and the return value (the errno problem) can be resolved with macros: %ifdef LINUX %macro system 0 call kernel %endmacro align 4 kernel: push ebx push ecx push edx push esi push edi push ebp mov ebx, [esp+32] mov ecx, [esp+36] mov edx, [esp+40] mov esi, [esp+44] mov ebp, [esp+48] int 80h pop ebp pop edi pop esi pop edx pop ecx pop ebx or eax, eax js .errno clc ret .errno: neg eax stc ret %else %macro system 0 int 80h %endmacro %endif Dealing with Other Portability Issues The above solutions can handle most cases of writing code portable between FreeBSD and Linux. Nevertheless, with some kernel services the differences are deeper. In that case, you need to write two different handlers for those particular system calls, and use conditional assembly. Luckily, most of your code does something other than calling the kernel, so usually you will only need a few such conditional sections in your code. Using a Library You can avoid portability issues in your main code altogether by writing a library of system calls. Create a separate library for FreeBSD, a different one for Linux, and yet other libraries for more operating systems. In your library, write a separate function (or procedure, if you prefer the traditional assembly language terminology) for each system call. Use the C calling convention of passing parameters. But still use EAX to pass the call number in. In that case, your FreeBSD library can be very simple, as many seemingly different functions can be just labels to the same code: sys.open: sys.close: [etc...] int 80h ret Your Linux library will require more different functions. But even here you can group system calls using the same number of parameters: sys.exit: sys.close: [etc... one-parameter functions] push ebx mov ebx, [esp+12] int 80h pop ebx jmp sys.return ... sys.return: or eax, eax js sys.err clc ret sys.err: neg eax stc ret The library approach may seem inconvenient at first because it requires you to produce a separate file your code depends on. But it has many advantages: For one, you only need to write it - once and can use it for all your programs. You can even let + once and can use it for all your programs. You can even let other assembly language programmers use it, or perhaps use one - written by someone else. But perhaps the greatest advantage of + written by someone else. But perhaps the greatest advantage of the library is that your code can be ported to other systems, even by other programmers, by simply writing a new library without any changes to your code. If you do not like the idea of having a library, you can at least place all your system calls in a separate assembly - language file and link it with your main program. Here, again, + language file and link it with your main program. Here, again, all porters have to do is create a new object file to link with your main program. Using an Include File If you are releasing your software as (or with) source code, you can use macros and place them in a separate file, which you include in your code. Porters of your software will simply write a new include - file. No library or external object file is necessary, yet your + file. No library or external object file is necessary, yet your code is portable without any need to edit the code. This is the approach we will use throughout this chapter. We will name our include file system.inc, and add to it whenever we deal with a new system call. We can start our system.inc by declaring the standard file descriptors: %define stdin 0 %define stdout 1 %define stderr 2 Next, we create a symbolic name for each system call: %define SYS_nosys 0 %define SYS_exit 1 %define SYS_fork 2 %define SYS_read 3 %define SYS_write 4 ; [etc...] We add a short, non-global procedure with a long name, so we do not accidentally reuse the name in our code: section .text align 4 access.the.bsd.kernel: int 80h ret We create a macro which takes one argument, the syscall number: %macro system 1 mov eax, %1 call access.the.bsd.kernel %endmacro Finally, we create macros for each syscall. These macros take no arguments. %macro sys.exit 0 system SYS_exit %endmacro %macro sys.fork 0 system SYS_fork %endmacro %macro sys.read 0 system SYS_read %endmacro %macro sys.write 0 system SYS_write %endmacro ; [etc...] Go ahead, enter it into your editor and save it as system.inc. We will add more to it as we discuss more syscalls. Our First Program We are now ready for our first program, the mandatory Hello, World! 1: %include 'system.inc' 2: 3: section .data 4: hello db 'Hello, World!', 0Ah 5: hbytes equ $-hello 6: 7: section .text 8: global _start 9: _start: 10: push dword hbytes 11: push dword hello 12: push dword stdout 13: sys.write 14: 15: push dword 0 16: sys.exit Here is what it does: Line 1 includes the defines, the macros, and the code from system.inc. Lines 3-5 are the data: Line 3 starts the data section/segment. Line 4 contains the string "Hello, World!" - followed by a new line (0Ah). Line 5 creates + followed by a new line (0Ah). Line 5 creates a constant that contains the length of the string from line 4 in bytes. - Lines 7-16 contain the code. Note that FreeBSD uses the + Lines 7-16 contain the code. Note that FreeBSD uses the elf file format for its executables, which requires every program to start at the point labeled _start (or, more precisely, the linker expects - that). This label has to be global. + that). This label has to be global. Lines 10-13 ask the system to write hbytes bytes of the hello string to stdout. Lines 15-16 ask the system to end the program with the return - value of 0. The 0. The SYS_exit syscall never returns, so the code ends there. If you have come to &unix; from &ms-dos; assembly language background, you may be used to writing - directly to the video hardware. You will never have to worry - about this in FreeBSD, or any other flavor of &unix;. As far as + directly to the video hardware. You will never have to worry + about this in FreeBSD, or any other flavor of &unix;. As far as you are concerned, you are writing to a file known as - stdout. This can be the video screen, or a + stdout. This can be the video screen, or a telnet terminal, or an actual file, - or even the input of another program. Which one it is, is for + or even the input of another program. Which one it is, is for the system to figure out. - Assembling the Code + + Assembling the Code - Type the code (except the line numbers) in an editor, and save - it in a file named hello.asm. You need - nasm to assemble it. + Type the code (except the line numbers) in an editor, and + save it in a file named hello.asm. You + need nasm to assemble it. - Installing <application>nasm</application> + + Installing <application>nasm</application> If you do not have nasm, type: -&prompt.user; su + &prompt.user; su Password:your root password &prompt.root; cd /usr/ports/devel/nasm &prompt.root; make install &prompt.root; exit &prompt.user; - -You may type make install clean instead of just -make install if you do not want to keep -nasm source code. - + You may type make install clean + instead of just make install if you do + not want to keep nasm source + code. - -Either way, FreeBSD will automatically download -nasm from the Internet, -compile it, and install it on your system. - + Either way, FreeBSD will automatically download + nasm from the Internet, compile it, + and install it on your system. - - -If your system is not FreeBSD, you need to get -nasm from its -home -page. You can still use it to assemble FreeBSD code. - - + + If your system is not FreeBSD, you need to get + nasm from its home + page. You can still use it to assemble FreeBSD + code. + - -Now you can assemble, link, and run the code: - + Now you can assemble, link, and run the code: -&prompt.user; nasm -f elf hello.asm + &prompt.user; nasm -f elf hello.asm &prompt.user; ld -s -o hello hello.o &prompt.user; ./hello Hello, World! &prompt.user; - - - - - + + -Writing &unix; Filters + Writing &unix; Filters - -A common type of &unix; application is a filter—a program -that reads data from the stdin, processes it -somehow, then writes the result to stdout. - + A common type of &unix; application is a filter—a + program that reads data from the stdin, + processes it somehow, then writes the result to + stdout. - -In this chapter, we shall develop a simple filter, and -learn how to read from stdin and write to -stdout. This filter will convert each byte -of its input into a hexadecimal number followed by a -blank space. - + In this chapter, we shall develop a simple filter, and + learn how to read from stdin and write to + stdout. This filter will convert each byte + of its input into a hexadecimal number followed by a blank + space. - -%include 'system.inc' + %include 'system.inc' section .data hex db '0123456789ABCDEF' buffer db 0, 0, ' ' section .text global _start _start: ; read a byte from stdin push dword 1 push dword buffer push dword stdin sys.read add esp, byte 12 or eax, eax je .done ; convert it to hex movzx eax, byte [buffer] mov edx, eax shr dl, 4 mov dl, [hex+edx] mov [buffer], dl and al, 0Fh mov al, [hex+eax] mov [buffer+1], al ; print it push dword 3 push dword buffer push dword stdout sys.write add esp, byte 12 jmp short _start .done: push dword 0 - sys.exit - - -In the data section we create an array called hex. -It contains the 16 hexadecimal digits in ascending order. -The array is followed by a buffer which we will use for -both input and output. The first two bytes of the buffer -are initially set to 0. This is where we will write -the two hexadecimal digits (the first byte also is -where we will read the input). The third byte is a -space. - + sys.exit - -The code section consists of four parts: Reading the byte, -converting it to a hexadecimal number, writing the result, -and eventually exiting the program. - + In the data section we create an array called + hex. It contains the 16 hexadecimal digits + in ascending order. The array is followed by a buffer which + we will use for both input and output. The first two bytes of + the buffer are initially set to 0. This + is where we will write the two hexadecimal digits (the first + byte also is where we will read the input). The third byte is + a space. - -To read the byte, we ask the system to read one byte -from stdin, and store it in the first byte -of the buffer. The system returns the number -of bytes read in EAX. This will be 1 -while data is coming, or 0, when no more input -data is available. Therefore, we check the value of -EAX. If it is 0, -we jump to .done, otherwise we continue. - + The code section consists of four parts: Reading the byte, + converting it to a hexadecimal number, writing the result, and + eventually exiting the program. - - -For simplicity sake, we are ignoring the possibility -of an error condition at this time. - - + To read the byte, we ask the system to read one byte from + stdin, and store it in the first byte of + the buffer. The system returns the number + of bytes read in EAX. This + will be 1 while data is coming, or + 0, when no more input data is available. + Therefore, we check the value of EAX. If it is + 0, we jump to .done, + otherwise we continue. - -The hexadecimal conversion reads the byte from the -buffer into EAX, or actually just -AL, while clearing the remaining bits of -EAX to zeros. We also copy the byte to -EDX because we need to convert the upper -four bits (nibble) separately from the lower -four bits. We store the result in the first two -bytes of the buffer. - + + For simplicity sake, we are ignoring the possibility of + an error condition at this time. + - -Next, we ask the system to write the three bytes -of the buffer, i.e., the two hexadecimal digits and -the blank space, to stdout. We then -jump back to the beginning of the program and -process the next byte. - + The hexadecimal conversion reads the byte from the + buffer into EAX, or actually just AL, while clearing the remaining + bits of EAX to zeros. We + also copy the byte to EDX + because we need to convert the upper four bits (nibble) + separately from the lower four bits. We store the result in + the first two bytes of the buffer. - -Once there is no more input left, we ask the system -to exit our program, returning a zero, which is -the traditional value meaning the program was -successful. - + Next, we ask the system to write the three bytes of the + buffer, i.e., the two hexadecimal digits and the blank space, + to stdout. We then jump back to the + beginning of the program and process the next byte. - -Go ahead, and save the code in a file named hex.asm, -then type the following (the ^D means press the -control key and type D while holding the -control key down): - + Once there is no more input left, we ask the system to + exit our program, returning a zero, which is the traditional + value meaning the program was successful. -&prompt.user; nasm -f elf hex.asm + Go ahead, and save the code in a file named + hex.asm, then type the following (the + ^D means press the control key and type + D while holding the control key + down): + + &prompt.user; nasm -f elf hex.asm &prompt.user; ld -s -o hex hex.o &prompt.user; ./hex Hello, World! 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come! 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D &prompt.user; - - -If you are migrating to &unix; from &ms-dos;, -you may be wondering why each line ends with 0A -instead of 0D 0A. -This is because &unix; does not use the cr/lf convention, but -a "new line" convention, which is 0A in hexadecimal. - - + + If you are migrating to &unix; from + &ms-dos;, you may be wondering why each + line ends with 0A instead of + 0D 0A. This is because &unix; does not + use the cr/lf convention, but a "new line" convention, which + is 0A in hexadecimal. + - -Can we improve this? Well, for one, it is a bit confusing because -once we have converted a line of text, our input no longer -starts at the beginning of the line. We can modify it to print -a new line instead of a space after each 0A: - + Can we improve this? Well, for one, it is a bit confusing + because once we have converted a line of text, our input no + longer starts at the beginning of the line. We can modify it + to print a new line instead of a space after each + 0A: - -%include 'system.inc' + %include 'system.inc' section .data hex db '0123456789ABCDEF' buffer db 0, 0, ' ' section .text global _start _start: mov cl, ' ' .loop: ; read a byte from stdin push dword 1 push dword buffer push dword stdin sys.read add esp, byte 12 or eax, eax je .done ; convert it to hex movzx eax, byte [buffer] mov [buffer+2], cl cmp al, 0Ah jne .hex mov [buffer+2], al .hex: mov edx, eax shr dl, 4 mov dl, [hex+edx] mov [buffer], dl and al, 0Fh mov al, [hex+eax] mov [buffer+1], al ; print it push dword 3 push dword buffer push dword stdout sys.write add esp, byte 12 jmp short .loop .done: push dword 0 - sys.exit - - -We have stored the space in the CL register. We can -do this safely because, unlike µsoft.windows;, &unix; system -calls do not modify the value of any register they do not use -to return a value in. - + sys.exit - -That means we only need to set CL once. We have, therefore, -added a new label .loop and jump to it for the next byte -instead of jumping at _start. We have also added the -.hex label so we can either have a blank space or a -new line as the third byte of the buffer. - + We have stored the space in the CL register. We can do this + safely because, unlike µsoft.windows;, &unix; system + calls do not modify the value of any register they do not use + to return a value in. - -Once you have changed hex.asm to reflect -these changes, type: - + That means we only need to set CL once. We have, therefore, + added a new label .loop and jump to it for + the next byte instead of jumping at _start. + We have also added the .hex label so we can + either have a blank space or a new line as the third byte of + the buffer. -&prompt.user; nasm -f elf hex.asm + Once you have changed hex.asm to + reflect these changes, type: + + &prompt.user; nasm -f elf hex.asm &prompt.user; ld -s -o hex hex.o &prompt.user; ./hex Hello, World! 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come! 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D &prompt.user; - -That looks better. But this code is quite inefficient! We -are making a system call for every single byte twice (once -to read it, another time to write the output). - + That looks better. But this code is quite inefficient! We + are making a system call for every single byte twice (once to + read it, another time to write the output). + - + + Buffered Input and Output - -Buffered Input and Output + We can improve the efficiency of our code by buffering our + input and output. We create an input buffer and read a whole + sequence of bytes at one time. Then we fetch them one by one + from the buffer. - -We can improve the efficiency of our code by buffering our -input and output. We create an input buffer and read a whole -sequence of bytes at one time. Then we fetch them one by one -from the buffer. - + We also create an output buffer. We store our output in + it until it is full. At that time we ask the kernel to write + the contents of the buffer to + stdout. - -We also create an output buffer. We store our output in it until -it is full. At that time we ask the kernel to write the contents -of the buffer to stdout. - + The program ends when there is no more input. But we + still need to ask the kernel to write the contents of our + output buffer to stdout one last time, + otherwise some of our output would make it to the output + buffer, but never be sent out. Do not forget that, or you + will be wondering why some of your output is missing. - -The program ends when there is no more input. But we still need -to ask the kernel to write the contents of our output buffer -to stdout one last time, otherwise some of our output -would make it to the output buffer, but never be sent out. -Do not forget that, or you will be wondering why some of your -output is missing. - + %include 'system.inc' - -%include 'system.inc' - %define BUFSIZE 2048 section .data hex db '0123456789ABCDEF' section .bss ibuffer resb BUFSIZE obuffer resb BUFSIZE section .text global _start _start: sub eax, eax sub ebx, ebx sub ecx, ecx mov edi, obuffer .loop: ; read a byte from stdin call getchar ; convert it to hex mov dl, al shr al, 4 mov al, [hex+eax] call putchar mov al, dl and al, 0Fh mov al, [hex+eax] call putchar mov al, ' ' cmp dl, 0Ah jne .put mov al, dl .put: call putchar jmp short .loop align 4 getchar: or ebx, ebx jne .fetch call read .fetch: lodsb dec ebx ret read: push dword BUFSIZE mov esi, ibuffer push esi push dword stdin sys.read add esp, byte 12 mov ebx, eax or eax, eax je .done sub eax, eax ret align 4 .done: call write ; flush output buffer push dword 0 sys.exit align 4 putchar: stosb inc ecx cmp ecx, BUFSIZE je write ret align 4 write: sub edi, ecx ; start of buffer push ecx push edi push dword stdout sys.write add esp, byte 12 sub eax, eax sub ecx, ecx ; buffer is empty now - ret - - -We now have a third section in the source code, named -.bss. This section is not included in our -executable file, and, therefore, cannot be initialized. We use -resb instead of db. -It simply reserves the requested size of uninitialized memory -for our use. - + ret - -We take advantage of the fact that the system does not modify the -registers: We use registers for what, otherwise, would have to be -global variables stored in the .data section. This is -also why the &unix; convention of passing parameters to system calls -on the stack is superior to the Microsoft convention of passing -them in the registers: We can keep the registers for our own use. - + We now have a third section in the source code, named + .bss. This section is not included in our + executable file, and, therefore, cannot be initialized. We + use resb instead of + db. It simply reserves + the requested size of uninitialized memory for our use. - -We use EDI and ESI as pointers to the next byte -to be read from or written to. We use EBX and -ECX to keep count of the number of bytes in the -two buffers, so we know when to dump the output to, or read more -input from, the system. - + We take advantage of the fact that the system does not + modify the registers: We use registers for what, otherwise, + would have to be global variables stored in the + .data section. This is also why the + &unix; convention of passing parameters to system calls on the + stack is superior to the Microsoft convention of passing them + in the registers: We can keep the registers for our own + use. - -Let us see how it works now: - + We use EDI and + ESI as pointers to the next + byte to be read from or written to. We use EBX and ECX to keep count of the number of + bytes in the two buffers, so we know when to dump the output + to, or read more input from, the system. -&prompt.user; nasm -f elf hex.asm + Let us see how it works now: + + &prompt.user; nasm -f elf hex.asm &prompt.user; ld -s -o hex hex.o &prompt.user; ./hex Hello, World! Here I come! 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D &prompt.user; - -Not what you expected? The program did not print the output -until we pressed ^D. That is easy to fix by -inserting three lines of code to write the output every time -we have converted a new line to 0A. I have marked -the three lines with > (do not copy the > in your -hex.asm). - + Not what you expected? The program did not print the + output until we pressed ^D. That is + easy to fix by inserting three lines of code to write the + output every time we have converted a new line to + 0A. I have marked the three lines with + > (do not copy the > in your + hex.asm). - -%include 'system.inc' + %include 'system.inc' %define BUFSIZE 2048 section .data hex db '0123456789ABCDEF' section .bss ibuffer resb BUFSIZE obuffer resb BUFSIZE section .text global _start _start: sub eax, eax sub ebx, ebx sub ecx, ecx mov edi, obuffer .loop: ; read a byte from stdin call getchar ; convert it to hex mov dl, al shr al, 4 mov al, [hex+eax] call putchar mov al, dl and al, 0Fh mov al, [hex+eax] call putchar mov al, ' ' cmp dl, 0Ah jne .put mov al, dl .put: call putchar > cmp al, 0Ah > jne .loop > call write jmp short .loop align 4 getchar: or ebx, ebx jne .fetch call read .fetch: lodsb dec ebx ret read: push dword BUFSIZE mov esi, ibuffer push esi push dword stdin sys.read add esp, byte 12 mov ebx, eax or eax, eax je .done sub eax, eax ret align 4 .done: call write ; flush output buffer push dword 0 sys.exit align 4 putchar: stosb inc ecx cmp ecx, BUFSIZE je write ret align 4 write: sub edi, ecx ; start of buffer push ecx push edi push dword stdout sys.write add esp, byte 12 sub eax, eax sub ecx, ecx ; buffer is empty now - ret - + ret - -Now, let us see how it works: - + Now, let us see how it works: -&prompt.user; nasm -f elf hex.asm + &prompt.user; nasm -f elf hex.asm &prompt.user; ld -s -o hex hex.o &prompt.user; ./hex Hello, World! 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come! 