Index: head/zh_TW.UTF-8/books/handbook/basics/chapter.xml =================================================================== --- head/zh_TW.UTF-8/books/handbook/basics/chapter.xml +++ head/zh_TW.UTF-8/books/handbook/basics/chapter.xml @@ -2925,146 +2925,6 @@ - - Binary 的格式 - - 若要知道為何 &os; 是採用 &man.elf.5; 格式，必先瞭解當前 &unix; - 系統中三種影響最為重大的可執行檔相關背景： - - - - &man.a.out.5; - - 最古老、經典 的 &unix; object 檔格式。 - It uses a short and compact header with a magic - number at the beginning that is often used to characterize - the format (see &man.a.out.5; for more details). It - contains three loaded segments: .text, .data, and .bss plus - a symbol table and a string table. - - - - COFF - - The SVR3 object format. The header now comprises a - section table, so you can have more than just .text, .data, - and .bss sections. - - - - &man.elf.5; - - The successor to COFF, featuring - multiple sections and 32-bit or 64-bit possible values. One - major drawback: ELF was also designed - with the assumption that there would be only one ABI per - system architecture. That assumption is actually quite - incorrect, and not even in the commercial SYSV world (which - has at least three ABIs: SVR4, Solaris, SCO) does it hold - true. - - FreeBSD tries to work around this problem somewhat by - providing a utility for branding a - known ELF executable with information - about the ABI it is compliant with. See the manual page for - &man.brandelf.1; for more information. - - - - FreeBSD comes from the classic camp and used - the &man.a.out.5; format, a technology tried and proven through - many generations of BSD releases, until the beginning of the 3.X - branch. Though it was possible to build and run native - ELF binaries (and kernels) on a FreeBSD - system for some time before that, FreeBSD initially resisted the - push to switch to ELF as the - default format. Why? Well, when the Linux camp made their - painful transition to ELF, it was not so much - to flee the a.out executable format as it - was their inflexible jump-table based shared library mechanism, - which made the construction of shared libraries very difficult - for vendors and developers alike. Since the - ELF tools available offered a solution to the - shared library problem and were generally seen as the way - forward anyway, the migration cost was accepted as - necessary and the transition made. FreeBSD's shared library - mechanism is based more closely on Sun's - &sunos; style shared library mechanism - and, as such, is very easy to use. - - So, why are there so many different formats? - - Back in the dim, dark past, there was simple hardware. This - simple hardware supported a simple, small system. a.out was - completely adequate for the job of representing binaries on this - simple system (a PDP-11). As people ported &unix; from this simple - system, they retained the a.out format because it was sufficient - for the early ports of &unix; to architectures like the Motorola - 68k, VAXen, etc. - - Then some bright hardware engineer decided that if he could - force software to do some sleazy tricks, then he would be able - to shave a few gates off the design and allow his CPU core to - run faster. While it was made to work with this new kind of - hardware (known these days as RISC), a.out - was ill-suited for this hardware, so many formats were developed - to get to a better performance from this hardware than the - limited, simple a.out format could - offer. Things like COFF, - ECOFF, and a few obscure others were invented - and their limitations explored before things seemed to settle on - ELF. - - In addition, program sizes were getting huge and disks (and - physical memory) were still relatively small so the concept of a - shared library was born. The VM system also became more - sophisticated. While each one of these advancements was done - using the a.out format, its usefulness was - stretched more and more with each new feature. In addition, - people wanted to dynamically load things at run time, or to junk - parts of their program after the init code had run to save in - core memory and swap space. Languages became more sophisticated - and people wanted code called before main automatically. Lots of - hacks were done to the a.out format to - allow all of these things to happen, and they basically worked - for a time. In time, a.out was not up to - handling all these problems without an ever increasing overhead - in code and complexity. While ELF solved many - of these problems, it would be painful to switch from the system - that basically worked. So ELF had to wait - until it was more painful to remain with - a.out than it was to migrate to - ELF. - - However, as time passed, the build tools that FreeBSD - derived their build tools from (the assembler and loader - especially) evolved in two parallel trees. The FreeBSD tree - added shared libraries and fixed some bugs. The GNU folks that - originally wrote these programs rewrote them and added simpler - support for building cross compilers, plugging in different - formats at will, and so on. Since many people wanted to build cross - compilers targeting FreeBSD, they were out of luck since the - older sources that FreeBSD had for as and ld were not up to the - task. The new GNU tools chain (binutils) does support cross - compiling, ELF, shared libraries, C++ - extensions, etc. In addition, many vendors are releasing - ELF binaries, and it is a good thing for - FreeBSD to run them. - - ELF is more expressive than a.out and - allows more extensibility in the base system. The - ELF tools are better maintained, and offer - cross compilation support, which is important to many people. - ELF may be a little slower than a.out, but - trying to measure it can be difficult. There are also numerous - details that are different between the two in how they map - pages, handle init code, etc. None of these are very important, - but they are differences. In time support for - a.out will be moved out of the GENERIC - kernel, and eventually removed from the kernel once the need to - run legacy a.out programs is past. - - 更多資訊