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D &prompt.user; - -Not bad for a 644-byte executable, is it! - + Not bad for a 644-byte executable, is it! - - -This approach to buffered input/output still -contains a hidden danger. I will discuss—and -fix—it later, when I talk about the -dark -side of buffering. - + + This approach to buffered input/output still + contains a hidden danger. I will discuss—and + fix—it later, when I talk about the dark side of + buffering. + - -How to Unread a Character + + How to Unread a Character - -This may be a somewhat advanced topic, mostly of interest to -programmers familiar with the theory of compilers. If you wish, -you may skip to the next -section, and perhaps read this later. - - - -While our sample program does not require it, more sophisticated -filters often need to look ahead. In other words, they may need -to see what the next character is (or even several characters). -If the next character is of a certain value, it is part of the -token currently being processed. Otherwise, it is not. - + + This may be a somewhat advanced topic, mostly of + interest to programmers familiar with the theory of + compilers. If you wish, you may skip to the next + section, and perhaps read this later. + - -For example, you may be parsing the input stream for a textual -string (e.g., when implementing a language compiler): If a -character is followed by another character, or perhaps a digit, -it is part of the token you are processing. If it is followed by -white space, or some other value, then it is not part of the -current token. - + While our sample program does not require it, more + sophisticated filters often need to look ahead. In other + words, they may need to see what the next character is (or + even several characters). If the next character is of a + certain value, it is part of the token currently being + processed. Otherwise, it is not. - -This presents an interesting problem: How to return the next -character back to the input stream, so it can be read again -later? - + For example, you may be parsing the input stream for a + textual string (e.g., when implementing a language + compiler): If a character is followed by another character, + or perhaps a digit, it is part of the token you are + processing. If it is followed by white space, or some other + value, then it is not part of the current token. - -One possible solution is to store it in a character variable, -then set a flag. We can modify getchar to check the flag, -and if it is set, fetch the byte from that variable instead of the -input buffer, and reset the flag. But, of course, that slows us -down. - + This presents an interesting problem: How to return the + next character back to the input stream, so it can be read + again later? - -The C language has an ungetc() function, just for that -purpose. Is there a quick way to implement it in our code? -I would like you to scroll back up and take a look at the -getchar procedure and see if you can find a nice and -fast solution before reading the next paragraph. Then come back -here and see my own solution. - + One possible solution is to store it in a character + variable, then set a flag. We can modify + getchar to check the flag, and if it is + set, fetch the byte from that variable instead of the input + buffer, and reset the flag. But, of course, that slows us + down. - -The key to returning a character back to the stream is in how -we are getting the characters to start with: - + The C language has an ungetc() + function, just for that purpose. Is there a quick way to + implement it in our code? I would like you to scroll back + up and take a look at the getchar + procedure and see if you can find a nice and fast solution + before reading the next paragraph. Then come back here and + see my own solution. - -First we check if the buffer is empty by testing the value -of EBX. If it is zero, we call the -read procedure. - + The key to returning a character back to the stream is + in how we are getting the characters to start with: - -If we do have a character available, we use lodsb, then -decrease the value of EBX. The lodsb -instruction is effectively identical to: - + First we check if the buffer is empty by testing the + value of EBX. If it is + zero, we call the read + procedure. - - mov al, [esi] - inc esi - + If we do have a character available, we use lodsb, then decrease the value of + EBX. The lodsb instruction is effectively + identical to: - -The byte we have fetched remains in the buffer until the next -time read is called. We do not know when that happens, -but we do know it will not happen until the next call to -getchar. Hence, to "return" the last-read byte back -to the stream, all we have to do is decrease the value of -ESI and increase the value of EBX: - + mov al, [esi] + inc esi - -ungetc: + The byte we have fetched remains in the buffer until the + next time read is called. We do not know + when that happens, but we do know it will not happen until the + next call to getchar. Hence, to "return" + the last-read byte back to the stream, all we have to do is + decrease the value of ESI + and increase the value of EBX: + + ungetc: dec esi inc ebx - ret - + ret - -But, be careful! We are perfectly safe doing this if our look-ahead -is at most one character at a time. If we are examining more than -one upcoming character and call ungetc several times -in a row, it will work most of the time, but not all the time -(and will be tough to debug). Why? - + But, be careful! We are perfectly safe doing this if our + look-ahead is at most one character at a time. If we are + examining more than one upcoming character and call + ungetc several times in a row, it will + work most of the time, but not all the time (and will be tough + to debug). Why? - -Because as long as getchar does not have to call -read, all of the pre-read bytes are still in the buffer, -and our ungetc works without a glitch. But the moment -getchar calls read, -the contents of the buffer change. - + Because as long as getchar does not + have to call read, all of the pre-read + bytes are still in the buffer, and our + ungetc works without a glitch. But the + moment getchar calls + read, the contents of the buffer + change. - -We can always rely on ungetc working properly on the last -character we have read with getchar, but not on anything -we have read before that. - + We can always rely on ungetc working + properly on the last character we have read with + getchar, but not on anything we have read + before that. - -If your program reads more than one byte ahead, you have at least -two choices: - + If your program reads more than one byte ahead, you have + at least two choices: - -If possible, modify the program so it only reads one byte ahead. -This is the simplest solution. - + If possible, modify the program so it only reads one byte + ahead. This is the simplest solution. - -If that option is not available, first of all determine the maximum -number of characters your program needs to return to the input -stream at one time. Increase that number slightly, just to be -sure, preferably to a multiple of 16—so it aligns nicely. -Then modify the .bss section of your code, and create -a small "spare" buffer right before your input buffer, -something like this: - + If that option is not available, first of all determine + the maximum number of characters your program needs to return + to the input stream at one time. Increase that number + slightly, just to be sure, preferably to a multiple of + 16—so it aligns nicely. Then modify the + .bss section of your code, and create a + small "spare" buffer right before your input buffer, something + like this: - -section .bss + section .bss resb 16 ; or whatever the value you came up with ibuffer resb BUFSIZE -obuffer resb BUFSIZE - +obuffer resb BUFSIZE - -You also need to modify your ungetc to pass the value -of the byte to unget in AL: - + You also need to modify your ungetc + to pass the value of the byte to unget in AL: - -ungetc: + ungetc: dec esi inc ebx mov [esi], al - ret - + ret - -With this modification, you can call ungetc -up to 17 times in a row safely (the first call will still -be within the buffer, the remaining 16 may be either within -the buffer or within the "spare"). - + With this modification, you can call + ungetc up to 17 times in a row safely + (the first call will still be within the buffer, the remaining + 16 may be either within the buffer or within the + "spare"). + + - + + Command Line Arguments - + Our hex program will be more + useful if it can read the names of an input and output file from + its command line, i.e., if it can process the command line + arguments. But... Where are they? -Command Line Arguments + Before a &unix; system starts a program, it pushes some data on the stack, then + jumps at the _start label of the program. + Yes, I said jumps, not calls. That means the data can be + accessed by reading [esp+offset], or by + simply popping it. - -Our hex program will be more useful if it can -read the names of an input and output file from its command -line, i.e., if it can process the command line arguments. -But... Where are they? - + The value at the top of the stack contains the number of + command line arguments. It is traditionally called + argc, for "argument count." - -Before a &unix; system starts a program, it pushes some -data on the stack, then jumps at the _start -label of the program. Yes, I said jumps, not calls. That means the -data can be accessed by reading [esp+offset], -or by simply popping it. - + Command line arguments follow next, all + argc of them. These are typically referred + to as argv, for "argument value(s)." That + is, we get argv[0], + argv[1], ..., + argv[argc-1]. These are not the actual + arguments, but pointers to arguments, i.e., memory addresses of + the actual arguments. The arguments themselves are + NUL-terminated character strings. - -The value at the top of the stack contains the number of -command line arguments. It is traditionally called -argc, for "argument count." - + The argv list is followed by a NULL + pointer, which is simply a 0. There is + more, but this is enough for our purposes right now. - -Command line arguments follow next, all argc of them. -These are typically referred to as argv, for -"argument value(s)." That is, we get argv[0], -argv[1], ..., -argv[argc-1]. These are not the actual -arguments, but pointers to arguments, i.e., memory addresses of -the actual arguments. The arguments themselves are -NUL-terminated character strings. - + + If you have come from the &ms-dos; + programming environment, the main difference is that each + argument is in a separate string. The second difference is + that there is no practical limit on how many arguments there + can be. + - -The argv list is followed by a NULL pointer, -which is simply a 0. There is more, but this is -enough for our purposes right now. - + Armed with this knowledge, we are almost ready for the next + version of hex.asm. First, however, we + need to add a few lines to + system.inc: - - -If you have come from the &ms-dos; programming -environment, the main difference is that each argument is in -a separate string. The second difference is that there is no -practical limit on how many arguments there can be. - - + First, we need to add two new entries to our list of system + call numbers: - -Armed with this knowledge, we are almost ready for the next -version of hex.asm. First, however, we need to -add a few lines to system.inc: - + %define SYS_open 5 +%define SYS_close 6 - -First, we need to add two new entries to our list of system -call numbers: - + Then we add two new macros at the end of the file: - -%define SYS_open 5 -%define SYS_close 6 - - - -Then we add two new macros at the end of the file: - - - -%macro sys.open 0 + %macro sys.open 0 system SYS_open %endmacro %macro sys.close 0 system SYS_close -%endmacro - +%endmacro - -Here, then, is our modified source code: - + Here, then, is our modified source code: - -%include 'system.inc' + %include 'system.inc' %define BUFSIZE 2048 section .data fd.in dd stdin fd.out dd stdout hex db '0123456789ABCDEF' section .bss ibuffer resb BUFSIZE obuffer resb BUFSIZE section .text align 4 err: push dword 1 ; return failure sys.exit align 4 global _start _start: add esp, byte 8 ; discard argc and argv[0] pop ecx jecxz .init ; no more arguments ; ECX contains the path to input file push dword 0 ; O_RDONLY push ecx sys.open jc err ; open failed add esp, byte 8 mov [fd.in], eax pop ecx jecxz .init ; no more arguments ; ECX contains the path to output file push dword 420 ; file mode (644 octal) push dword 0200h | 0400h | 01h ; O_CREAT | O_TRUNC | O_WRONLY push ecx sys.open jc err add esp, byte 12 mov [fd.out], eax .init: sub eax, eax sub ebx, ebx sub ecx, ecx mov edi, obuffer .loop: ; read a byte from input file or stdin call getchar ; convert it to hex mov dl, al shr al, 4 mov al, [hex+eax] call putchar mov al, dl and al, 0Fh mov al, [hex+eax] call putchar mov al, ' ' cmp dl, 0Ah jne .put mov al, dl .put: call putchar cmp al, dl jne .loop call write jmp short .loop align 4 getchar: or ebx, ebx jne .fetch call read .fetch: lodsb dec ebx ret read: push dword BUFSIZE mov esi, ibuffer push esi push dword [fd.in] sys.read add esp, byte 12 mov ebx, eax or eax, eax je .done sub eax, eax ret align 4 .done: call write ; flush output buffer ; close files push dword [fd.in] sys.close push dword [fd.out] sys.close ; return success push dword 0 sys.exit align 4 putchar: stosb inc ecx cmp ecx, BUFSIZE je write ret align 4 write: sub edi, ecx ; start of buffer push ecx push edi push dword [fd.out] sys.write add esp, byte 12 sub eax, eax sub ecx, ecx ; buffer is empty now - ret - + ret - -In our .data section we now have two new variables, -fd.in and fd.out. We store the input and -output file descriptors here. - + In our .data section we now have two + new variables, fd.in and + fd.out. We store the input and output file + descriptors here. - -In the .text section we have replaced the references -to stdin and stdout with -[fd.in] and [fd.out]. - + In the .text section we have replaced + the references to stdin and + stdout with [fd.in] and + [fd.out]. - -The .text section now starts with a simple error -handler, which does nothing but exit the program with a return -value of 1. -The error handler is before _start so we are -within a short distance from where the errors occur. - + The .text section now starts with a + simple error handler, which does nothing but exit the program + with a return value of 1. The error + handler is before _start so we are within + a short distance from where the errors occur. - -Naturally, the program execution still begins at _start. -First, we remove argc and argv[0] from the -stack: They are of no interest to us (in this program, that is). - + Naturally, the program execution still begins at + _start. First, we remove + argc and argv[0] from + the stack: They are of no interest to us (in this program, + that is). - -We pop argv[1] to ECX. This -register is particularly suited for pointers, as we can handle -NULL pointers with jecxz. If argv[1] -is not NULL, we try to open the file named in the first -argument. Otherwise, we continue the program as before: Reading -from stdin, writing to stdout. -If we fail to open the input file (e.g., it does not exist), -we jump to the error handler and quit. - + We pop argv[1] to ECX. This register is + particularly suited for pointers, as we can handle NULL + pointers with jecxz. If + argv[1] is not NULL, we try to open the + file named in the first argument. Otherwise, we continue the + program as before: Reading from stdin, + writing to stdout. If we fail to open the + input file (e.g., it does not exist), we jump to the error + handler and quit. - -If all went well, we now check for the second argument. If -it is there, we open the output file. Otherwise, we send -the output to stdout. If we fail to open the output -file (e.g., it exists and we do not have the write permission), -we, again, jump to the error handler. - + If all went well, we now check for the second argument. + If it is there, we open the output file. Otherwise, we send + the output to stdout. If we fail to open + the output file (e.g., it exists and we do not have the write + permission), we, again, jump to the error handler. - -The rest of the code is the same as before, except we close -the input and output files before exiting, and, as mentioned, -we use [fd.in] and [fd.out]. - + The rest of the code is the same as before, except we + close the input and output files before exiting, and, as + mentioned, we use [fd.in] and + [fd.out]. - -Our executable is now a whopping 768 bytes long. - + Our executable is now a whopping 768 bytes long. - -Can we still improve it? Of course! Every program can be improved. -Here are a few ideas of what we could do: - + Can we still improve it? Of course! Every program can be + improved. Here are a few ideas of what we could do: - - - -Have our error handler print a message to -stderr. - - + + + Have our error handler print a message to + stderr. + - - -Add error handlers to the read -and write functions. - - + + Add error handlers to the read + and write functions. + - - -Close stdin when we open an input file, -stdout when we open an output file. - - + + Close stdin when we open an input + file, stdout when we open an output + file. + - - -Add command line switches, such as -i -and -o, so we can list the input and -output files in any order, or perhaps read from -stdin and write to a file. - - + + Add command line switches, such as + -i and -o, + so we can list the input and output files in any order, + or perhaps read from stdin and write to + a file. + - - -Print a usage message if command line arguments are incorrect. - - + + Print a usage message if command line arguments are + incorrect. + - - -I shall leave these enhancements as an exercise to the reader: -You already know everything you need to know to implement them. - + - + I shall leave these enhancements as an exercise to the + reader: You already know everything you need to know to + implement them. + - -&unix; Environment + + &unix; Environment - -An important &unix; concept is the environment, which is defined by -environment variables. Some are set by the system, others -by you, yet others by the shell, or any program -that loads another program. - + An important &unix; concept is the environment, which is + defined by environment variables. Some + are set by the system, others by you, yet others by the + shell, or any program that loads + another program. - -How to Find Environment Variables + + How to Find Environment Variables - -I said earlier that when a program starts executing, the stack -contains argc followed by the NULL-terminated -argv array, followed by something else. The -"something else" is the environment, or, -to be more precise, a NULL-terminated array of pointers to -environment variables. This is often referred -to as env. - + I said earlier that when a program starts executing, the + stack contains argc followed by the + NULL-terminated argv array, followed by + something else. The "something else" is the + environment, or, to be more precise, a + NULL-terminated array of pointers to environment + variables. This is often referred to as + env. - -The structure of env is the same as that of -argv, a list of memory addresses followed by a -NULL (0). In this case, there is no -"envc"—we figure out where the array ends -by searching for the final NULL. - + The structure of env is the same as + that of argv, a list of memory addresses + followed by a NULL (0). In this case, + there is no "envc"—we figure out + where the array ends by searching for the final NULL. - -The variables usually come in the name=value -format, but sometimes the =value part -may be missing. We need to account for that possibility. - + The variables usually come in the + name=value format, but sometimes the + =value part may be missing. We need to + account for that possibility. + - + + webvars - -webvars + I could just show you some code that prints the + environment the same way the &unix; + env command does. But I thought + it would be more interesting to write a simple assembly + language CGI utility. - -I could just show you some code that prints the environment -the same way the &unix; env command does. But -I thought it would be more interesting to write a simple -assembly language CGI utility. - + + CGI: a Quick Overview - -CGI: A Quick Overview + I have a detailed + CGI tutorial on my web site, + but here is a very quick overview of + CGI: - -I have a -detailed -CGI tutorial on my web site, -but here is a very quick overview of CGI: - + + + The web server communicates with the + CGI program by setting + environment variables. + - - - -The web server communicates with the CGI -program by setting environment variables. - - + + The CGI program sends its + output to stdout. The web server + reads it from there. + - - -The CGI program -sends its output to stdout. -The web server reads it from there. - - + + It must start with an HTTP + header followed by two blank lines. + - - -It must start with an HTTP -header followed by two blank lines. - - + + It then prints the HTML code, + or whatever other type of data it is producing. + + - - -It then prints the HTML -code, or whatever other type of data it is producing. - - + + While certain environment + variables use standard names, others vary, + depending on the web server. That makes + webvars quite a useful + diagnostic tool. + + - - - -While certain environment variables use -standard names, others vary, depending on the web server. That -makes webvars -quite a useful diagnostic tool. - - + + The Code - + Our webvars program, then, + must send out the HTTP header followed + by some HTML mark-up. It then must + read the environment variables one by + one and send them out as part of the + HTML page. - -The Code + The code follows. I placed comments and explanations + right inside the code: - -Our webvars program, then, must send out -the HTTP header followed by some -HTML mark-up. It then must read -the environment variables one by one -and send them out as part of the -HTML page. - - - -The code follows. I placed comments and explanations -right inside the code: - - - -;;;;;;; webvars.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;;;;; webvars.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; Copyright (c) 2000 G. Adam Stanislav ; All rights reserved. ; ; Redistribution and use in source and binary forms, with or without ; modification, are permitted provided that the following conditions ; are met: ; 1. Redistributions of source code must retain the above copyright ; notice, this list of conditions and the following disclaimer. ; 2. Redistributions in binary form must reproduce the above copyright ; notice, this list of conditions and the following disclaimer in the ; documentation and/or other materials provided with the distribution. ; ; THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND ; ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE ; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ; ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE ; FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL ; DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS ; OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) ; HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT ; LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY ; OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF ; SUCH DAMAGE. ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; Version 1.0 ; ; Started: 8-Dec-2000 ; Updated: 8-Dec-2000 ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; %include 'system.inc' section .data http db 'Content-type: text/html', 0Ah, 0Ah db '<?xml version="1.0" encoding="utf-8"?>', 0Ah db '<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Strict//EN" ' db '"DTD/xhtml1-strict.dtd">', 0Ah db '<html xmlns="http://www.w3.org/1999/xhtml" ' db 'xml.lang="en" lang="en">', 0Ah db '<head>', 0Ah db '<title>Web Environment</title>', 0Ah db '<meta name="author" content="G. Adam Stanislav" />', 0Ah db '</head>', 0Ah, 0Ah db '<body bgcolor="#ffffff" text="#000000" link="#0000ff" ' db 'vlink="#840084" alink="#0000ff">', 0Ah db '<div class="webvars">', 0Ah db '<h1>Web Environment</h1>', 0Ah db '<p>The following <b>environment variables</b> are defined ' db 'on this web server:</p>', 0Ah, 0Ah db '<table align="center" width="80" border="0" cellpadding="10" ' db 'cellspacing="0" class="webvars">', 0Ah httplen equ $-http left db '<tr>', 0Ah db '<td class="name"><tt>' leftlen equ $-left middle db '</tt></td>', 0Ah db '<td class="value"><tt><b>' midlen equ $-middle undef db '<i>(undefined)</i>' undeflen equ $-undef right db '</b></tt></td>', 0Ah db '</tr>', 0Ah rightlen equ $-right wrap db '</table>', 0Ah db '</div>', 0Ah db '</body>', 0Ah db '</html>', 0Ah, 0Ah wraplen equ $-wrap section .text global _start _start: ; First, send out all the http and xhtml stuff that is ; needed before we start showing the environment push dword httplen push dword http push dword stdout sys.write ; Now find how far on the stack the environment pointers ; are. We have 12 bytes we have pushed before "argc" mov eax, [esp+12] ; We need to remove the following from the stack: ; ; The 12 bytes we pushed for sys.write ; The 4 bytes of argc ; The EAX*4 bytes of argv ; The 4 bytes of the NULL after argv ; ; Total: ; 20 + eax * 4 ; ; Because stack grows down, we need to ADD that many bytes ; to ESP. lea esp, [esp+20+eax*4] cld ; This should already be the case, but let's be sure. ; Loop through the environment, printing it out .loop: pop edi or edi, edi ; Done yet? je near .wrap ; Print the left part of HTML push dword leftlen push dword left push dword stdout sys.write ; It may be tempting to search for the '=' in the env string next. ; But it is possible there is no '=', so we search for the ; terminating NUL first. mov esi, edi ; Save start of string sub ecx, ecx not ecx ; ECX = FFFFFFFF sub eax, eax repne scasb not ecx ; ECX = string length + 1 mov ebx, ecx ; Save it in EBX ; Now is the time to find '=' mov edi, esi ; Start of string mov al, '=' repne scasb not ecx add ecx, ebx ; Length of name push ecx push esi push dword stdout sys.write ; Print the middle part of HTML table code push dword midlen push dword middle push dword stdout sys.write ; Find the length of the value not ecx lea ebx, [ebx+ecx-1] ; Print "undefined" if 0 or ebx, ebx jne .value mov ebx, undeflen mov edi, undef .value: push ebx push edi push dword stdout sys.write ; Print the right part of the table row push dword rightlen push dword right push dword stdout sys.write ; Get rid of the 60 bytes we have pushed add esp, byte 60 ; Get the next variable jmp .loop .wrap: ; Print the rest of HTML push dword wraplen push dword wrap push dword stdout sys.write ; Return success push dword 0 - sys.exit - + sys.exit - -This code produces a 1,396-byte executable. Most of it is data, -i.e., the HTML mark-up we need to send out. - + This code produces a 1,396-byte executable. Most of it is + data, i.e., the HTML mark-up we need to + send out. - -Assemble and link it as usual: - + Assemble and link it as usual: -&prompt.user; nasm -f elf webvars.asm + &prompt.user; nasm -f elf webvars.asm &prompt.user; ld -s -o webvars webvars.o - -To use it, you need to upload webvars to your -web server. Depending on how your web server is set up, you -may have to store it in a special cgi-bin directory, -or perhaps rename it with a .cgi extension. - + To use it, you need to upload webvars + to your web server. Depending on how your web server is set + up, you may have to store it in a special + cgi-bin directory, or perhaps rename it + with a .cgi extension. - -Then you need to use your browser to view its output. -To see its output on my web server, please go to -http://www.int80h.org/webvars/. -If curious about the additional environment variables -present in a password protected web directory, go to -http://www.int80h.org/private/, -using the name asm and password -programmer. - + Then you need to use your browser to view its output. To + see its output on my web server, please go to http://www.int80h.org/webvars/. + If curious about the additional environment variables present + in a password protected web directory, go to http://www.int80h.org/private/, + using the name asm and password + programmer. + + + - + + Working with Files - + We have already done some basic file work: We know how to + open and close them, how to read and write them using buffers. + But &unix; offers much more functionality when it comes to + files. We will examine some of it in this section, and end up + with a nice file conversion utility. - + Indeed, let us start at the end, that is, with the file + conversion utility. It always makes programming easier when we + know from the start what the end product is supposed to + do. - -Working with Files + One of the first programs I wrote for &unix; was tuc, + a text-to-&unix; file converter. It converts a text file from + other operating systems to a &unix; text file. In other words, + it changes from different kind of line endings to the newline + convention of &unix;. It saves the output in a different file. + Optionally, it converts a &unix; text file to a + DOS text file. - -We have already done some basic file work: We know how -to open and close them, how to read and write them using -buffers. But &unix; offers much more functionality when it -comes to files. We will examine some of it in this section, -and end up with a nice file conversion utility. - + I have used tuc extensively, but + always only to convert from some other OS to + &unix;, never the other way. I have always wished it would just + overwrite the file instead of me having to send the output to a + different file. Most of the time, I end up using it like + this: - -Indeed, let us start at the end, that is, with the file -conversion utility. It always makes programming easier -when we know from the start what the end product is -supposed to do. - - - -One of the first programs I wrote for &unix; was -tuc, -a text-to-&unix; file converter. It converts a text -file from other operating systems to a &unix; text file. -In other words, it changes from different kind of line endings -to the newline convention of &unix;. It saves the output -in a different file. Optionally, it converts a &unix; text -file to a DOS text file. - - - -I have used tuc extensively, but always -only to convert from some other OS -to &unix;, never the other way. I have always wished -it would just overwrite the file instead of me having -to send the output to a different file. Most of the time, -I end up using it like this: - - -&prompt.user; tuc myfile tempfile + &prompt.user; tuc myfile tempfile &prompt.user; mv tempfile myfile - -It would be nice to have a ftuc, -i.e., fast tuc, and use it like this: - + It would be nice to have a ftuc, + i.e., fast tuc, and use it like + this: -&prompt.user; ftuc myfile + &prompt.user; ftuc myfile - -In this chapter, then, we will write -ftuc in assembly language -(the original tuc -is in C), and study various -file-oriented kernel services in the process. - + In this chapter, then, we will write + ftuc in assembly language (the + original tuc is in C), and study + various file-oriented kernel services in the process. - -At first sight, such a file conversion is very -simple: All you have to do is strip the carriage -returns, right? - + At first sight, such a file conversion is very simple: All + you have to do is strip the carriage returns, right? - -If you answered yes, think again: That approach will -work most of the time (at least with MS -DOS text files), but will fail occasionally. - + If you answered yes, think again: That approach will work + most of the time (at least with MS DOS text + files), but will fail occasionally. - -The problem is that not all non &unix; text files end their -line with the carriage return / line feed sequence. Some -use carriage returns without line feeds. Others combine several -blank lines into a single carriage return followed by several -line feeds. And so on. - + The problem is that not all non &unix; text files end their + line with the carriage return / line feed sequence. Some use + carriage returns without line feeds. Others combine several + blank lines into a single carriage return followed by several + line feeds. And so on. - -A text file converter, then, must be able to handle -any possible line endings: - + A text file converter, then, must be able to handle any + possible line endings: - - - -carriage return / line feed - - + + + carriage return / line feed + - - -carriage return - - + + carriage return + - - -line feed / carriage return - - + + line feed / carriage return + - - -line feed - - + + line feed + + - - -It should also handle files that use some kind of a -combination of the above (e.g., carriage return followed -by several line feeds). - + It should also handle files that use some kind of a + combination of the above (e.g., carriage return followed by + several line feeds). - -Finite State Machine + + Finite State Machine - -The problem is easily solved by the use of a technique -called finite state machine, originally developed -by the designers of digital electronic circuits. A -finite state machine is a digital circuit -whose output is dependent not only on its input but on -its previous input, i.e., on its state. The microprocessor -is an example of a finite state machine: Our -assembly language code is assembled to machine language in which -some assembly language code produces a single byte -of machine language, while others produce several bytes. -As the microprocessor fetches the bytes from the memory -one by one, some of them simply change its state rather than -produce some output. When all the bytes of the op code are -fetched, the microprocessor produces some output, or changes -the value of a register, etc. - + The problem is easily solved by the use of a technique + called finite state machine, originally + developed by the designers of digital electronic circuits. A + finite state machine is a digital circuit + whose output is dependent not only on its input but on its + previous input, i.e., on its state. The microprocessor is an + example of a finite state machine: Our + assembly language code is assembled to machine language in + which some assembly language code produces a single byte of + machine language, while others produce several bytes. As the + microprocessor fetches the bytes from the memory one by one, + some of them simply change its state rather than produce some + output. When all the bytes of the op code are fetched, the + microprocessor produces some output, or changes the value of + a register, etc. - -Because of that, all software is essentially a sequence of state -instructions for the microprocessor. Nevertheless, the concept -of finite state machine is useful in software design as well. - + Because of that, all software is essentially a sequence of + state instructions for the microprocessor. Nevertheless, the + concept of finite state machine is useful + in software design as well. - -Our text file converter can be designed as a finite state machine with three -possible states. We could call them states 0-2, -but it will make our life easier if we give them symbolic names: - + Our text file converter can be designer as a + finite state machine with three possible + states. We could call them states 0-2, but it will make our + life easier if we give them symbolic names: - - - -ordinary - - + + + ordinary + - - -cr - - + + cr + - - -lf - - + + lf + + - - -Our program will start in the ordinary -state. During this state, the program action depends on -its input as follows: - + Our program will start in the ordinary + state. During this state, the program action depends on its + input as follows: - - - -If the input is anything other than a carriage return -or line feed, the input is simply passed on to the output. The -state remains unchanged. - - + + + If the input is anything other than a carriage return + or line feed, the input is simply passed on to the output. + The state remains unchanged. + - - -If the input is a carriage return, the state is changed -to cr. The input is then discarded, i.e., -no output is made. - - + + If the input is a carriage return, the state is + changed to cr. The input is then + discarded, i.e., no output is made. + - - -If the input is a line feed, the state is changed to -lf. The input is then discarded. - - + + If the input is a line feed, the state is changed to + lf. The input is then discarded. + + - - -Whenever we are in the cr state, it is -because the last input was a carriage return, which was -unprocessed. What our software does in this state again -depends on the current input: - + Whenever we are in the cr state, it is + because the last input was a carriage return, which was + unprocessed. What our software does in this state again + depends on the current input: - - - -If the input is anything other than a carriage return -or line feed, output a line feed, then output the input, then -change the state to ordinary. - - + + + If the input is anything other than a carriage return + or line feed, output a line feed, then output the input, + then change the state to ordinary. + - - -If the input is a carriage return, we have received -two (or more) carriage returns in a row. We discard the -input, we output a line feed, and leave the state unchanged. - - + + If the input is a carriage return, we have received + two (or more) carriage returns in a row. We discard the + input, we output a line feed, and leave the state + unchanged. + - - -If the input is a line feed, we output the line feed -and change the state to ordinary. Note that -this is not the same as the first case above – if we tried -to combine them, we would be outputting two line feeds -instead of one. - - + + If the input is a line feed, we output the line feed + and change the state to ordinary. Note + that this is not the same as the first case above – + if we tried to combine them, we would be outputting two + line feeds instead of one. + + - - -Finally, we are in the lf state after -we have received a line feed that was not preceded by a -carriage return. This will happen when our file already is -in &unix; format, or whenever several lines in a row are -expressed by a single carriage return followed by several -line feeds, or when line ends with a line feed / -carriage return sequence. Here is how we need to handle -our input in this state: - + Finally, we are in the lf state after we + have received a line feed that was not preceded by a + carriage return. This will happen when our file already is in + &unix; format, or whenever several lines in a row are + expressed by a single carriage return followed by several line + feeds, or when line ends with a line feed / carriage return + sequence. Here is how we need to handle our input in this + state: - - - -If the input is anything other than a carriage return or -line feed, we output a line feed, then output the input, then -change the state to ordinary. This is exactly -the same action as in the cr state upon -receiving the same kind of input. - - + + + If the input is anything other than a carriage return + or line feed, we output a line feed, then output the + input, then change the state to ordinary. + This is exactly the same action as in the + cr state upon receiving the same kind of + input. + - - -If the input is a carriage return, we discard the input, -we output a line feed, then change the state to ordinary. - - + + If the input is a carriage return, we discard the + input, we output a line feed, then change the state to + ordinary. + - - -If the input is a line feed, we output the line feed, -and leave the state unchanged. - - + + If the input is a line feed, we output the line feed, + and leave the state unchanged. + + - - -The Final State + + The Final State - -The above finite state machine works for the entire file, but leaves -the possibility that the final line end will be ignored. That will -happen whenever the file ends with a single carriage return or -a single line feed. I did not think of it when I wrote -tuc, just to discover that -occasionally it strips the last line ending. - + The above finite state machine + works for the entire file, but leaves the possibility that + the final line end will be ignored. That will happen + whenever the file ends with a single carriage return or a + single line feed. I did not think of it when I wrote + tuc, just to discover that + occasionally it strips the last line ending. - -This problem is easily fixed by checking the state after the -entire file was processed. If the state is not -ordinary, we simply -need to output one last line feed. - + This problem is easily fixed by checking the state after + the entire file was processed. If the state is not + ordinary, we simply need to output one last + line feed. - - -Now that we have expressed our algorithm as a finite state machine, -we could easily design a dedicated digital electronic -circuit (a "chip") to do the conversion for us. Of course, -doing so would be considerably more expensive than writing -an assembly language program. - - + + Now that we have expressed our algorithm as a + finite state machine, we could easily + design a dedicated digital electronic circuit (a "chip") + to do the conversion for us. Of course, doing so would be + considerably more expensive than writing an assembly + language program. + + - + + The Output Counter - -The Output Counter + Because our file conversion program may be combining two + characters into one, we need to use an output counter. We + initialize it to 0, and increase it + every time we send a character to the output. At the end of + the program, the counter will tell us what size we need to + set the file to. + + - -Because our file conversion program may be combining two -characters into one, we need to use an output counter. We -initialize it to 0, and increase it -every time we send a character to the output. At the end of -the program, the counter will tell us what size we need -to set the file to. - + + Implementing FSM in Software - + The hardest part of working with a finite state + machine is analyzing the problem and expressing + it as a finite state machine. That + accomplished, the software almost writes itself. - + In a high-level language, such as C, there are several + main approaches. One is to use a switch statement which chooses + what function should be run. For example, - -Implementing FSM in Software - - -The hardest part of working with a finite state machine -is analyzing the problem and expressing it as a -finite state machine. That accomplished, -the software almost writes itself. - - - -In a high-level language, such as C, there are several main -approaches. One is to use a switch statement -which chooses what function should be run. For example, - - - - switch (state) { + switch (state) { default: case REGULAR: regular(inputchar); break; case CR: cr(inputchar); break; case LF: lf(inputchar); break; - } - + } - -Another approach is by using an array of function pointers, -something like this: - + Another approach is by using an array of function + pointers, something like this: - - (output[state])(inputchar); - + (output[state])(inputchar); - -Yet another is to have state be a -function pointer, set to point at the appropriate function: - + Yet another is to have state be a + function pointer, set to point at the appropriate + function: - - (*state)(inputchar); - - -This is the approach we will use in our program because it is very easy to do in assembly language, and very fast, too. We will simply keep the address of the right procedure in EBX, and then just issue: + (*state)(inputchar); - - call ebx - + This is the approach we will use in our program because it + is very easy to do in assembly language, and very fast, too. + We will simply keep the address of the right procedure in + EBX, and then just + issue: - -This is possibly faster than hardcoding the address in the code -because the microprocessor does not have to fetch the address from -the memory—it is already stored in one of its registers. I said -possibly because with the caching modern -microprocessors do, either way may be equally fast. - + call ebx - + This is possibly faster than hardcoding the address in the + code because the microprocessor does not have to fetch the + address from the memory—it is already stored in one of + its registers. I said possibly because + with the caching modern microprocessors do, either way may be + equally fast. + - -Memory Mapped Files + + Memory Mapped Files - -Because our program works on a single file, we cannot use the -approach that worked for us before, i.e., to read from an input -file and to write to an output file. - + Because our program works on a single file, we cannot use + the approach that worked for us before, i.e., to read from an + input file and to write to an output file. - -&unix; allows us to map a file, or a section of a file, -into memory. To do that, we first need to open the file with the -appropriate read/write flags. Then we use the mmap -system call to map it into the memory. One nice thing about -mmap is that it automatically works with -virtual memory: We can map more of the file into the memory than -we have physical memory available, yet still access it through -regular memory op codes, such as mov, -lods, and stos. -Whatever changes we make to the memory image of the file will be -written to the file by the system. We do not even have to keep -the file open: As long as it stays mapped, we can -read from it and write to it. - + &unix; allows us to map a file, or a section of a file, + into memory. To do that, we first need to open the file with + the appropriate read/write flags. Then we use the mmap system call to map it into + the memory. One nice thing about mmap is that it automatically + works with virtual memory: We can map more of the file into + the memory than we have physical memory available, yet still + access it through regular memory op codes, such as mov, lods, and stos. Whatever changes we make to + the memory image of the file will be written to the file by + the system. We do not even have to keep the file open: As + long as it stays mapped, we can read from it and write to + it. - -The 32-bit Intel microprocessors can access up to four -gigabytes of memory – physical or virtual. The FreeBSD system -allows us to use up to a half of it for file mapping. - + The 32-bit Intel microprocessors can access up to four + gigabytes of memory – physical or virtual. The FreeBSD + system allows us to use up to a half of it for file + mapping. - -For simplicity sake, in this tutorial we will only convert files -that can be mapped into the memory in their entirety. There are -probably not too many text files that exceed two gigabytes in size. -If our program encounters one, it will simply display a message -suggesting we use the original -tuc instead. - + For simplicity sake, in this tutorial we will only convert + files that can be mapped into the memory in their entirety. + There are probably not too many text files that exceed two + gigabytes in size. If our program encounters one, it will + simply display a message suggesting we use the original + tuc instead. - -If you examine your copy of syscalls.master, -you will find two separate syscalls named mmap. -This is because of evolution of &unix;: There was the traditional -BSD mmap, -syscall 71. That one was superseded by the &posix; mmap, -syscall 197. The FreeBSD system supports both because -older programs were written by using the original BSD -version. But new software uses the &posix; version, -which is what we will use. - + If you examine your copy of + syscalls.master, you will find two + separate syscalls named mmap. This is because of + evolution of &unix;: There was the traditional + BSD + mmap, syscall 71. That one was + superseded by the &posix; mmap, syscall 197. The FreeBSD + system supports both because older programs were written by + using the original BSD version. But new + software uses the &posix; version, which is + what we will use. - -The syscalls.master file lists -the &posix; version like this: - + The syscalls.master lists the + &posix; version like this: - -197 STD BSD { caddr_t mmap(caddr_t addr, size_t len, int prot, \ - int flags, int fd, long pad, off_t pos); } - + 197 STD BSD { caddr_t mmap(caddr_t addr, size_t len, int prot, \ + int flags, int fd, long pad, off_t pos); } - -This differs slightly from what -mmap2 -says. That is because -mmap2 -describes the C version. - + This differs slightly from what + mmap2 + says. That is because + mmap2 + describes the C version. - -The difference is in the long pad argument, which is not present in the C version. However, the FreeBSD syscalls add a 32-bit pad after pushing a 64-bit argument. In this case, off_t is a 64-bit value. + The difference is in the long pad + argument, which is not present in the C version. However, the + FreeBSD syscalls add a 32-bit pad after pushing a 64-bit argument. In this + case, off_t is a 64-bit value. - -When we are finished working with a memory-mapped file, -we unmap it with the munmap syscall: - + When we are finished working with a memory-mapped file, we + unmap it with the munmap syscall: - - -For an in-depth treatment of mmap, see -W. Richard Stevens' -Unix -Network Programming, Volume 2, Chapter 12. - - + + For an in-depth treatment of mmap, see W. Richard Stevens' + Unix + Network Programming, Volume 2, Chapter 12. + + - + + Determining File Size - -Determining File Size + Because we need to tell mmap how many bytes of the file to + map into the memory, and because we want to map the entire + file, we need to determine the size of the file. - -Because we need to tell mmap how many bytes -of the file to map into the memory, and because we want to map -the entire file, we need to determine the size of the file. - + We can use the fstat + syscall to get all the information about an open file that the + system can give us. That includes the file size. - -We can use the fstat syscall to get all -the information about an open file that the system can give us. -That includes the file size. - + Again, syscalls.master lists two + versions of fstat, a + traditional one (syscall 62), and a &posix; + one (syscall 189). Naturally, we will use the + &posix; version: - -Again, syscalls.master lists two versions -of fstat, a traditional one -(syscall 62), and a &posix; one -(syscall 189). Naturally, we will use the -&posix; version: - + 189 STD POSIX { int fstat(int fd, struct stat *sb); } - -189 STD POSIX { int fstat(int fd, struct stat *sb); } - + This is a very straightforward call: We pass to it the + address of a stat + structure and the descriptor of an open file. It will fill + out the contents of the stat structure. - -This is a very straightforward call: We pass to it the address -of a stat structure and the descriptor -of an open file. It will fill out the contents of the -stat structure. - + I do, however, have to say that I tried to declare the + stat structure in the + .bss section, and fstat did not like it: It set the + carry flag indicating an error. After I changed the code to + allocate the structure on the stack, everything was working + fine. + - -I do, however, have to say that I tried to declare the -stat structure in the -.bss section, and -fstat did not like it: It set the carry -flag indicating an error. After I changed the code to allocate -the structure on the stack, everything was working fine. - + + Changing the File Size - + Because our program may combine carriage return / line + feed sequences into straight line feeds, our output may be + smaller than our input. However, since we are placing our + output into the same file we read the input from, we may have + to change the size of the file. - -Changing the File Size + The ftruncate system + call allows us to do just that. Despite its somewhat + misleading name, the ftruncate system call can be used + to both truncate the file (make it smaller) and to grow + it. - -Because our program may combine carriage return / line feed -sequences into straight line feeds, our output may be smaller -than our input. However, since we are placing our output into -the same file we read the input from, we may have to change the -size of the file. - + And yes, we will find two versions of ftruncate in + syscalls.master, an older one (130), and + a newer one (201). We will use the newer one: - -The ftruncate system call allows us to do -just that. Despite its somewhat misleading name, the -ftruncate system call can be used to both -truncate the file (make it smaller) and to grow it. - + 201 STD BSD { int ftruncate(int fd, int pad, off_t length); } - -And yes, we will find two versions of ftruncate -in syscalls.master, an older one -(130), and a newer one (201). We will use -the newer one: - + Please note that this one contains a int + pad again. + - -201 STD BSD { int ftruncate(int fd, int pad, off_t length); } - + + ftuc - -Please note that this one contains a int pad again. - + We now know everything we need to write + ftuc. We start by adding some new + lines in system.inc. First, we define + some constants and structures, somewhere at or near the + beginning of the file: - - - -ftuc - - -We now know everything we need to write ftuc. -We start by adding some new lines in system.inc. -First, we define some constants and structures, somewhere at -or near the beginning of the file: - - - -;;;;;;; open flags + ;;;;;;; open flags %define O_RDONLY 0 %define O_WRONLY 1 %define O_RDWR 2 ;;;;;;; mmap flags %define PROT_NONE 0 %define PROT_READ 1 %define PROT_WRITE 2 %define PROT_EXEC 4 ;; %define MAP_SHARED 0001h %define MAP_PRIVATE 0002h ;;;;;;; stat structure struc stat st_dev resd 1 ; = 0 st_ino resd 1 ; = 4 st_mode resw 1 ; = 8, size is 16 bits st_nlink resw 1 ; = 10, ditto st_uid resd 1 ; = 12 st_gid resd 1 ; = 16 st_rdev resd 1 ; = 20 st_atime resd 1 ; = 24 st_atimensec resd 1 ; = 28 st_mtime resd 1 ; = 32 st_mtimensec resd 1 ; = 36 st_ctime resd 1 ; = 40 st_ctimensec resd 1 ; = 44 st_size resd 2 ; = 48, size is 64 bits st_blocks resd 2 ; = 56, ditto st_blksize resd 1 ; = 64 st_flags resd 1 ; = 68 st_gen resd 1 ; = 72 st_lspare resd 1 ; = 76 st_qspare resd 4 ; = 80 -endstruc - +endstruc - -We define the new syscalls: - + We define the new syscalls: - -%define SYS_mmap 197 + %define SYS_mmap 197 %define SYS_munmap 73 %define SYS_fstat 189 -%define SYS_ftruncate 201 - +%define SYS_ftruncate 201 - -We add the macros for their use: - + We add the macros for their use: - -%macro sys.mmap 0 + %macro sys.mmap 0 system SYS_mmap %endmacro %macro sys.munmap 0 system SYS_munmap %endmacro %macro sys.ftruncate 0 system SYS_ftruncate %endmacro %macro sys.fstat 0 system SYS_fstat -%endmacro - +%endmacro - -And here is our code: - + And here is our code: - -;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; ;; Started: 21-Dec-2000 ;; Updated: 22-Dec-2000 ;; ;; Copyright 2000 G. Adam Stanislav. ;; All rights reserved. ;; ;;;;;;; v.1 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; %include 'system.inc' section .data db 'Copyright 2000 G. Adam Stanislav.', 0Ah db 'All rights reserved.', 0Ah usg db 'Usage: ftuc filename', 0Ah usglen equ $-usg co db "ftuc: Can't open file.", 0Ah colen equ $-co fae db 'ftuc: File access error.', 0Ah faelen equ $-fae ftl db 'ftuc: File too long, use regular tuc instead.', 0Ah ftllen equ $-ftl mae db 'ftuc: Memory allocation error.', 0Ah maelen equ $-mae section .text align 4 memerr: push dword maelen push dword mae jmp short error align 4 toolong: push dword ftllen push dword ftl jmp short error align 4 facerr: push dword faelen push dword fae jmp short error align 4 cantopen: push dword colen push dword co jmp short error align 4 usage: push dword usglen push dword usg error: push dword stderr sys.write push dword 1 sys.exit align 4 global _start _start: pop eax ; argc pop eax ; program name pop ecx ; file to convert jecxz usage pop eax or eax, eax ; Too many arguments? jne usage ; Open the file push dword O_RDWR push ecx sys.open jc cantopen mov ebp, eax ; Save fd sub esp, byte stat_size mov ebx, esp ; Find file size push ebx push ebp ; fd sys.fstat jc facerr mov edx, [ebx + st_size + 4] ; File is too long if EDX != 0 ... or edx, edx jne near toolong mov ecx, [ebx + st_size] ; ... or if it is above 2 GB or ecx, ecx js near toolong ; Do nothing if the file is 0 bytes in size jecxz .quit ; Map the entire file in memory push edx push edx ; starting at offset 0 push edx ; pad push ebp ; fd push dword MAP_SHARED push dword PROT_READ | PROT_WRITE push ecx ; entire file size push edx ; let system decide on the address sys.mmap jc near memerr mov edi, eax mov esi, eax push ecx ; for SYS_munmap push edi ; Use EBX for state machine mov ebx, ordinary mov ah, 0Ah cld .loop: lodsb call ebx loop .loop cmp ebx, ordinary je .filesize ; Output final lf mov al, ah stosb inc edx .filesize: ; truncate file to new size push dword 0 ; high dword push edx ; low dword push eax ; pad push ebp sys.ftruncate ; close it (ebp still pushed) sys.close add esp, byte 16 sys.munmap .quit: push dword 0 sys.exit align 4 ordinary: cmp al, 0Dh je .cr cmp al, ah je .lf stosb inc edx ret align 4 .cr: mov ebx, cr ret align 4 .lf: mov ebx, lf ret align 4 cr: cmp al, 0Dh je .cr cmp al, ah je .lf xchg al, ah stosb inc edx xchg al, ah ; fall through .lf: stosb inc edx mov ebx, ordinary ret align 4 .cr: mov al, ah stosb inc edx ret align 4 lf: cmp al, ah je .lf cmp al, 0Dh je .cr xchg al, ah stosb inc edx xchg al, ah stosb inc edx mov ebx, ordinary ret align 4 .cr: mov ebx, ordinary mov al, ah ; fall through .lf: stosb inc edx - ret - + ret - -Do not use this program on files stored on a disk formatted -by &ms-dos; or &windows;. There seems to be a -subtle bug in the FreeBSD code when using mmap -on these drives mounted under FreeBSD: If the file is over -a certain size, mmap will just fill the memory -with zeros, and then copy them to the file overwriting -its contents. - - - + + Do not use this program on files stored on a disk + formatted by &ms-dos; or &windows;. + There seems to be a subtle bug in the FreeBSD code when + using mmap on these + drives mounted under FreeBSD: If the file is over a certain + size, mmap will just + fill the memory with zeros, and then copy them to the file + overwriting its contents. + + + - + + One-Pointed Mind - -One-Pointed Mind + As a student of Zen, I like the idea of a one-pointed mind: + Do one thing at a time, and do it well. - -As a student of Zen, I like the idea of a one-pointed mind: -Do one thing at a time, and do it well. - + This, indeed, is very much how &unix; works as well. While + a typical &windows; application is attempting to do everything + imaginable (and is, therefore, riddled with bugs), a typical + &unix; program does only one thing, and it does it well. - -This, indeed, is very much how &unix; works as well. While -a typical &windows; application is attempting to do everything -imaginable (and is, therefore, riddled with bugs), a -typical &unix; program does only one thing, and it does it -well. - + The typical &unix; user then essentially assembles his own + applications by writing a shell script which combines the + various existing programs by piping the output of one program to + the input of another. - -The typical &unix; user then essentially assembles his own -applications by writing a shell script which combines the -various existing programs by piping the output of one -program to the input of another. - + When writing your own &unix; software, it is generally a + good idea to see what parts of the problem you need to solve can + be handled by existing programs, and only write your own + programs for that part of the problem that you do not have an + existing solution for. - -When writing your own &unix; software, it is generally a -good idea to see what parts of the problem you need to -solve can be handled by existing programs, and only -write your own programs for that part of the problem -that you do not have an existing solution for. - + + CSV -CSV + I will illustrate this principle with a specific real-life + example I was faced with recently: - -I will illustrate this principle with a specific real-life -example I was faced with recently: - + I needed to extract the 11th field of each record from a + database I downloaded from a web site. The database was a + CSV file, i.e., a list of + comma-separated values. That is quite + a standard format for sharing data among people who may be + using different database software. - -I needed to extract the 11th field of each record from a -database I downloaded from a web site. The database was a -CSV file, i.e., a list of -comma-separated values. That is quite -a standard format for sharing data among people who may be -using different database software. - + The first line of the file contains the list of various + fields separated by commas. The rest of the file contains the + data listed line by line, with values separated by + commas. - -The first line of the file contains the list of various fields -separated by commas. The rest of the file contains the data -listed line by line, with values separated by commas. - + I tried awk, using the comma as + a separator. But because several lines contained a quoted + comma, awk was extracting the wrong + field from those lines. - -I tried awk, using the comma as a separator. -But because several lines contained a quoted comma, -awk was extracting the wrong field -from those lines. - + Therefore, I needed to write my own software to extract + the 11th field from the CSV file. However, + going with the &unix; spirit, I only needed to write a simple + filter that would do the following: - -Therefore, I needed to write my own software to extract the 11th -field from the CSV file. However, going with the &unix; -spirit, I only needed to write a simple filter that would do the -following: - + + + Remove the first line from the file; + - - - -Remove the first line from the file; - - + + Change all unquoted commas to a different + character; + - - -Change all unquoted commas to a different character; - - + + Remove all quotation marks. + + - - -Remove all quotation marks. - - + Strictly speaking, I could use + sed to remove the first line from + the file, but doing so in my own program was very easy, so I + decided to do it and reduce the size of the pipeline. - - -Strictly speaking, I could use sed to remove -the first line from the file, but doing so in my own program -was very easy, so I decided to do it and reduce the size of -the pipeline. - + At any rate, writing a program like this took me about + 20 minutes. Writing a program that extracts the 11th field + from the CSV file would take a lot longer, + and I could not reuse it to extract some other field from some + other database. - -At any rate, writing a program like this took me about -20 minutes. Writing a program that extracts the 11th field -from the CSV file would take a lot longer, -and I could not reuse it to extract some other field from some -other database. - + This time I decided to let it do a little more work than a + typical tutorial program would: - -This time I decided to let it do a little more work than -a typical tutorial program would: - + + + It parses its command line for options; + - - - -It parses its command line for options; - - + + It displays proper usage if it finds wrong + arguments; + - - -It displays proper usage if it finds wrong arguments; - - + + It produces meaningful error messages. + + - - -It produces meaningful error messages. - - + Here is its usage message: - - -Here is its usage message: - + Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>] -Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>] + All parameters are optional, and can appear in any + order. - -All parameters are optional, and can appear in any order. - + The -t parameter declares what to + replace the commas with. The tab is the + default here. For example, -t; will + replace all unquoted commas with semicolons. - -The -t parameter declares what to replace -the commas with. The tab is the default here. -For example, -t; will replace all unquoted -commas with semicolons. - + I did not need the -c option, but + it may come in handy in the future. It lets me declare that I + want a character other than a comma replaced with something + else. For example, -c@ will replace + all at signs (useful if you want to split a list of email + addresses to their user names and domains). - -I did not need the -c option, but it may -come in handy in the future. It lets me declare that I want a -character other than a comma replaced with something else. -For example, -c@ will replace all at signs -(useful if you want to split a list of email addresses -to their user names and domains). - + The -p option preserves the first + line, i.e., it does not delete it. By default, we delete the + first line because in a CSV file it + contains the field names rather than data. - -The -p option preserves the first line, i.e., -it does not delete it. By default, we delete the first -line because in a CSV file it contains the field -names rather than data. - + The -i and + -o options let me specify the input and + the output files. Defaults are stdin and + stdout, so this is a regular &unix; + filter. - -The -i and -o -options let me specify the input and the output files. Defaults -are stdin and stdout, -so this is a regular &unix; filter. - + I made sure that both -i filename + and -ifilename are accepted. I also + made sure that only one input and one output files may be + specified. - -I made sure that both -i filename and --ifilename are accepted. I also made -sure that only one input and one output files may be -specified. - + To get the 11th field of each record, I can now do: - -To get the 11th field of each record, I can now do: - + &prompt.user; csv '-t;' data.csv | awk '-F;' '{print $11}' -&prompt.user; csv '-t;' data.csv | awk '-F;' '{print $11}' + The code stores the options (except for the file + descriptors) in EDX: The + comma in DH, the new + separator in DL, and the + flag for the -p option in the highest + bit of EDX, so a check for + its sign will give us a quick decision what to do. - -The code stores the options (except for the file descriptors) -in EDX: The comma in DH, the new -separator in DL, and the flag for -the -p option in the highest bit of -EDX, so a check for its sign will give us a -quick decision what to do. - + Here is the code: - -Here is the code: - - - -;;;;;;; csv.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;;;;; csv.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; Convert a comma-separated file to a something-else separated file. ; ; Started: 31-May-2001 ; Updated: 1-Jun-2001 ; ; Copyright (c) 2001 G. Adam Stanislav ; All rights reserved. ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; %include 'system.inc' %define BUFSIZE 2048 section .data fd.in dd stdin fd.out dd stdout usg db 'Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]', 0Ah usglen equ $-usg iemsg db "csv: Can't open input file", 0Ah iemlen equ $-iemsg oemsg db "csv: Can't create output file", 0Ah oemlen equ $-oemsg section .bss ibuffer resb BUFSIZE obuffer resb BUFSIZE section .text align 4 ierr: push dword iemlen push dword iemsg push dword stderr sys.write push dword 1 ; return failure sys.exit align 4 oerr: push dword oemlen push dword oemsg push dword stderr sys.write push dword 2 sys.exit align 4 usage: push dword usglen push dword usg push dword stderr sys.write push dword 3 sys.exit align 4 global _start _start: add esp, byte 8 ; discard argc and argv[0] mov edx, (',' << 8) | 9 .arg: pop ecx or ecx, ecx je near .init ; no more arguments ; ECX contains the pointer to an argument cmp byte [ecx], '-' jne usage inc ecx mov ax, [ecx] .o: cmp al, 'o' jne .i ; Make sure we are not asked for the output file twice cmp dword [fd.out], stdout jne usage ; Find the path to output file - it is either at [ECX+1], ; i.e., -ofile -- ; or in the next argument, ; i.e., -o file inc ecx or ah, ah jne .openoutput pop ecx jecxz usage .openoutput: push dword 420 ; file mode (644 octal) push dword 0200h | 0400h | 01h ; O_CREAT | O_TRUNC | O_WRONLY push ecx sys.open jc near oerr add esp, byte 12 mov [fd.out], eax jmp short .arg .i: cmp al, 'i' jne .p ; Make sure we are not asked twice cmp dword [fd.in], stdin jne near usage ; Find the path to the input file inc ecx or ah, ah jne .openinput pop ecx or ecx, ecx je near usage .openinput: push dword 0 ; O_RDONLY push ecx sys.open jc near ierr ; open failed add esp, byte 8 mov [fd.in], eax jmp .arg .p: cmp al, 'p' jne .t or ah, ah jne near usage or edx, 1 << 31 jmp .arg .t: cmp al, 't' ; redefine output delimiter jne .c or ah, ah je near usage mov dl, ah jmp .arg .c: cmp al, 'c' jne near usage or ah, ah je near usage mov dh, ah jmp .arg align 4 .init: sub eax, eax sub ebx, ebx sub ecx, ecx mov edi, obuffer ; See if we are to preserve the first line or edx, edx js .loop .firstline: ; get rid of the first line call getchar cmp al, 0Ah jne .firstline .loop: ; read a byte from stdin call getchar ; is it a comma (or whatever the user asked for)? cmp al, dh jne .quote ; Replace the comma with a tab (or whatever the user wants) mov al, dl .put: call putchar jmp short .loop .quote: cmp al, '"' jne .put ; Print everything until you get another quote or EOL. If it ; is a quote, skip it. If it is EOL, print it. .qloop: call getchar cmp al, '"' je .loop cmp al, 0Ah je .put call putchar jmp short .qloop align 4 getchar: or ebx, ebx jne .fetch call read .fetch: lodsb dec ebx ret read: jecxz .read call write .read: push dword BUFSIZE mov esi, ibuffer push esi push dword [fd.in] sys.read add esp, byte 12 mov ebx, eax or eax, eax je .done sub eax, eax ret align 4 .done: call write ; flush output buffer ; close files push dword [fd.in] sys.close push dword [fd.out] sys.close ; return success push dword 0 sys.exit align 4 putchar: stosb inc ecx cmp ecx, BUFSIZE je write ret align 4 write: jecxz .ret ; nothing to write sub edi, ecx ; start of buffer push ecx push edi push dword [fd.out] sys.write add esp, byte 12 sub eax, eax sub ecx, ecx ; buffer is empty now .ret: - ret - + ret - -Much of it is taken from hex.asm above. But there -is one important difference: I no longer call write -whenever I am outputting a line feed. Yet, the code can be -used interactively. - + Much of it is taken from hex.asm + above. But there is one important difference: I no longer + call write whenever I am outputting a + line feed. Yet, the code can be used interactively. - -I have found a better solution for the interactive problem -since I first started writing this chapter. I wanted to -make sure each line is printed out separately only when needed. -After all, there is no need to flush out every line when used -non-interactively. - + I have found a better solution for the interactive problem + since I first started writing this chapter. I wanted to + make sure each line is printed out separately only when + needed. After all, there is no need to flush out every line + when used non-interactively. - -The new solution I use now is to call write every -time I find the input buffer empty. That way, when running in -the interactive mode, the program reads one line from the user's -keyboard, processes it, and sees its input buffer is empty. It -flushes its output and reads the next line. - + The new solution I use now is to call + write every time I find the input buffer + empty. That way, when running in the interactive mode, the + program reads one line from the user's keyboard, processes + it, and sees its input buffer is empty. It flushes its + output and reads the next line. - -The Dark Side of Buffering - -This change prevents a mysterious lockup -in a very specific case. I refer to it as the -dark side of buffering, mostly -because it presents a danger that is not -quite obvious. - + + The Dark Side of Buffering - -It is unlikely to happen with a program like the -csv above, so let us consider yet -another filter: In this case we expect our input -to be raw data representing color values, such as -the red, green, and -blue intensities of a pixel. Our -output will be the negative of our input. - + This change prevents a mysterious lockup in a very + specific case. I refer to it as the dark side + of buffering, mostly because it presents a + danger that is not quite obvious. - -Such a filter would be very simple to write. -Most of it would look just like all the other -filters we have written so far, so I am only -going to show you its inner loop: - + It is unlikely to happen with a program like the + csv above, so let us consider + yet another filter: In this case we expect our input to be + raw data representing color values, such as the + red, green, and + blue intensities of a pixel. Our + output will be the negative of our input. - -.loop: + Such a filter would be very simple to write. Most of + it would look just like all the other filters we have + written so far, so I am only going to show you its inner + loop: + + .loop: call getchar not al ; Create a negative call putchar - jmp short .loop - - -Because this filter works with raw data, -it is unlikely to be used interactively. - + jmp short .loop - -But it could be called by image manipulation software. -And, unless it calls write before each call -to read, chances are it will lock up. - + Because this filter works with raw data, it is + unlikely to be used interactively. - -Here is what might happen: - + But it could be called by image manipulation software. + And, unless it calls write before + each call to read, chances are it + will lock up. - -The image editor will load our filter using the -C function popen(). - - - -It will read the first row of pixels from -a bitmap or pixmap. - - - -It will write the first row of pixels to -the pipe leading to -the fd.in of our filter. - - - -Our filter will read each pixel -from its input, turn it to a negative, -and write it to its output buffer. - - - -Our filter will call getchar -to fetch the next pixel. - - - -getchar will find an empty -input buffer, so it will call -read. - - - -read will call the -SYS_read system call. - - - -The kernel will suspend -our filter until the image editor -sends more data to the pipe. - - - -The image editor will read from the -other pipe, connected to the -fd.out of our filter so it can set the first row of the -output image before -it sends us the second row of the input. - - - -The kernel suspends -the image editor until it receives -some output from our filter, so it -can pass it on to the image editor. - - - - -At this point our filter waits for the image -editor to send it more data to process, while -the image editor is waiting for our filter -to send it the result of the processing -of the first row. But the result sits in -our output buffer. - + Here is what might happen: - -The filter and the image editor will continue -waiting for each other forever (or, at least, -until they are killed). Our software has just -entered a -race condition. - + + + The image editor will load our filter using the C + function popen(). + - -This problem does not exist if our filter flushes -its output buffer before asking the -kernel for more input data. - + + It will read the first row of pixels from a bitmap + or pixmap. + - + + It will write the first row of pixels to the + pipe leading to the + fd.in of our filter. + - + + Our filter will read each pixel from its input, + turn it to a negative, and write it to its output + buffer. + - + + Our filter will call getchar + to fetch the next pixel. + - -Using the <acronym>FPU</acronym> - -Strangely enough, most of assembly language literature does not -even mention the existence of the FPU, -or floating point unit, let alone discuss -programming it. - + + getchar will find an empty + input buffer, so it will call + read. + - -Yet, never does assembly language shine more than when -we create highly optimized FPU -code by doing things that can be done only in assembly language. + + read will call the SYS_read system + call. + -Organization of the <acronym>FPU</acronym> - -The FPU consists of 8 80–bit floating–point registers. -These are organized in a stack fashion—you can -push a value on TOS -(top of stack) and you can -pop it. - + + The kernel will suspend our + filter until the image editor sends more data to the + pipe. + - -That said, the assembly language op codes are not push -and pop because those are already taken. + + The image editor will read from the other pipe, + connected to the fd.out of our + filter so it can set the first row of the output image + before it sends us the second row + of the input. + - -You can push a value on TOS -by using fld, fild, -and fbld. Several other op codes -let you push many common -constants—such as pi—on -the TOS. - + + The kernel suspends the image + editor until it receives some output from our filter, + so it can pass it on to the image editor. + + - -Similarly, you can pop a value by -using fst, fstp, -fist, fistp, and -fbstp. Actually, only the op -codes that end with a p will -literally pop the value, -the rest will store it -somewhere else without removing it from -the TOS. - + At this point our filter waits for the image editor to + send it more data to process, while the image editor is + waiting for our filter to send it the result of the + processing of the first row. But the result sits in our + output buffer. - -We can transfer the data between the -TOS and the computer memory either as -a 32–bit, 64–bit, or 80–bit real, -a 16–bit, 32–bit, or 64–bit integer, -or an 80–bit packed decimal. - + The filter and the image editor will continue waiting + for each other forever (or, at least, until they are + killed). Our software has just entered a race + condition. - -The 80–bit packed decimal is -a special case of binary coded -decimal which is very convenient when -converting between the ASCII -representation of data and the internal -data of the FPU. It allows us to use -18 significant digits. - + This problem does not exist if our filter flushes its + output buffer before asking the + kernel for more input data. + + + - -No matter how we represent data in the memory, -the FPU always stores it in the 80–bit -real format in its registers. - + + Using the <acronym>FPU</acronym> - -Its internal precision is at least 19 decimal -digits, so even if we choose to display results -as ASCII in the full -18–digit precision, we are still showing -correct results. - + Strangely enough, most of assembly language literature + does not even mention the existence of the + FPU, or floating point + unit, let alone discuss programming it. - -We can perform mathematical operations on the -TOS: We can calculate its -sine, we can scale it -(i.e., we can multiply or divide it by a power -of 2), we can calculate its base–2 -logarithm, and many other things. - + Yet, never does assembly language shine more than when we + create highly optimized FPU code by doing + things that can be done only in assembly + language. - -We can also multiply or -divide it by, add -it to, or subtract it from, -any of the FPU registers (including -itself). - + + Organization of the <acronym>FPU</acronym> - -The official Intel op code for the -TOS is st, and -for the registers -st(0)–st(7). -st and st(0), then, -refer to the same register. - + The FPU consists of 8 80–bit + floating–point registers. These are organized in a + stack fashion—you can push a + value on TOS (top of + stack) and you can pop + it. - -For whatever reasons, the original author of -nasm has decided to use -different op codes, namely -st0–st7. -In other words, there are no parentheses, -and the TOS is always -st0, never just st. - + That said, the assembly language op codes are not + push and pop because those are already + taken. - -The Packed Decimal Format - -The packed decimal format -uses 10 bytes (80 bits) of -memory to represent 18 digits. The -number represented there is always an -integer. - + You can push a value on + TOS by using fld, fild, and fbld. Several other op codes let + you push many common + constants—such as + pi—on the + TOS. - - -You can use it to get decimal places -by multiplying the TOS -by a power of 10 first. - - + Similarly, you can pop a value by + using fst, fstp, fist, fistp, and fbstp. Actually, only the op + codes that end with a p will literally + pop the value, the rest will + store it somewhere else without + removing it from the TOS. - -The highest bit of the highest byte -(byte 9) is the sign bit: -If it is set, the number is negative, -otherwise, it is positive. -The rest of the bits of this byte are unused/ignored. - + We can transfer the data between the + TOS and the computer memory either as a + 32–bit, 64–bit, or 80–bit + real, a 16–bit, 32–bit, or + 64–bit integer, or an + 80–bit packed decimal. - -The remaining 9 bytes store the 18 digits -of the number: 2 digits per byte. + The 80–bit packed decimal is + a special case of binary coded decimal + which is very convenient when converting between the + ASCII representation of data and the + internal data of the FPU. It allows us + to use 18 significant digits. - -The more significant digit is -stored in the high nibble -(4 bits), the less significant -digit in the low nibble. - + No matter how we represent data in the memory, the + FPU always stores it in the 80–bit + real format in its registers. - -That said, you might think that -1234567 -would be stored in the memory like this (using -hexadecimal notation): - + Its internal precision is at least 19 decimal digits, so + even if we choose to display results as + ASCII in the full 18–digit + precision, we are still showing correct results. - -80 00 00 00 00 00 01 23 45 67 - - -Alas it is not! As with everything else of Intel make, -even the packed decimal is -little–endian. + We can perform mathematical operations on the + TOS: We can calculate its + sine, we can scale + it (i.e., we can multiply or divide it by a power of 2), we + can calculate its base–2 + logarithm, and many other + things. - -That means our -1234567 -is stored like this: - + We can also multiply or + divide it by, add + it to, or subtract it from, any of the + FPU registers (including itself). - -67 45 23 01 00 00 00 00 00 80 - - -Remember that, or you will be pulling your hair out -in desperation! - + The official Intel op code for the + TOS is st, and for the + registers st(0)–st(7). st and st(0), then, + refer to the same register. - - -The book to read—if you can find it—is Richard Startz' -8087/80287/80387 -for the IBM PC & Compatibles. -Though it does seem to take the fact about the -little–endian storage of the packed -decimal for granted. I kid you not about the -desperation of trying to figure out what was wrong -with the filter I show below before -it occurred to me I should try the -little–endian order even for this type of data. - - + For whatever reasons, the original author of + nasm has decided to use + different op codes, namely st0–st7. In other words, there are + no parentheses, and the TOS is always + st0, never just + st. - + + The Packed Decimal Format - + The packed decimal format uses 10 + bytes (80 bits) of memory to represent 18 digits. The + number represented there is always an + integer. - -Excursion to Pinhole Photography - -To write meaningful software, we must not only -understand our programming tools, but also the -field we are creating software for. - + + You can use it to get decimal places by multiplying + the TOS by a power of 10 + first. + - -Our next filter will help us whenever we want -to build a pinhole camera, -so, we need some background in pinhole -photography before we can continue. - + The highest bit of the highest byte (byte 9) is the + sign bit: If it is set, the number is + negative, otherwise, it is + positive. The rest of the bits of + this byte are unused/ignored. - -The Camera - -The easiest way to describe any camera ever built -is as some empty space enclosed in some -lightproof material, with a small hole in the -enclosure. - + The remaining 9 bytes store the 18 digits of the + number: 2 digits per byte. - -The enclosure is usually sturdy (e.g., a box), -though sometimes it is flexible (the bellows). -It is quite dark inside the camera. However, the -hole lets light rays in through a single point -(though in some cases there may be several). -These light rays form an image, a representation -of whatever is outside the camera, in front of the -hole. - + The more significant digit is + stored in the high nibble (4 bits), + the less significant digit in the low + nibble. - -If some light sensitive material (such as film) -is placed inside the camera, it can capture the -image. + That said, you might think that + -1234567 would be stored in the + memory like this (using hexadecimal notation): - -The hole often contains a lens, or -a lens assembly, often called the objective. - + 80 00 00 00 00 00 01 23 45 67 - + Alas it is not! As with everything else of Intel make, + even the packed decimal is + little–endian. - -The Pinhole - -But, strictly speaking, the lens is not necessary: -The original cameras did not use a lens but a -pinhole. Even today, pinholes -are used, both as a tool to study how cameras -work, and to achieve a special kind of image. - + That means our -1234567 is stored + like this: - -The image produced by the pinhole -is all equally sharp. Or blurred. -There is an ideal size for a pinhole: If it is -either larger or smaller, the image loses its -sharpness. + 67 45 23 01 00 00 00 00 00 80 - + Remember that, or you will be pulling your hair out in + desperation! - -Focal Length - -This ideal pinhole diameter is a function -of the square root of focal -length, which is the distance of the -pinhole from the film. - + + The book to read—if you can find it—is + Richard Startz' 8087/80287/80387 + for the IBM PC & Compatibles. Though it does + seem to take the fact about the little–endian + storage of the packed decimal for + granted. I kid you not about the desperation of trying + to figure out what was wrong with the filter I show + below before it occurred to me I + should try the little–endian order even for this + type of data. + + + - - D = PC * sqrt(FL) - - -In here, D is the -ideal diameter of the pinhole, -FL is the focal length, -and PC is a pinhole -constant. According to Jay Bender, -its value is 0.04, while -Kenneth Connors has determined it to -be 0.037. Others have -proposed other values. Plus, this -value is for the daylight only: Other types -of light will require a different constant, -whose value can only be determined by -experimentation. - + + Excursion to Pinhole Photography - + To write meaningful software, we must not only + understand our programming tools, but also the + field we are creating software for. - -The F–Number - -The f–number is a very useful measure of -how much light reaches the film. A light -meter can determine that, for example, -to expose a film of specific sensitivity -with f5.6 may require the exposure to last -1/1000 sec. + Our next filter will help us whenever we want to build a + pinhole camera, so, we need some + background in pinhole photography + before we can continue. - -It does not matter whether it is a 35–mm -camera, or a 6x9cm camera, etc. -As long as we know the f–number, we can determine -the proper exposure. - + + The Camera - -The f–number is easy to calculate: - + The easiest way to describe any camera ever built is + as some empty space enclosed in some lightproof material, + with a small hole in the enclosure. - - F = FL / D - - -In other words, the f–number equals the focal -length divided by the diameter of the pinhole. -It also means a higher f–number either implies -a smaller pinhole or a larger focal distance, -or both. That, in turn, implies, the higher -the f–number, the longer the exposure has to be. - + The enclosure is usually sturdy (e.g., a box), though + sometimes it is flexible (the bellows). It is quite dark + inside the camera. However, the hole lets light rays in + through a single point (though in some cases there may be + several). These light rays form an image, a + representation of whatever is outside the camera, in front + of the hole. - -Furthermore, while pinhole diameter and focal -distance are one–dimensional measurements, -both, the film and the pinhole, are two–dimensional. -That means that -if you have measured the exposure at f–number -A as t, then the exposure -at f–number B is: + If some light sensitive material (such as film) is + placed inside the camera, it can capture the image. - - t * (B / A)² - - + The hole often contains a lens, + or a lens assembly, often called the + objective. + - -Normalized F–Number - -While many modern cameras can change the diameter -of their pinhole, and thus their f–number, quite -smoothly and gradually, such was not always the case. - + + The Pinhole - -To allow for different f–numbers, cameras typically -contained a metal plate with several holes of -different sizes drilled to them. - + But, strictly speaking, the lens is not necessary: The + original cameras did not use a lens but a + pinhole. Even today, + pinholes are used, both as a tool to + study how cameras work, and to achieve a special kind of + image. - -Their sizes were chosen according to the above -formula in such a way that the resultant f–number -was one of standard f–numbers used on all cameras -everywhere. For example, a very old Kodak Duaflex IV -camera in my possession has three such holes for -f–numbers 8, 11, and 16. - + The image produced by the pinhole + is all equally sharp. Or blurred. + There is an ideal size for a pinhole: If it is either + larger or smaller, the image loses its sharpness. + - -A more recently made camera may offer f–numbers of -2.8, 4, 5.6, 8, 11, -16, 22, and 32 (as well as others). -These numbers were not chosen arbitrarily: They all are -powers of the square root of 2, though they may -be rounded somewhat. - + + Focal Length - + This ideal pinhole diameter is a function of the + square root of focal length, which is + the distance of the pinhole from the film. - -The F–Stop - -A typical camera is designed in such a way that setting -any of the normalized f–numbers changes the feel of the -dial. It will naturally stop in that -position. Because of that, these positions of the dial -are called f–stops. + D = PC * sqrt(FL) - -Since the f–numbers at each stop are powers of the -square root of 2, moving the dial by 1 -stop will double the amount of light required for -proper exposure. Moving it by 2 stops will -quadruple the required exposure. Moving the dial by -3 stops will require the increase in exposure -8 times, etc. - + In here, D is the ideal diameter of + the pinhole, FL is the focal length, + and PC is a pinhole constant. + According to Jay Bender, its value is + 0.04, while Kenneth Connors has + determined it to be 0.037. Others + have proposed other values. Plus, this value is for the + daylight only: Other types of light will require a + different constant, whose value can only be determined by + experimentation. + - + + The F–Number - + The f–number is a very useful measure of how + much light reaches the film. A light meter can determine + that, for example, to expose a film of specific + sensitivity with f5.6 mkay require the exposure to last + 1/1000 sec. - -Designing the Pinhole Software - -We are now ready to decide what exactly we want our -pinhole software to do. - + It does not matter whether it is a 35–mm + camera, or a 6x9cm camera, etc. As long as we know the + f–number, we can determine the proper + exposure. - -Processing Program Input - -Since its main purpose is to help us design a working -pinhole camera, we will use the focal -length as the input to the program. This is something -we can determine without software: Proper focal length -is determined by the size of the film and by the need -to shoot "regular" pictures, wide angle pictures, or -telephoto pictures. - + The f–number is easy to calculate: - -Most of the programs we have written so far worked with -individual characters, or bytes, as their input: The -hex program converted individual bytes -into a hexadecimal number, the csv -program either let a character through, or deleted it, -or changed it to a different character, etc. - + F = FL / D - -One program, ftuc used the state machine -to consider at most two input bytes at a time. - + In other words, the f–number equals the focal + length divided by the diameter of the pinhole. It also + means a higher f–number either implies a smaller + pinhole or a larger focal distance, or both. That, in + turn, implies, the higher the f–number, the longer + the exposure has to be. - -But our pinhole program cannot just -work with individual characters, it has to deal with -larger syntactic units. - + Furthermore, while pinhole diameter and focal distance + are one–dimensional measurements, both, the film and + the pinhole, are two–dimensional. That means that + if you have measured the exposure at f–number + A as t, then the + exposure at f–number B is: - -For example, if we want the program to calculate the -pinhole diameter (and other values we will discuss -later) at the focal lengths of 100 mm, -150 mm, and 210 mm, we may want -to enter something like this: + t * (B / A)² + -100, 150, 210 - -Our program needs to consider more than a single byte of -input at a time. When it sees the first 1, -it must understand it is seeing the first digit of a -decimal number. When it sees the 0 and -the other 0, it must know it is seeing -more digits of the same number. - + + Normalized F–Number - -When it encounters the first comma, it must know it is -no longer receiving the digits of the first number. -It must be able to convert the digits of the first number -into the value of 100. And the digits of the -second number into the value of 150. And, -of course, the digits of the third number into the -numeric value of 210. - + While many modern cameras can change the diameter of + their pinhole, and thus their f–number, quite + smoothly and gradually, such was not always the + case. - -We need to decide what delimiters to accept: Do the -input numbers have to be separated by a comma? If so, -how do we treat two numbers separated by something else? - + To allow for different f–numbers, cameras + typically contained a metal plate with several holes of + different sizes drilled to them. - -Personally, I like to keep it simple. Something either -is a number, so I process it. Or it is not a number, -so I discard it. I do not like the computer complaining -about me typing in an extra character when it is -obvious that it is an extra character. Duh! - + Their sizes were chosen according to the above formula + in such a way that the resultant f–number was one + of standard f–numbers used on all cameras + everywhere. For example, a very old Kodak Duaflex IV + camera in my possession has three such holes for + f–numbers 8, 11, and 16. - -Plus, it allows me to break up the monotony of computing -and type in a query instead of just a number: - + A more recently made camera may offer f–numbers + of 2.8, 4, 5.6, 8, 11, 16, 22, and 32 (as well as others). + These numbers were not chosen arbitrarily: They all are + powers of the square root of 2, though they may be rounded + somewha. + -What is the best pinhole diameter for the focal length of 150? - -There is no reason for the computer to spit out -a number of complaints: - + + The F–Stop -Syntax error: What + A typical camera is designed in such a way that + setting any of the normalized f–numbers changes the + feel of the dial. It will naturally + stop in that position. Because of + that, these positions of the dial are called + f–stops. + + Since the f–numbers at each stop are powers of + the square root of 2, moving the dial by 1 stop will + double the amount of light required for proper exposure. + Moving it by 2 stops will quadruple the required exposure. + Moving the dial by 3 stops will require the increase in + exposure 8 times, etc. + + + + + Designing the Pinhole Software + + We are now ready to decide what exactly we want our + pinhole software to do. + + + Processing Program Input + + Since its main purpose is to help us design a working + pinhole camera, we will use the focal + length as the input to the program. This is + something we can determine without software: Proper focal + length is determined by the size of the film and by the + need to shoot "regular" pictures, wide angle pictures, or + telephoto pictures. + + Most of the programs we have written so far worked + with individual characters, or bytes, as their input: The + hex program converted + individual bytes into a hexadecimal number, the + csv program either let a + character through, or deleted it, or changed it to a + different character, etc. + + One program, ftuc used the + state machine to consider at most two input bytes at a + time. + + But our pinhole program + cannot just work with individual characters, it has to + deal with larger syntactic units. + + For example, if we want the program to calculate the + pinhole diameter (and other values we will discuss later) + at the focal lengths of 100 mm, + 150 mm, and 210 + mm, we may want to enter something like + this: + + 100, 150, 210 + + Our program needs to consider more than a single byte + of input at a time. When it sees the first + 1, it must understand it is seeing + the first digit of a decimal number. When it sees the + 0 and the other + 0, it must know it is seeing more + digits of the same number. + + When it encounters the first comma, it must know it is + no longer receiving the digits of the first number. It + must be able to convert the digits of the first number + into the value of 100. And the + digits of the second number into the value of + 150. And, of course, the digits of + the third number into the numeric value of + 210. + + We need to decide what delimiters to accept: Do the + input numbers have to be separated by a comma? If so, how + do we treat two numbers separated by something + else? + + Personally, I like to keep it simple. Something + either is a number, so I process it. Or it is not a + number, so I discard it. I do not like the computer + complaining about me typing in an extra character when it + is obvious that it is an extra + character. Duh! + + Plus, it allows me to break up the monotony of + computing and type in a query instead of just a + number: + + What is the best pinhole diameter for the + focal length of 150? + + There is no reason for the computer to spit out a + number of complaints: + + Syntax error: What Syntax error: is Syntax error: the Syntax error: best - -Et cetera, et cetera, et cetera. - -Secondly, I like the # character to denote -the start of a comment which extends to the end of the -line. This does not take too much effort to code, and -lets me treat input files for my software as executable -scripts. - + Et cetera, et cetera, et cetera. - -In our case, we also need to decide what units the -input should come in: We choose millimeters -because that is how most photographers measure -the focus length. - + Secondly, I like the # character + to denote the start of a comment which extends to the end + of the line. This does not take too much effort to code, + and lets me treat input files for my software as + executable scripts. - -Finally, we need to decide whether to allow the use -of the decimal point (in which case we must also -consider the fact that much of the world uses a -decimal comma). + In our case, we also need to decide what units the + input should come in: We choose + millimeters because that is how most + photographers measure the focus length. - -In our case allowing for the decimal point/comma -would offer a false sense of precision: There is -little if any noticeable difference between the -focus lengths of 50 and 51, -so allowing the user to input something like -50.5 is not a good idea. This is -my opinion, mind you, but I am the one writing -this program. You can make other choices in yours, -of course. - + Finally, we need to decide whether to allow the use of + the decimal point (in which case we must also consider the + fact that much of the world uses a decimal + comma). - + In our case allowing for the decimal point/comma would + offer a false sense of precision: There is little if any + noticeable difference between the focus lengths of + 50 and 51, so + allowing the user to input something like + 50.5 is not a good idea. This is + my opinion, mind you, but I am the one writing this + program. You can make other choices in yours, of + course. + - -Offering Options - -The most important thing we need to know when building -a pinhole camera is the diameter of the pinhole. Since -we want to shoot sharp images, we will use the above -formula to calculate the pinhole diameter from focal length. -As experts are offering several different values for the -PC constant, we will need to have the choice. - + + Offering Options - -It is traditional in &unix; programming to have two main ways -of choosing program parameters, plus to have a default for -the time the user does not make a choice. - + The most important thing we need to know when building + a pinhole camera is the diameter of the pinhole. Since we + want to shoot sharp images, we will use the above formula + to calculate the pinhole diameter from focal length. As + experts are offering several different values for the + PC constant, we will need to have the + choice. - -Why have two ways of choosing? + It is traditional in &unix; programming to have two + main ways of choosing program parameters, plus to have a + default for the time the user does not make a + choice. - -One is to allow a (relatively) permanent -choice that applies automatically each time the -software is run without us having to tell it over and -over what we want it to do. - + Why have two ways of choosing? - -The permanent choices may be stored in a configuration -file, typically found in the user's home directory. -The file usually has the same name as the application -but is started with a dot. Often "rc" -is added to the file name. So, ours could be -~/.pinhole or ~/.pinholerc. -(The ~/ means current user's -home directory.) - + One is to allow a (relatively) + permanent choice that applies + automatically each time the software is run without us + having to tell it over and over what we want it to + do. - -The configuration file is used mostly by programs -that have many configurable parameters. Those -that have only one (or a few) often use a different -method: They expect to find the parameter in an -environment variable. In our case, -we might look at an environment variable named -PINHOLE. - + The permanent choices may be stored in a configuration + file, typically found in the user's home directory. The + file usually has the same name as the application but is + started with a dot. Often "rc" is + added to the file name. So, ours could be + ~/.pinhole or + ~/.pinholerc. (The + ~/ means current user's home + directory.) - -Usually, a program uses one or the other of the -above methods. Otherwise, if a configuration -file said one thing, but an environment variable -another, the program might get confused (or just -too complicated). - + The configuration file is used mostly by programs that + have many configurable parameters. Those that have only + one (or a few) often use a different method: They expect + to find the parameter in an environment + variable. In our case, we might look at an + environment variable named + PINHOLE. - -Because we only need to choose one -such parameter, we will go with the second method -and search the environment for a variable named -PINHOLE. + Usually, a program uses one or the other of the above + methods. Otherwise, if a configuration file said one + thing, but an environment variable another, the program + might get confused (or just too complicated). - -The other way allows us to make ad hoc -decisions: "Though I usually want -you to use 0.039, this time I want 0.03872." -In other words, it allows us to override -the permanent choice. - + Because we only need to choose + one such parameter, we will go with + the second method and search the environment for a + variable named PINHOLE. - -This type of choice is usually done with command -line parameters. - + The other way allows us to make ad + hoc decisions: "Though I usually + want you to use 0.039, this time I want + 0.03872." In other words, it allows us to + override the permanent choice. - -Finally, a program always needs a -default. The user may not make -any choices. Perhaps he does not know what -to choose. Perhaps he is "just browsing." -Preferably, the default will be the value -most users would choose anyway. That way -they do not need to choose. Or, rather, they -can choose the default without an additional -effort. - + This type of choice is usually done with command line + parameters. - -Given this system, the program may find conflicting -options, and handle them this way: - + Finally, a program always needs a + default. The user may not make any + choices. Perhaps he does not know what to choose. + Perhaps he is "just browsing." Preferably, the default + will be the value most users would choose anyway. That + way they do not need to choose. Or, rather, they can + choose the default without an additional effort. - -If it finds an ad hoc choice -(e.g., command line parameter), it should -accept that choice. It must ignore any permanent -choice and any default. - - - -Otherwise, if it finds -a permanent option (e.g., an environment -variable), it should accept it, and ignore -the default. - - -Otherwise, it should use -the default. - - - - -We also need to decide what format -our PC option should have. - + Given this system, the program may find conflicting + options, and handle them this way: - -At first site, it seems obvious to use the -PINHOLE=0.04 format for the -environment variable, and -p0.04 -for the command line. - + + + If it finds an ad hoc choice + (e.g., command line parameter), it should accept that + choice. It must ignore any permanent choice and any + default. + - -Allowing that is actually a security risk. -The PC constant is a very small -number. Naturally, we will test our software -using various small values of PC. -But what will happen if someone runs the program -choosing a huge value? - + + Otherwise, if it finds a + permanent option (e.g., an environment variable), it + should accept it, and ignore the default. + - -It may crash the program because we have not -designed it to handle huge numbers. - + + Otherwise, it should use the + default. + + - -Or, we may spend more time on the program so -it can handle huge numbers. We might do that -if we were writing commercial software for -computer illiterate audience. - + We also need to decide what + format our PC + option should have. - -Or, we might say, "Tough! -The user should know better."" - + At first site, it seems obvious to use the + PINHOLE=0.04 format for the + environment variable, and -p0.04 + for the command line. - -Or, we just may make it impossible for the user -to enter a huge number. This is the approach we -will take: We will use an implied 0. -prefix. - + Allowing that is actually a security risk. The + PC constant is a very small number. + Naturally, we will test our software using various small + values of PC. But what will happen + if someone runs the program choosing a huge value? - -In other words, if the user wants 0.04, -we will expect him to type -p04, -or set PINHOLE=04 in his environment. -So, if he says -p9999999, we will -interpret it as 0.9999999—still -ridiculous but at least safer. - + It may crash the program because we have not designed + it to handle huge numbers. - -Secondly, many users will just want to go with either -Bender's constant or Connors' constant. -To make it easier on them, we will interpret --b as identical to -p04, -and -c as identical to -p037. - + Or, we may spend more time on the program so it can + handle huge numbers. We might do that if we were writing + commercial software for computer illiterate + audience. - + Or, we might say, "Tough! The user should + know better."" - -The Output - -We need to decide what we want our software to -send to the output, and in what format. - + Or, we just may make it impossible for the user to + enter a huge number. This is the approach we will take: + We will use an implied 0. + prefix. - -Since our input allows for an unspecified number -of focal length entries, it makes sense to use -a traditional database–style output of showing -the result of the calculation for each -focal length on a separate line, while -separating all values on one line by a -tab character. - + In other words, if the user wants + 0.04, we will expect him to type + -p04, or set + PINHOLE=04 in his environment. So, if + he says -p9999999, we will + interpret it as 0.9999999—still + ridiculous but at least safer. - -Optionally, we should also allow the user -to specify the use of the CSV -format we have studied earlier. In this case, -we will print out a line of comma–separated -names describing each field of every line, -then show our results as before, but substituting -a comma for the tab. + Secondly, many users will just want to go with either + Bender's constant or Connors' constant. To make it easier + on them, we will interpret -b as + identical to -p04, and + -c as identical to + -p037. + - -We need a command line option for the CSV -format. We cannot use -c because -that already means use Connors' constant. -For some strange reason, many web sites refer to -CSV files as "Excel -spreadsheet" (though the CSV -format predates Excel). We will, therefore, use -the -e switch to inform our software -we want the output in the CSV format. - + + The Output - -We will start each line of the output with the -focal length. This may sound repetitious at first, -especially in the interactive mode: The user -types in the focal length, and we are repeating it. - + We need to decide what we want our software to send to + the output, and in what format. - -But the user can type several focal lengths on one -line. The input can also come in from a file or -from the output of another program. In that case -the user does not see the input at all. - + Since our input allows for an unspecified number of + focal length entries, it makes sense to use a traditional + database–style output of showing the result of the + calculation for each focal length on a separate line, + while separating all values on one line by a + tab character. - -By the same token, the output can go to a file -which we will want to examine later, or it could -go to the printer, or become the input of another -program. - + Optionally, we should also allow the user to specify + the use of the CSV format we have + studied earlier. In this case, we will print out a line + of comma–separated names describing each field of + every line, then show our results as before, but + substituting a comma for the + tab. - -So, it makes perfect sense to start each line with -the focal length as entered by the user. - + We need a command line option for the + CSV format. We cannot use + -c because that already means + use Connors' constant. For some + strange reason, many web sites refer to + CSV files as "Excel + spreadsheet" (though the + CSV format predates Excel). + We will, therefore, use the -e + switch to inform our software we want the output in the + CSV format. - -No, wait! Not as entered by the user. What if the user -types in something like this: + We will start each line of the output with the focal + length. This may sound repetitious at first, especially + in the interactive mode: The user types in the focal + length, and we are repeating it. -00000000150 - -Clearly, we need to strip those leading zeros. + But the user can type several focal lengths on one + line. The input can also come in from a file or from the + output of another program. In that case the user does not + see the input at all. - -So, we might consider reading the user input as is, -converting it to binary inside the FPU, -and printing it out from there. - + By the same token, the output can go to a file which + we will want to examine later, or it could go to the + printer, or become the input of another program. - -But... + So, it makes perfect sense to start each line with + the focal length as entered by the user. - -What if the user types something like this: - + No, wait! Not as entered by the user. What if the + user types in something like this: -17459765723452353453534535353530530534563507309676764423 - -Ha! The packed decimal FPU format -lets us input 18–digit numbers. But the -user has entered more than 18 digits. How -do we handle that? - + 00000000150 - -Well, we could modify our code to read -the first 18 digits, enter it to the FPU, -then read more, multiply what we already have on the -TOS by 10 raised to the number -of additional digits, then add to it. - + Clearly, we need to strip those leading zeros. - -Yes, we could do that. But in this -program it would be ridiculous (in a different one it may be just the thing to do): Even the circumference of the Earth expressed in -millimeters only takes 11 digits. Clearly, -we cannot build a camera that large (not yet, -anyway). - + So, we might consider reading the user input as is, + converting it to binary inside the FPU, + and printing it out from there. - -So, if the user enters such a huge number, he is -either bored, or testing us, or trying to break -into the system, or playing games—doing -anything but designing a pinhole camera. - + But... - -What will we do? + What if the user types something like this: - -We will slap him in the face, in a manner of speaking: + 17459765723452353453534535353530530534563507309676764423 -17459765723452353453534535353530530534563507309676764423 ??? ??? ??? ??? ??? - -To achieve that, we will simply ignore any leading zeros. -Once we find a non–zero digit, we will initialize a -counter to 0 and start taking three steps: - + Ha! The packed decimal FPU format + lets us input 18–digit numbers. But the user has + entered more than 18 digits. How do we handle + that? - - -Send the digit to the output. - - - -Append the digit to a buffer we will use later to -produce the packed decimal we can send to the -FPU. - - - -Increase the counter. - - - - -Now, while we are taking these three steps, -we also need to watch out for one of two -conditions: + Well, we could modify our code to + read the first 18 digits, enter it to the + FPU, then read more, multiply what we + already have on the TOS by 10 raised to + the number of additional digits, then + add to it. - - - -If the counter grows above 18, -we stop appending to the buffer. We -continue reading the digits and sending -them to the output. - - + Yes, we could do that. But in + this program it would be ridiculous + (in a different one it may be just the thing to do): Even + the circumference of the Earth expressed in millimeters + only takes 11 digits. Clearly, we cannot build a camera + that large (not yet, anyway). - - -If, or rather when, -the next input character is not -a digit, we are done inputting -for now. - + So, if the user enters such a huge number, he is + either bored, or testing us, or trying to break into the + system, or playing games—doing anything but + designing a pinhole camera. - -Incidentally, we can simply -discard the non–digit, unless it -is a #, which we must -return to the input stream. It -starts a comment, so we must see it -after we are done producing output -and start looking for more input. - - + What will we do? - - -That still leaves one possibility -uncovered: If all the user enters -is a zero (or several zeros), we -will never find a non–zero to -display. + We will slap him in the face, in a manner of + speaking: - -We can determine this has happened -whenever our counter stays at 0. -In that case we need to send 0 -to the output, and perform another -"slap in the face": - + 17459765723452353453534535353530530534563507309676764423 ??? ??? ??? ??? ??? -0 ??? ??? ??? ??? ??? - -Once we have displayed the focal -length and determined it is valid -(greater than 0 -but not exceeding 18 digits), -we can calculate the pinhole diameter. - + To achieve that, we will simply ignore any leading + zeros. Once we find a non–zero digit, we will + initialize a counter to 0 and start + taking three steps: - -It is not by coincidence that pinhole -contains the word pin. Indeed, -many a pinhole literally is a pin -hole, a hole carefully punched with the -tip of a pin. - + + + Send the digit to the output. + - -That is because a typical pinhole is very -small. Our formula gets the result in -millimeters. We will multiply it by 1000, -so we can output the result in microns. - + + Append the digit to a buffer we will use later to + produce the packed decimal we can send to the + FPU. + - -At this point we have yet another trap to face: -Too much precision. - + + Increase the counter. + + - -Yes, the FPU was designed -for high precision mathematics. But we -are not dealing with high precision -mathematics. We are dealing with physics -(optics, specifically). - + Now, while we are taking these three steps, we also + need to watch out for one of two conditions: - -Suppose we want to convert a truck into -a pinhole camera (we would not be the -first ones to do that!). Suppose its box is -12 -meters long, so we have the focal length -of 12000. Well, using Bender's constant, it gives us square root of -12000 multiplied by 0.04, -which is 4.381780460 millimeters, -or 4381.780460 microns. - + + + If the counter grows above 18, we stop appending + to the buffer. We continue reading the digits and + sending them to the output. + - -Put either way, the result is absurdly precise. -Our truck is not exactly 12000 -millimeters long. We did not measure its length -with such a precision, so stating we need a pinhole -with the diameter of 4.381780460 -millimeters is, well, deceiving. 4.4 -millimeters would do just fine. - + + If, or rather when, the next + input character is not a digit, we are done inputting + for now. - - -I "only" used ten digits in the above example. -Imagine the absurdity of going for all 18! - - + Incidentally, we can simply discard the + non–digit, unless it is a + #, which we must return to the + input stream. It starts a comment, so we must see it + after we are done producing output and start looking + for more input. + + - -We need to limit the number of significant -digits of our result. One way of doing it -is by using an integer representing microns. -So, our truck would need a pinhole with the diameter -of 4382 microns. Looking at that number, we still decide that 4400 microns, -or 4.4 millimeters is close enough. - + That still leaves one possibility uncovered: If all + the user enters is a zero (or several zeros), we will + never find a non–zero to display. - -Additionally, we can decide that no matter how -big a result we get, we only want to display four -significant digits (or any other number -of them, of course). Alas, the FPU -does not offer rounding to a specific number -of digits (after all, it does not view the -numbers as decimal but as binary). - + We can determine this has happened whenever our + counter stays at 0. In that case we + need to send 0 to the output, and + perform another "slap in the face": - -We, therefore, must devise an algorithm to reduce -the number of significant digits. - + 0 ??? ??? ??? ??? ??? - -Here is mine (I think it is awkward—if -you know a better one, please, let me know): + Once we have displayed the focal length and determined + it is valid (greater than 0 but not + exceeding 18 digits), we can calculate the pinhole + diameter. - - -Initialize a counter to 0. - - - -While the number is greater than or equal to -10000, divide it by -10 and increase the counter. - - - -Output the result. - - -While the counter is greater than 0, -output 0 and decrease the counter. - - - - - -The 10000 is only good if you want -four significant digits. For any other -number of significant digits, replace -10000 with 10 -raised to the number of significant digits. - - + It is not by coincidence that + pinhole contains the word + pin. Indeed, many a pinhole + literally is a pin hole, a hole + carefully punched with the tip of a pin. - -We will, then, output the pinhole diameter -in microns, rounded off to four significant -digits. - + That is because a typical pinhole is very small. Our + formula gets the result in millimeters. We will multiply + it by 1000, so we can output the + result in microns. - -At this point, we know the focal -length and the pinhole -diameter. That means we have enough -information to also calculate the -f–number. - + At this point we have yet another trap to face: + Too much precision. - -We will display the f–number, rounded to -four significant digits. Chances are the -f–number will tell us very little. To make -it more meaningful, we can find the nearest -normalized f–number, i.e., -the nearest power of the square root -of 2. - + Yes, the FPU was designed for high + precision mathematics. But we are not dealing with high + precision mathematics. We are dealing with physics + (optics, specifically). - -We do that by multiplying the actual f–number -by itself, which, of course, will give us -its square. We will then calculate -its base–2 logarithm, which is much -easier to do than calculating the -base–square–root–of–2 logarithm! -We will round the result to the nearest integer. -Next, we will raise 2 to the result. Actually, -the FPU gives us a good shortcut -to do that: We can use the fscale -op code to "scale" 1, which is -analogous to shifting an -integer left. Finally, we calculate the square -root of it all, and we have the nearest -normalized f–number. - + Suppose we want to convert a truck into a pinhole + camera (we would not be the first ones to do that!). + Suppose its box is 12 meters long, + so we have the focal length of 12000. + Well, using Bender's constant, it gives us square root of + 12000 multiplied by + 0.04, which is + 4.381780460 millimeters, or + 4381.780460 microns. - -If all that sounds overwhelming—or too much -work, perhaps—it may become much clearer -if you see the code. It takes 9 op -codes altogether: + Put either way, the result is absurdly precise. Our + truck is not exactly + 12000 millimeters long. We did not + measure its length with such a precision, so stating we + need a pinhole with the diameter of + 4.381780460 millimeters is, well, + deceiving. 4.4 millimeters would do + just fine. - - fmul st0, st0 + + I "only" used ten digits in the above example. + Imagine the absurdity of going for all 18! + + + We need to limit the number of significant digits of + our result. One way of doing it is by using an integer + representing microns. So, our truck would need a pinhole + with the diameter of 4382 microns. + Looking at that number, we still decide that + 4400 microns, or + 4.4 millimeters is close + enough. + + Additionally, we can decide that no matter how big a + result we get, we only want to display four significant + digits (or any other number of them, of course). Alas, + the FPU does not offer rounding to a + specific number of digits (after all, it does not view the + numbers as decimal but as binary). + + We, therefore, must devise an algorithm to reduce + the number of significant digits. + + Here is mine (I think it is awkward—if you know + a better one, please, let me + know): + + + + Initialize a counter to + 0. + + + + While the number is greater than or equal to + 10000, divide it by + 10 and increase the + counter. + + + + Output the result. + + + + While the counter is greater than + 0, output 0 + and decrease the counter. + + + + + The 10000 is only good if you + want four significant digits. For + any other number of significant digits, replace + 10000 with 10 + raised to the number of significant digits. + + + We will, then, output the pinhole diameter in microns, + rounded off to four significant digits. + + At this point, we know the focal + length and the pinhole + diameter. That means we have enough + information to also calculate the + f–number. + + We will display the f–number, rounded to four + significant digits. Chances are the f–number will + tell us very little. To make it more meaningful, we can + find the nearest normalized + f–number, i.e., the nearest power of the + square root of 2. + + We do that by multiplying the actual f–number by + itself, which, of course, will give us its + square. We will then calculate its + base–2 logarithm, which is much easier to do than + calculating the + base–square–root–of–2 logarithm! + We will round the result to the nearest integer. Next, we + will raise 2 to the result. Actually, the + FPU gives us a good shortcut to do + that: We can use the fscale op code to "scale" 1, + which is analogous to shifting an integer left. + Finally, we calculate the square root of it all, and we + have the nearest normalized f–number. + + If all that sounds overwhelming—or too much + work, perhaps—it may become much clearer if you see + the code. It takes 9 op codes altogether: + + fmul st0, st0 fld1 fld st1 fyl2x frndint fld1 fscale fsqrt - fstp st1 - - -The first line, fmul st0, st0, squares -the contents of the TOS -(top of the stack, same as st, -called st0 by nasm). -The fld1 pushes 1 -on the TOS. + fstp st1 - -The next line, fld st1, pushes -the square back to the TOS. -At this point the square is both in st -and st(2) (it will become -clear why we leave a second copy on the stack -in a moment). st(1) contains -1. - + The first line, fmul st0, st0, squares the + contents of the TOS (top of the stack, + same as st, called + st0 by + nasm). The fld1 pushes + 1 on the + TOS. - -Next, fyl2x calculates base–2 -logarithm of st multiplied by -st(1). That is why we placed 1 on st(1) before. + The next line, fld + st1, pushes the square back to the + TOS. At this point the square is + both in st and st(2) (it will become clear + why we leave a second copy on the stack in a moment). + st(1) contains + 1. - -At this point, st contains -the logarithm we have just calculated, -st(1) contains the square -of the actual f–number we saved for later. - + Next, fyl2x + calculates base–2 logarithm of st multiplied by + st(1). That is why we + placed 1 on st(1) before. - -frndint rounds the TOS -to the nearest integer. fld1 pushes -a 1. fscale shifts the -1 we have on the TOS -by the value in st(1), -effectively raising 2 to st(1). - + At this point, st + contains the logarithm we have just calculated, st(1) contains the square of + the actual f–number we saved for later. - -Finally, fsqrt calculates -the square root of the result, i.e., -the nearest normalized f–number. - + frndint rounds the + TOS to the nearest integer. fld1 pushes a + 1. fscale shifts the + 1 we have on the + TOS by the value in st(1), effectively raising 2 + to st(1). - -We now have the nearest normalized -f–number on the TOS, -the base–2 logarithm rounded to the -nearest integer in st(1), -and the square of the actual f–number -in st(2). We are saving -the value in st(2) for later. - + Finally, fsqrt + calculates the square root of the result, i.e., the + nearest normalized f–number. - -But we do not need the contents of -st(1) anymore. The last -line, fstp st1, places the -contents of st to -st(1), and pops. As a -result, what was st(1) -is now st, what was st(2) -is now st(1), etc. -The new st contains the -normalized f–number. The new -st(1) contains the square -of the actual f–number we have -stored there for posterity. - + We now have the nearest normalized f–number on + the TOS, the base–2 logarithm + rounded to the nearest integer in st(1), and the square of the + actual f–number in st(2). We are saving the + value in st(2) for + later. - -At this point, we are ready to output -the normalized f–number. Because it is -normalized, we will not round it off to -four significant digits, but will -send it out in its full precision. - + But we do not need the contents of st(1) anymore. The last line, + fstp st1, places the + contents of st to + st(1), and pops. As a + result, what was st(1) + is now st, what was + st(2) is now st(1), etc. The new st contains the normalized + f–number. The new st(1) contains the square of + the actual f–number we have stored there for + posterity. - -The normalized f-number is useful as long -as it is reasonably small and can be found -on our light meter. Otherwise we need a -different method of determining proper -exposure. - + At this point, we are ready to output the normalized + f–number. Because it is normalized, we will not + round it off to four significant digits, but will send it + out in its full precision. - -Earlier we have figured out the formula -of calculating proper exposure at an arbitrary -f–number from that measured at a different -f–number. - + The normalized f-number is useful as long as it is + reasonably small and can be found on our light meter. + Otherwise we need a different method of determining proper + exposure. - -Every light meter I have ever seen can determine -proper exposure at f5.6. We will, therefore, -calculate an "f5.6 multiplier," -i.e., by how much we need to multiply the exposure measured -at f5.6 to determine the proper exposure -for our pinhole camera. - + Earlier we have figured out the formula of calculating + proper exposure at an arbitrary f–number from that + measured at a different f–number. - -From the above formula we know this factor can be -calculated by dividing our f–number (the -actual one, not the normalized one) by -5.6, and squaring the result. - + Every light meter I have ever seen can determine + proper exposure at f5.6. We will, therefore, calculate an + "f5.6 multiplier," i.e., by how much + we need to multiply the exposure measured at f5.6 to + determine the proper exposure for our pinhole + camera. - -Mathematically, dividing the square of our -f–number by the square of 5.6 -will give us the same result. - + From the above formula we know this factor can be + calculated by dividing our f–number (the actual one, + not the normalized one) by 5.6, and + squaring the result. - -Computationally, we do not want to square -two numbers when we can only square one. -So, the first solution seems better at first. - + Mathematically, dividing the square of our + f–number by the square of 5.6 + will give us the same result. - -But... + Computationally, we do not want to square two numbers + when we can only square one. So, the first solution seems + better at first. - -5.6 is a constant. -We do not have to have our FPU -waste precious cycles. We can just tell it -to divide the square of the f–number by -whatever 5.6² equals to. -Or we can divide the f–number by 5.6, -and then square the result. The two ways -now seem equal. - + But... - -But, they are not! + 5.6 is a + constant. We do not have to have our + FPU waste precious cycles. We can just + tell it to divide the square of the f–number by + whatever 5.6² equals to. Or we + can divide the f–number by 5.6, + and then square the result. The two ways now seem + equal. - -Having studied the principles of photography -above, we remember that the 5.6 -is actually square root of 2 raised to -the fifth power. An irrational -number. The square of this number is -exactly 32. - + But, they are not! - -Not only is 32 an integer, -it is a power of 2. We do not need -to divide the square of the f–number by -32. We only need to use -fscale to shift it right by -five positions. In the FPU -lingo it means we will fscale it -with st(1) equal to --5. That is much -faster than a division. - + Having studied the principles of photography above, we + remember that the 5.6 is actually + square root of 2 raised to the fifth power. An + irrational number. The square of + this number is exactly + 32. - -So, now it has become clear why we have -saved the square of the f–number on the -top of the FPU stack. -The calculation of the f5.6 multiplier -is the easiest calculation of this -entire program! We will output it rounded -to four significant digits. - + Not only is 32 an integer, it is + a power of 2. We do not need to divide the square of the + f–number by 32. We only need + to use fscale to shift + it right by five positions. In the FPU + lingo it means we will fscale it with st(1) equal to + -5. That is much + faster than a division. - -There is one more useful number we can calculate: -The number of stops our f–number is from f5.6. -This may help us if our f–number is just outside -the range of our light meter, but we have -a shutter which lets us set various speeds, -and this shutter uses stops. - + So, now it has become clear why we have saved the + square of the f–number on the top of the + FPU stack. The calculation of the + f5.6 multiplier is the easiest calculation of this entire + program! We will output it rounded to four significant + digits. - -Say, our f–number is 5 stops from -f5.6, and the light meter says -we should use 1/1000 sec. -Then we can set our shutter speed to 1/1000 -first, then move the dial by 5 stops. - + There is one more useful number we can calculate: The + number of stops our f–number is from f5.6. This may + help us if our f–number is just outside the range of + our light meter, but we have a shutter which lets us set + various speeds, and this shutter uses stops. - -This calculation is quite easy as well. All -we have to do is to calculate the base-2 -logarithm of the f5.6 multiplier -we had just calculated (though we need its -value from before we rounded it off). We then -output the result rounded to the nearest integer. -We do not need to worry about having more than -four significant digits in this one: The result -is most likely to have only one or two digits -anyway. + Say, our f–number is 5 stops from f5.6, and the + light meter says we should use 1/1000 sec. Then we can + set our shutter speed to 1/1000 first, then move the dial + by 5 stops. - + This calculation is quite easy as well. All we have + to do is to calculate the base-2 logarithm of the f5.6 + multiplier we had just calculated (though we need its + value from before we rounded it off). We then output the + result rounded to the nearest integer. We do not need to + worry about having more than four significant digits in + this one: The result is most likely to have only one or + two digits anyway. + + - + + FPU Optimizations - -FPU Optimizations - -In assembly language we can optimize the FPU -code in ways impossible in high languages, -including C. - + In assembly language we can optimize the + FPU code in ways impossible in high + languages, including C. - -Whenever a C function needs to calculate -a floating–point value, it loads all necessary -variables and constants into FPU -registers. It then does whatever calculation is -required to get the correct result. Good C -compilers can optimize that part of the code really -well. - + Whenever a C function needs to calculate a + floating–point value, it loads all necessary variables + and constants into FPU registers. It + then does whatever calculation is required to get the + correct result. Good C compilers can optimize that part of + the code really well. - -It "returns" the value by leaving -the result on the TOS. -However, before it returns, it cleans up. -Any variables and constants it used in its -calculation are now gone from the FPU. - + It "returns" the value by leaving the result on the + TOS. However, before it returns, it + cleans up. Any variables and constants it used in its + calculation are now gone from the + FPU. - -It cannot do what we just did above: We calculated -the square of the f–number and kept it on the -stack for later use by another function. - + It cannot do what we just did above: We calculated the + square of the f–number and kept it on the stack for + later use by another function. - -We knew we would need that value -later on. We also knew we had enough room on the -stack (which only has room for 8 numbers) -to store it there. - + We knew we would need that value + later on. We also knew we had enough room on the + stack (which only has room for 8 numbers) to store it + there. - -A C compiler has no way of knowing -that a value it has on the stack will be -required again in the very near future. - + A C compiler has no way of knowing that a value it has + on the stack will be required again in the very near + future. - -Of course, the C programmer may know it. -But the only recourse he has is to store the -value in a memory variable. - + Of course, the C programmer may know it. But the only + recourse he has is to store the value in a memory + variable. - -That means, for one, the value will be changed -from the 80-bit precision used internally -by the FPU to a C double -(64 bits) or even single (32 -bits). - + That means, for one, the value will be changed from the + 80-bit precision used internally by the + FPU to a C double + (64 bits) or even single (32 + bits). - -That also means that the value must be moved -from the TOS into the memory, -and then back again. Alas, of all FPU -operations, the ones that access the computer -memory are the slowest. - + That also means that the value must be moved from the + TOS into the memory, and then back again. + Alas, of all FPU operations, the ones + that access the computer memory are the slowest. - -So, whenever programming the FPU -in assembly language, look for the ways of keeping -intermediate results on the FPU -stack. - + So, whenever programming the FPU in + assembly language, look for the ways of keeping intermediate + results on the FPU stack. - -We can take that idea even further! In our -program we are using a constant -(the one we named PC). - + We can take that idea even further! In our program we + are using a constant (the one we named + PC). - -It does not matter how many pinhole diameters -we are calculating: 1, 10, 20, -1000, we are always using the same constant. -Therefore, we can optimize our program by keeping -the constant on the stack all the time. - + It does not matter how many pinhole diameters we are + calculating: 1, 10, 20, 1000, we are always using the same + constant. Therefore, we can optimize our program by keeping + the constant on the stack all the time. - -Early on in our program, we are calculating the -value of the above constant. We need to divide -our input by 10 for every digit in the -constant. - + Early on in our program, we are calculating the value of + the above constant. We need to divide our input by + 10 for every digit in the + constant. - -It is much faster to multiply than to divide. -So, at the start of our program, we divide 10 -into 1 to obtain 0.1, which we -then keep on the stack: Instead of dividing the -input by 10 for every digit, -we multiply it by 0.1. - + It is much faster to multiply than to divide. So, at + the start of our program, we divide 10 + into 1 to obtain + 0.1, which we then keep on the stack: + Instead of dividing the input by 10 for + every digit, we multiply it by + 0.1. - -By the way, we do not input 0.1 directly, -even though we could. We have a reason for that: -While 0.1 can be expressed with just one -decimal place, we do not know how many binary -places it takes. We, therefore, let the FPU -calculate its binary value to its own high precision. - + By the way, we do not input 0.1 + directly, even though we could. We have a reason for that: + While 0.1 can be expressed with just + one decimal place, we do not know how many + binary places it takes. We, therefore, + let the FPU calculate its binary value to + its own high precision. - -We are using other constants: We multiply the pinhole -diameter by 1000 to convert it from -millimeters to microns. We compare numbers to -10000 when we are rounding them off to -four significant digits. So, we keep both, 1000 -and 10000, on the stack. And, of course, -we reuse the 0.1 when rounding off numbers -to four digits. - + We are using other constants: We multiply the pinhole + diameter by 1000 to convert it from + millimeters to microns. We compare numbers to + 10000 when we are rounding them off to + four significant digits. So, we keep both, + 1000 and 10000, on + the stack. And, of course, we reuse the + 0.1 when rounding off numbers to four + digits. - -Last but not least, we keep -5 on the stack. -We need it to scale the square of the f–number, -instead of dividing it by 32. It is not -by coincidence we load this constant last. That makes -it the top of the stack when only the constants -are on it. So, when the square of the f–number is -being scaled, the -5 is at st(1), -precisely where fscale expects it to be. - + Last but not least, we keep -5 on + the stack. We need it to scale the square of the + f–number, instead of dividing it by + 32. It is not by coincidence we load + this constant last. That makes it the top of the stack when + only the constants are on it. So, when the square of the + f–number is being scaled, the -5 + is at st(1), precisely + where fscale expects it + to be. - -It is common to create certain constants from -scratch instead of loading them from the memory. -That is what we are doing with -5: - + It is common to create certain constants from scratch + instead of loading them from the memory. That is what we + are doing with -5: - - fld1 ; TOS = 1 + fld1 ; TOS = 1 fadd st0, st0 ; TOS = 2 fadd st0, st0 ; TOS = 4 fld1 ; TOS = 1 faddp st1, st0 ; TOS = 5 - fchs ; TOS = -5 - - -We can generalize all these optimizations into one rule: -Keep repeat values on the stack! - + fchs ; TOS = -5 - - -&postscript; is a stack–oriented -programming language. There are many more books -available about &postscript; than about the -FPU assembly language: Mastering -&postscript; will help you master the FPU. - - + We can generalize all these optimizations into one rule: + Keep repeat values on the stack! - + + &postscript; is a + stack–oriented programming language. There are many + more books available about &postscript; than about the + FPU assembly language: Mastering + &postscript; will help you master the + FPU. + + - -<application>pinhole</application>—The Code - -;;;;;;; pinhole.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + <application>pinhole</application>—The Code + + ;;;;;;; pinhole.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; Find various parameters of a pinhole camera construction and use ; ; Started: 9-Jun-2001 ; Updated: 10-Jun-2001 ; ; Copyright (c) 2001 G. Adam Stanislav ; All rights reserved. ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; %include 'system.inc' %define BUFSIZE 2048 section .data align 4 ten dd 10 thousand dd 1000 tthou dd 10000 fd.in dd stdin fd.out dd stdout envar db 'PINHOLE=' ; Exactly 8 bytes, or 2 dwords long pinhole db '04,', ; Bender's constant (0.04) connors db '037', 0Ah ; Connors' constant usg db 'Usage: pinhole [-b] [-c] [-e] [-p <value>] [-o <outfile>] [-i <infile>]', 0Ah usglen equ $-usg iemsg db "pinhole: Can't open input file", 0Ah iemlen equ $-iemsg oemsg db "pinhole: Can't create output file", 0Ah oemlen equ $-oemsg pinmsg db "pinhole: The PINHOLE constant must not be 0", 0Ah pinlen equ $-pinmsg toobig db "pinhole: The PINHOLE constant may not exceed 18 decimal places", 0Ah biglen equ $-toobig huhmsg db 9, '???' separ db 9, '???' sep2 db 9, '???' sep3 db 9, '???' sep4 db 9, '???', 0Ah huhlen equ $-huhmsg header db 'focal length in millimeters,pinhole diameter in microns,' db 'F-number,normalized F-number,F-5.6 multiplier,stops ' db 'from F-5.6', 0Ah headlen equ $-header section .bss ibuffer resb BUFSIZE obuffer resb BUFSIZE dbuffer resb 20 ; decimal input buffer bbuffer resb 10 ; BCD buffer section .text align 4 huh: call write push dword huhlen push dword huhmsg push dword [fd.out] sys.write add esp, byte 12 ret align 4 perr: push dword pinlen push dword pinmsg push dword stderr sys.write push dword 4 ; return failure sys.exit align 4 consttoobig: push dword biglen push dword toobig push dword stderr sys.write push dword 5 ; return failure sys.exit align 4 ierr: push dword iemlen push dword iemsg push dword stderr sys.write push dword 1 ; return failure sys.exit align 4 oerr: push dword oemlen push dword oemsg push dword stderr sys.write push dword 2 sys.exit align 4 usage: push dword usglen push dword usg push dword stderr sys.write push dword 3 sys.exit align 4 global _start _start: add esp, byte 8 ; discard argc and argv[0] sub esi, esi .arg: pop ecx or ecx, ecx je near .getenv ; no more arguments ; ECX contains the pointer to an argument cmp byte [ecx], '-' jne usage inc ecx mov ax, [ecx] inc ecx .o: cmp al, 'o' jne .i ; Make sure we are not asked for the output file twice cmp dword [fd.out], stdout jne usage ; Find the path to output file - it is either at [ECX+1], ; i.e., -ofile -- ; or in the next argument, ; i.e., -o file or ah, ah jne .openoutput pop ecx jecxz usage .openoutput: push dword 420 ; file mode (644 octal) push dword 0200h | 0400h | 01h ; O_CREAT | O_TRUNC | O_WRONLY push ecx sys.open jc near oerr add esp, byte 12 mov [fd.out], eax jmp short .arg .i: cmp al, 'i' jne .p ; Make sure we are not asked twice cmp dword [fd.in], stdin jne near usage ; Find the path to the input file or ah, ah jne .openinput pop ecx or ecx, ecx je near usage .openinput: push dword 0 ; O_RDONLY push ecx sys.open jc near ierr ; open failed add esp, byte 8 mov [fd.in], eax jmp .arg .p: cmp al, 'p' jne .c or ah, ah jne .pcheck pop ecx or ecx, ecx je near usage mov ah, [ecx] .pcheck: cmp ah, '0' jl near usage cmp ah, '9' ja near usage mov esi, ecx jmp .arg .c: cmp al, 'c' jne .b or ah, ah jne near usage mov esi, connors jmp .arg .b: cmp al, 'b' jne .e or ah, ah jne near usage mov esi, pinhole jmp .arg .e: cmp al, 'e' jne near usage or ah, ah jne near usage mov al, ',' mov [huhmsg], al mov [separ], al mov [sep2], al mov [sep3], al mov [sep4], al jmp .arg align 4 .getenv: ; If ESI = 0, we did not have a -p argument, ; and need to check the environment for "PINHOLE=" or esi, esi jne .init sub ecx, ecx .nextenv: pop esi or esi, esi je .default ; no PINHOLE envar found ; check if this envar starts with 'PINHOLE=' mov edi, envar mov cl, 2 ; 'PINHOLE=' is 2 dwords long rep cmpsd jne .nextenv ; Check if it is followed by a digit mov al, [esi] cmp al, '0' jl .default cmp al, '9' jbe .init ; fall through align 4 .default: ; We got here because we had no -p argument, ; and did not find the PINHOLE envar. mov esi, pinhole ; fall through align 4 .init: sub eax, eax sub ebx, ebx sub ecx, ecx sub edx, edx mov edi, dbuffer+1 mov byte [dbuffer], '0' ; Convert the pinhole constant to real .constloop: lodsb cmp al, '9' ja .setconst cmp al, '0' je .processconst jb .setconst inc dl .processconst: inc cl cmp cl, 18 ja near consttoobig stosb jmp short .constloop align 4 .setconst: or dl, dl je near perr finit fild dword [tthou] fld1 fild dword [ten] fdivp st1, st0 fild dword [thousand] mov edi, obuffer mov ebp, ecx call bcdload .constdiv: fmul st0, st2 loop .constdiv fld1 fadd st0, st0 fadd st0, st0 fld1 faddp st1, st0 fchs ; If we are creating a CSV file, ; print header cmp byte [separ], ',' jne .bigloop push dword headlen push dword header push dword [fd.out] sys.write .bigloop: call getchar jc near done ; Skip to the end of the line if you got '#' cmp al, '#' jne .num call skiptoeol jmp short .bigloop .num: ; See if you got a number cmp al, '0' jl .bigloop cmp al, '9' ja .bigloop ; Yes, we have a number sub ebp, ebp sub edx, edx .number: cmp al, '0' je .number0 mov dl, 1 .number0: or dl, dl ; Skip leading 0's je .nextnumber push eax call putchar pop eax inc ebp cmp ebp, 19 jae .nextnumber mov [dbuffer+ebp], al .nextnumber: call getchar jc .work cmp al, '#' je .ungetc cmp al, '0' jl .work cmp al, '9' ja .work jmp short .number .ungetc: dec esi inc ebx .work: ; Now, do all the work or dl, dl je near .work0 cmp ebp, 19 jae near .toobig call bcdload ; Calculate pinhole diameter fld st0 ; save it fsqrt fmul st0, st3 fld st0 fmul st5 sub ebp, ebp ; Round off to 4 significant digits .diameter: fcom st0, st7 fstsw ax sahf jb .printdiameter fmul st0, st6 inc ebp jmp short .diameter .printdiameter: call printnumber ; pinhole diameter ; Calculate F-number fdivp st1, st0 fld st0 sub ebp, ebp .fnumber: fcom st0, st6 fstsw ax sahf jb .printfnumber fmul st0, st5 inc ebp jmp short .fnumber .printfnumber: call printnumber ; F number ; Calculate normalized F-number fmul st0, st0 fld1 fld st1 fyl2x frndint fld1 fscale fsqrt fstp st1 sub ebp, ebp call printnumber ; Calculate time multiplier from F-5.6 fscale fld st0 ; Round off to 4 significant digits .fmul: fcom st0, st6 fstsw ax sahf jb .printfmul inc ebp fmul st0, st5 jmp short .fmul .printfmul: call printnumber ; F multiplier ; Calculate F-stops from 5.6 fld1 fxch st1 fyl2x sub ebp, ebp call printnumber mov al, 0Ah call putchar jmp .bigloop .work0: mov al, '0' call putchar align 4 .toobig: call huh jmp .bigloop align 4 done: call write ; flush output buffer ; close files push dword [fd.in] sys.close push dword [fd.out] sys.close finit ; return success push dword 0 sys.exit align 4 skiptoeol: ; Keep reading until you come to cr, lf, or eof call getchar jc done cmp al, 0Ah jne .cr ret .cr: cmp al, 0Dh jne skiptoeol ret align 4 getchar: or ebx, ebx jne .fetch call read .fetch: lodsb dec ebx clc ret read: jecxz .read call write .read: push dword BUFSIZE mov esi, ibuffer push esi push dword [fd.in] sys.read add esp, byte 12 mov ebx, eax or eax, eax je .empty sub eax, eax ret align 4 .empty: add esp, byte 4 stc ret align 4 putchar: stosb inc ecx cmp ecx, BUFSIZE je write ret align 4 write: jecxz .ret ; nothing to write sub edi, ecx ; start of buffer push ecx push edi push dword [fd.out] sys.write add esp, byte 12 sub eax, eax sub ecx, ecx ; buffer is empty now .ret: ret align 4 bcdload: ; EBP contains the number of chars in dbuffer push ecx push esi push edi lea ecx, [ebp+1] lea esi, [dbuffer+ebp-1] shr ecx, 1 std mov edi, bbuffer sub eax, eax mov [edi], eax mov [edi+4], eax mov [edi+2], ax .loop: lodsw sub ax, 3030h shl al, 4 or al, ah mov [edi], al inc edi loop .loop fbld [bbuffer] cld pop edi pop esi pop ecx sub eax, eax ret align 4 printnumber: push ebp mov al, [separ] call putchar ; Print the integer at the TOS mov ebp, bbuffer+9 fbstp [bbuffer] ; Check the sign mov al, [ebp] dec ebp or al, al jns .leading ; We got a negative number (should never happen) mov al, '-' call putchar .leading: ; Skip leading zeros mov al, [ebp] dec ebp or al, al jne .first cmp ebp, bbuffer jae .leading ; We are here because the result was 0. ; Print '0' and return mov al, '0' jmp putchar .first: ; We have found the first non-zero. ; But it is still packed test al, 0F0h jz .second push eax shr al, 4 add al, '0' call putchar pop eax and al, 0Fh .second: add al, '0' call putchar .next: cmp ebp, bbuffer jb .done mov al, [ebp] push eax shr al, 4 add al, '0' call putchar pop eax and al, 0Fh add al, '0' call putchar dec ebp jmp short .next .done: pop ebp or ebp, ebp je .ret .zeros: mov al, '0' call putchar dec ebp jne .zeros .ret: - ret - - -The code follows the same format as all the other -filters we have seen before, with one subtle -exception: - + ret -
- -We are no longer assuming that the end of input -implies the end of things to do, something we -took for granted in the character–oriented -filters. - + The code follows the same format as all the other filters + we have seen before, with one subtle exception: - -This filter does not process characters. It -processes a language -(albeit a very simple -one, consisting only of numbers). - +
+ We are no longer assuming that the end of input implies + the end of things to do, something we took for granted in + the character–oriented + filters. - -When we have no more input, it can mean one -of two things: + This filter does not process characters. It processes a + language (albeit a very simple one, + consisting only of numbers). - - -We are done and can quit. This is the -same as before. - - + When we have no more input, it can mean one of two + things: - - -The last character we have read was a digit. -We have stored it at the end of our -ASCII–to–float conversion -buffer. We now need to convert -the contents of that buffer into a -number and write the last line of our -output. - - + + + We are done and can quit. This is the same as + before. + - - -For that reason, we have modified our getchar -and our read routines to return with -the carry flag clear whenever we are -fetching another character from the input, or the -carry flag set whenever there is no more -input. - + + The last character we have read was a digit. We + have stored it at the end of our + ASCII–to–float + conversion buffer. We now need to convert the contents + of that buffer into a number and write the last line of + our output. + + - -Of course, we are still using assembly language magic -to do that! Take a good look at getchar. -It always returns with the -carry flag clear. - + For that reason, we have modified our + getchar and our + read routines to return with the + carry flag + clear whenever we are fetching another + character from the input, or the carry flag + set whenever there is no more + input. - -Yet, our main code relies on the carry -flag to tell it when to quit—and it works. - + Of course, we are still using assembly language magic to + do that! Take a good look at getchar. + It always returns with the carry flag + clear. - -The magic is in read. Whenever it -receives more input from the system, it just -returns to getchar, which -fetches a character from the input buffer, -clears the carry flag -and returns. - + Yet, our main code relies on the carry flag to tell it when to + quit—and it works. - -But when read receives no more -input from the system, it does not -return to getchar at all. -Instead, the add esp, byte 4 -op code adds 4 to ESP, -sets the carry -flag, and returns. - + The magic is in read. Whenever it + receives more input from the system, it just returns to + getchar, which fetches a character from + the input buffer, clears the carry flag and returns. - -So, where does it return to? Whenever a -program uses the call op code, -the microprocessor pushes the -return address, i.e., it stores it on -the top of the stack (not the FPU -stack, the system stack, which is in the memory). -When a program uses the ret -op code, the microprocessor pops -the return value from the stack, and jumps -to the address that was stored there. - + But when read receives no more + input from the system, it does not + return to getchar at all. Instead, the + add esp, byte 4 op code + adds 4 to ESP, sets + the carry flag, and + returns. - -But since we added 4 to -ESP (which is the stack -pointer register), we have effectively -given the microprocessor a minor case -of amnesia: It no longer -remembers it was getchar -that called read. - + So, where does it return to? Whenever a program uses the + call op code, the + microprocessor pushes the + return address, i.e., it stores it on the top of the stack + (not the FPU stack, the system stack, + which is in the memory). When a program uses the ret op code, the microprocessor + pops the return value + from the stack, and jumps to the address that was stored + there. - -And since getchar never -pushed anything before -calling read, -the top of the stack now contains the -return address to whatever or whoever -called getchar. -As far as that caller is concerned, -he called getchar, -which returned with the -carry flag set! - + But since we added 4 to ESP (which is the stack pointer + register), we have effectively given the microprocessor a + minor case of amnesia: It no longer + remembers it was getchar that called + read. -
- -Other than that, the bcdload -routine is caught up in the middle of a -Lilliputian conflict between the Big–Endians -and the Little–Endians. - + And since getchar never pushed anything before calling + read, the top of the stack now contains + the return address to whatever or whoever called + getchar. As far as that caller is + concerned, he called + getchar, which returned with the carry flag set! +
- -It is converting the text representation -of a number into that number: The text -is stored in the big–endian order, but -the packed decimal is little–endian. - + Other than that, the bcdload routine + is caught up in the middle of a Lilliputian conflict between + the Big–Endians and the Little–Endians. - -To solve the conflict, we use the std -op code early on. We cancel it with cld -later on: It is quite important we do not -call anything that may depend on -the default setting of the direction -flag while std is active. - + It is converting the text representation of a number into + that number: The text is stored in the big–endian order, + but the packed decimal is + little–endian. - -Everything else in this code should be quite -clear, providing you have read the entire chapter -that precedes it. - + To solve the conflict, we use the std + op code early on. We cancel it with cld + later on: It is quite important we do not + call anything that may depend on the + default setting of the direction flag + while std is active. - -It is a classical example of the adage that -programming requires a lot of thought and only -a little coding. Once we have thought through every -tiny detail, the code almost writes itself. - + Everything else in this code should be quit eclear, + providing you have read the entire chapter that precedes + it. - + It is a classical example of the adage that programming + requires a lot of thought and only a little coding. Once we + have thought through every tiny detail, the code almost writes + itself. + - -Using <application>pinhole</application> - -Because we have decided to make the program -ignore any input except for numbers -(and even those inside a comment), we can -actually perform textual queries. -We do not have to, but we can. - + + Using <application>pinhole</application> - -In my humble opinion, forming a textual query, -instead of having to follow a very strict -syntax, makes software much more user friendly. - + Because we have decided to make the program + ignore any input except for numbers (and + even those inside a comment), we can actually perform + textual queries. We do not + have to, but we + can. - -Suppose we want to build a pinhole camera to use the -4x5 inch film. The standard focal -length for that film is about 150mm. We want -to fine–tune our focal length so the -pinhole diameter is as round a number as possible. -Let us also suppose we are quite comfortable with -cameras but somewhat intimidated by computers. -Rather than just have to type in a bunch of numbers, -we want to ask a couple of questions. - + In my humble opinion, forming a textual query, instead of + having to follow a very strict syntax, makes software much + more user friendly. - -Our session might look like this: + Suppose we want to build a pinhole camera to use the 4x5 + inch film. The standard focal length for that film is about + 150mm. We want to fine–tune our + focal length so the pinhole diameter is as round a number as + possible. Let us also suppose we are quite comfortable with + cameras but somewhat intimidated by computers. Rather than + just have to type in a bunch of numbers, we want to + ask a couple of questions. -&prompt.user; pinhole + Our session might look like this: + &prompt.user; pinhole + Computer, What size pinhole do I need for the focal length of 150? 150 490 306 362 2930 12 Hmmm... How about 160? 160 506 316 362 3125 12 Let's make it 155, please. 155 498 311 362 3027 12 Ah, let's try 157... 157 501 313 362 3066 12 156? 156 500 312 362 3047 12 That's it! Perfect! Thank you very much! ^D - -We have found that while for the focal length -of 150, our pinhole diameter should be 490 -microns, or 0.49 mm, if we go with the almost -identical focal length of 156 mm, we can -get away with a pinhole diameter of exactly -one half of a millimeter. - - + We have found that while for the focal length of 150, our + pinhole diameter should be 490 microns, or 0.49 mm, if we go + with the almost identical focal length of 156 mm, we can get + away with a pinhole diameter of exactly one half of a + millimeter. + - -Scripting - -Because we have chosen the # -character to denote the start of a comment, -we can treat our pinhole -software as a scripting language. - + + Scripting - -You have probably seen shell -scripts that start with: + Because we have chosen the # + character to denote the start of a comment, we can treat our + pinhole software as a + scripting language. - -#! /bin/sh - - -...or... + You have probably seen shell + scripts that start with: - -#!/bin/sh - -...because the blank space after the #! -is optional. - + #! /bin/sh - -Whenever &unix; is asked to run an executable -file which starts with the #!, -it assumes the file is a script. It adds the -command to the rest of the first line of the -script, and tries to execute that. - + ...or... - -Suppose now that we have installed pinhole -in /usr/local/bin/, we can now -write a script to calculate various pinhole -diameters suitable for various focal lengths -commonly used with the 120 film. + #!/bin/sh - -The script might look something like this: + ...because the blank space after the + #! is optional. - -#! /usr/local/bin/pinhole -b -i + Whenever &unix; is asked to run an executable + file which starts with the #!, + it assumes the file is a script. It adds the + command to the rest of the first line of the + script, and tries to execute that. + + Suppose now that we have installed + pinhole in + /usr/local/bin/, we can now write a + script to calculate various pinhole diameters suitable for + various focal lengths commonly used with the 120 film. + + The script might look something like this: + + #! /usr/local/bin/pinhole -b -i # Find the best pinhole diameter # for the 120 film ### Standard 80 ### Wide angle 30, 40, 50, 60, 70 ### Telephoto -100, 120, 140 - - -Because 120 is a medium size film, -we may name this file medium. - +100, 120, 140 - -We can set its permissions to execute, -and run it as if it were a program: - + Because 120 is a medium size film, we may name this file + medium. -&prompt.user; chmod 755 medium + We can set its permissions to execute, and run it as if it + were a program: + + &prompt.user; chmod 755 medium &prompt.user; ./medium - -&unix; will interpret that last command as: -&prompt.user; /usr/local/bin/pinhole -b -i ./medium - -It will run that command and display: - + &unix; will interpret that last command as: -80 358 224 256 1562 11 + &prompt.user; /usr/local/bin/pinhole -b -i ./medium + + It will run that command and display: + + 80 358 224 256 1562 11 30 219 137 128 586 9 40 253 158 181 781 10 50 283 177 181 977 10 60 310 194 181 1172 10 70 335 209 181 1367 10 100 400 250 256 1953 11 120 438 274 256 2344 11 140 473 296 256 2734 11 - -Now, let us enter: + Now, let us enter: -&prompt.user; ./medium -c - -&unix; will treat that as: + &prompt.user; ./medium -c -&prompt.user; /usr/local/bin/pinhole -b -i ./medium -c - -That gives it two conflicting options: --b and -c -(Use Bender's constant and use Connors' -constant). We have programmed it so -later options override early ones—our -program will calculate everything -using Connors' constant: - + &unix; will treat that as: -80 331 242 256 1826 11 + &prompt.user; /usr/local/bin/pinhole -b -i ./medium -c + + That gives it two conflicting options: + -b and -c + (Use Bender's constant and use Connors' constant). We have + programmed it so later options override early ones—our + program will calculate everything using Connors' + constant: + + 80 331 242 256 1826 11 30 203 148 128 685 9 40 234 171 181 913 10 50 262 191 181 1141 10 60 287 209 181 1370 10 70 310 226 256 1598 11 100 370 270 256 2283 11 120 405 296 256 2739 11 140 438 320 362 3196 12 - -We decide we want to go with Bender's -constant after all. We want to save its -values as a comma–separated file: - -&prompt.user; ./medium -b -e > bender + We decide we want to go with Bender's constant after all. + We want to save its values as a comma–separated + file: + + &prompt.user; ./medium -b -e > bender &prompt.user; cat bender focal length in millimeters,pinhole diameter in microns,F-number,normalized F-number,F-5.6 multiplier,stops from F-5.6 80,358,224,256,1562,11 30,219,137,128,586,9 40,253,158,181,781,10 50,283,177,181,977,10 60,310,194,181,1172,10 70,335,209,181,1367,10 100,400,250,256,1953,11 120,438,274,256,2344,11 140,473,296,256,2734,11 &prompt.user; - + + - + + Caveats - -Caveats + Assembly language programmers who "grew up" under + &ms-dos; and &windows; often tend to take + shortcuts. Reading the keyboard scan codes and writing + directly to video memory are two classical examples of + practices which, under &ms-dos; are not + frowned upon but considered the right thing to do. - -Assembly language programmers who "grew up" under -&ms-dos; and &windows; often tend to take shortcuts. -Reading the keyboard scan codes and writing directly to video -memory are two classical examples of practices which, under -&ms-dos; are not frowned upon but considered the -right thing to do. - + The reason? Both the PC BIOS and + &ms-dos; are notoriously slow when + performing these operations. - -The reason? Both the PC BIOS and -&ms-dos; are notoriously -slow when performing these operations. - + You may be tempted to continue similar practices in the + &unix; environment. For example, I have seen a web site which + explains how to access the keyboard scan codes on a popular + &unix; clone. - -You may be tempted to continue similar practices in the -&unix; environment. For example, I have seen a web site which -explains how to access the keyboard scan codes on a popular &unix; clone. - + That is generally a very bad idea in + &unix; environment! Let me explain why. - -That is generally a very bad idea -in &unix; environment! Let me explain why. - + + &unix; Is Protected - -&unix; Is Protected + For one thing, it may simply not be possible. &unix; + runs in protected mode. Only the kernel and device drivers + are allowed to access hardware directly. Perhaps a + particular &unix; clone will let you read the keyboard scan + codes, but chances are a real &unix; operating system will + not. And even if one version may let you do it, the next + one may not, so your carefully crafted software may become a + dinosaur overnight. + - -For one thing, it may simply not be possible. &unix; runs in -protected mode. Only the kernel and device drivers are allowed -to access hardware directly. Perhaps a particular &unix; clone -will let you read the keyboard scan codes, but chances are a real -&unix; operating system will not. And even if one version may let you -do it, the next one may not, so your carefully crafted software may -become a dinosaur overnight. - + + &unix; Is an Abstraction - + But there is a much more important reason not to try + accessing the hardware directly (unless, of course, you are + writing a device driver), even on the &unix; like systems + that let you do it: - -&unix; Is an Abstraction + &unix; is an abstraction! - -But there is a much more important reason not to try -accessing the hardware directly (unless, of course, -you are writing a device driver), even on the &unix; like -systems that let you do it: - + There is a major difference in the philosophy of design + between &ms-dos; and &unix;. + &ms-dos; was designed as a single-user + system. It is run on a computer with a keyboard and a video + screen attached directly to that computer. User input is + almost guaranteed to come from that keyboard. Your + program's output virtually always ends up on that + screen. - -&unix; is an abstraction! - + This is NEVER guaranteed under &unix;. It is quite + common for a &unix; user to pipe and redirect program input + and output: - -There is a major difference in the philosophy of design -between &ms-dos; and &unix;. -&ms-dos; was designed as a single-user -system. It is run on a computer with a keyboard and a video -screen attached directly to that computer. User input is almost -guaranteed to come from that keyboard. Your program's output -virtually always ends up on that screen. - + &prompt.user; program1 | program2 | program3 > file1 - -This is NEVER guaranteed under &unix;. It is quite common -for a &unix; user to pipe and redirect program input and output: - + If you have written program2, + your input does not come from the keyboard but from the + output of program1. Similarly, + your output does not go to the screen but becomes the input + for program3 whose output, in + turn, goes to file1. -&prompt.user; program1 | program2 | program3 > file1 + But there is more! Even if you made sure that your input + comes from, and your output goes to, the terminal, there is + no guarantee the terminal is a PC: It may not have its video + memory where you expect it, nor may its keyboard be + producing PC-style scan codes. It may be + a &macintosh;, or any other computer. - -If you have written program2, your input -does not come from the keyboard but from the output of -program1. Similarly, your output does not -go to the screen but becomes the input for -program3 whose output, in turn, -goes to file1. - + Now you may be shaking your head: My software is in + PC assembly language, how can it run on a + &macintosh;? But I did not say your software would be + running on a &macintosh;, only that its terminal may be a + &macintosh;. - -But there is more! Even if you made sure that your input comes -from, and your output goes to, the terminal, there is no guarantee -the terminal is a PC: It may not have its video memory -where you expect it, nor may its keyboard be producing -PC-style scan codes. It may be a &macintosh;, -or any other computer. - + Under &unix;, the terminal does not have to be directly + attached to the computer that runs your software, it can + even be on another continent, or, for that matter, on + another planet. It is perfectly possible that a &macintosh; + user in Australia connects to a &unix; system in North + America (or anywhere else) via + telnet. The software then runs + on one computer, while the terminal is on a different + computer: If you try to read the scan codes, you will get + the wrong input! - -Now you may be shaking your head: My software is in -PC assembly language, how can -it run on a &macintosh;? But I did not say your software -would be running on a &macintosh;, only that its terminal -may be a &macintosh;. - + Same holds true about any other hardware: A file you are + reading may be on a disk you have no direct access to. A + camera you are reading images from may be on a + space shuttle, connected to you via satellites. - -Under &unix;, the terminal does not have to be directly -attached to the computer that runs your software, it can -even be on another continent, or, for that matter, on another -planet. It is perfectly possible that a &macintosh; user in -Australia connects to a &unix; system in North America (or -anywhere else) via telnet. The -software then runs on one computer, while the terminal is -on a different computer: If you try to read the scan codes, -you will get the wrong input! - + That is why under &unix; you must never make any + assumptions about where your data is coming from and going + to. Always let the system handle the physical access to the + hardware. - -Same holds true about any other hardware: A file you are reading -may be on a disk you have no direct access to. A camera you are -reading images from may be on a space shuttle, connected to you -via satellites. - + + These are caveats, not absolute rules. Exceptions are + possible. For example, if a text editor has determined it + is running on a local machine, it may want to read the + scan codes directly for improved control. I am not + mentioning these caveats to tell you what to do or what + not to do, just to make you aware of certain pitfalls that + await you if you have just arrived to &unix; form + &ms-dos;. Of course, creative people + often break rules, and it is OK as long as they know they + are breaking them and why. + + + - -That is why under &unix; you must never make any assumptions about -where your data is coming from and going to. Always let the -system handle the physical access to the hardware. - + + Acknowledgements - - -These are caveats, not absolute rules. Exceptions are possible. -For example, if a text editor has determined it is running -on a local machine, it may want to read the scan codes -directly for improved control. I am not mentioning these caveats -to tell you what to do or what not to do, just to make you aware -of certain pitfalls that await you if you have just arrived to &unix; -form &ms-dos;. Of course, creative people often break -rules, and it is OK as long as they know they are breaking -them and why. - - + This tutorial would never have been possible without the + help of many experienced FreeBSD programmers from the + &a.hackers;, many of whom have patiently answered my + questions, and pointed me in the right direction in my + attempts to explore the inner workings of &unix; system + programming in general and FreeBSD in particular. - + Thomas M. Sommers opened the door for me . His How + do I write "Hello, world" in FreeBSD assembler? web + page was my first encounter with an example of assembly + language programming under FreeBSD. - + Jake Burkholder has kept the door open by willingly + answering all of my questions and supplying me with example + assembly language source code. - - -Acknowledgements - - -This tutorial would never have been possible without the -help of many experienced FreeBSD programmers from the -&a.hackers;, many of whom have patiently -answered my questions, and pointed me in the right direction -in my attempts to explore the inner workings of &unix; -system programming in general and FreeBSD in particular. - - - -Thomas M. Sommers opened the door for me. His How -do I write "Hello, world" in FreeBSD assembler? -web page was my first encounter with an example of -assembly language programming under FreeBSD. - - - -Jake Burkholder has kept the door open by willingly -answering all of my questions and supplying me with -example assembly language source code. - - - -Copyright © 2000-2001 G. Adam Stanislav. All rights reserved. - - - - - - + Copyright © 2000-2001 G. Adam Stanislav. All rights + reserved. + +