diff --git a/sys/amd64/amd64/trap.c b/sys/amd64/amd64/trap.c
--- a/sys/amd64/amd64/trap.c
+++ b/sys/amd64/amd64/trap.c
@@ -1011,7 +1011,7 @@
 {
 	struct proc *p;
 	struct trapframe *frame;
-	register_t *argp;
+	syscallarg_t *argp;
 	caddr_t params;
 	int reg, regcnt, error;
 
diff --git a/sys/arm/arm/syscall.c b/sys/arm/arm/syscall.c
--- a/sys/arm/arm/syscall.c
+++ b/sys/arm/arm/syscall.c
@@ -100,7 +100,7 @@
 cpu_fetch_syscall_args(struct thread *td)
 {
 	struct proc *p;
-	register_t *ap;
+	syscallarg_t *ap;
 	struct syscall_args *sa;
 	u_int nap;
 	int error;
@@ -124,10 +124,10 @@
 	else
 		sa->callp = &p->p_sysent->sv_table[sa->code];
 	error = 0;
-	memcpy(sa->args, ap, nap * sizeof(register_t));
+	memcpy(sa->args, ap, nap * sizeof(*sa->args));
 	if (sa->callp->sy_narg > nap) {
 		error = copyin((void *)td->td_frame->tf_usr_sp, sa->args +
-		    nap, (sa->callp->sy_narg - nap) * sizeof(register_t));
+		    nap, (sa->callp->sy_narg - nap) * sizeof(*sa->args));
 	}
 	if (error == 0) {
 		td->td_retval[0] = 0;
diff --git a/sys/arm64/arm64/trap.c b/sys/arm64/arm64/trap.c
--- a/sys/arm64/arm64/trap.c
+++ b/sys/arm64/arm64/trap.c
@@ -134,7 +134,7 @@
 cpu_fetch_syscall_args(struct thread *td)
 {
 	struct proc *p;
-	register_t *ap, *dst_ap;
+	syscallarg_t *ap, *dst_ap;
 	struct syscall_args *sa;
 
 	p = td->td_proc;
@@ -159,7 +159,7 @@
 	KASSERT(sa->callp->sy_narg <= nitems(sa->args),
 	    ("Syscall %d takes too many arguments", sa->code));
 
-	memcpy(dst_ap, ap, (nitems(sa->args) - 1) * sizeof(register_t));
+	memcpy(dst_ap, ap, (nitems(sa->args) - 1) * sizeof(*dst_ap));
 
 	td->td_retval[0] = 0;
 	td->td_retval[1] = 0;
diff --git a/sys/kern/kern_ktrace.c b/sys/kern/kern_ktrace.c
--- a/sys/kern/kern_ktrace.c
+++ b/sys/kern/kern_ktrace.c
@@ -524,7 +524,7 @@
 }
 
 void
-ktrsyscall(int code, int narg, register_t args[])
+ktrsyscall(int code, int narg, syscallarg_t args[])
 {
 	struct ktr_request *req;
 	struct ktr_syscall *ktp;
diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -474,7 +474,7 @@
 		struct dbreg dbreg;
 		struct fpreg fpreg;
 		struct reg reg;
-		char args[sizeof(td->td_sa.args)];
+		syscallarg_t args[nitems(td->td_sa.args)];
 		struct ptrace_sc_ret psr;
 		int ptevents;
 	} r;
@@ -1010,7 +1010,7 @@
 		/* See the explanation in linux_ptrace_get_syscall_info(). */
 		bcopy(td2->td_sa.args, addr, SV_PROC_ABI(td->td_proc) ==
 		    SV_ABI_LINUX ? sizeof(td2->td_sa.args) :
-		    td2->td_sa.callp->sy_narg * sizeof(register_t));
+		    td2->td_sa.callp->sy_narg * sizeof(syscallarg_t));
 		break;
 
 	case PT_GET_SC_RET:
diff --git a/sys/riscv/riscv/trap.c b/sys/riscv/riscv/trap.c
--- a/sys/riscv/riscv/trap.c
+++ b/sys/riscv/riscv/trap.c
@@ -94,7 +94,7 @@
 cpu_fetch_syscall_args(struct thread *td)
 {
 	struct proc *p;
-	register_t *ap, *dst_ap;
+	syscallarg_t *ap, *dst_ap;
 	struct syscall_args *sa;
 
 	p = td->td_proc;
@@ -119,7 +119,7 @@
 	KASSERT(sa->callp->sy_narg <= nitems(sa->args),
 	    ("Syscall %d takes too many arguments", sa->code));
 
-	memcpy(dst_ap, ap, (NARGREG - 1) * sizeof(register_t));
+	memcpy(dst_ap, ap, (NARGREG - 1) * sizeof(*dst_ap));
 
 	td->td_retval[0] = 0;
 	td->td_retval[1] = 0;
diff --git a/sys/sys/ktrace.h b/sys/sys/ktrace.h
--- a/sys/sys/ktrace.h
+++ b/sys/sys/ktrace.h
@@ -284,7 +284,7 @@
 void	ktrfault(vm_offset_t, int);
 void	ktrfaultend(int);
 void	ktrgenio(int, enum uio_rw, struct uio *, int);
-void	ktrsyscall(int, int narg, register_t args[]);
+void	ktrsyscall(int, int narg, syscallarg_t args[]);
 void	ktrsysctl(int *name, u_int namelen);
 void	ktrsysret(int, int, register_t);
 void	ktrprocctor(struct proc *);
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -357,7 +357,7 @@
 	} td_state;			/* (t) thread state */
 	/* Note: td_state must be accessed using TD_{GET,SET}_STATE(). */
 	union {
-		register_t	tdu_retval[2];
+		syscallarg_t	tdu_retval[2];
 		off_t		tdu_off;
 	} td_uretoff;			/* (k) Syscall aux returns. */
 #define td_retval	td_uretoff.tdu_retval
diff --git a/sys/sys/ptrace.h b/sys/sys/ptrace.h
--- a/sys/sys/ptrace.h
+++ b/sys/sys/ptrace.h
@@ -160,7 +160,7 @@
 
 /* Argument structure for PT_GET_SC_RET. */
 struct ptrace_sc_ret {
-	register_t	sr_retval[2];	/* Only valid if sr_error == 0. */
+	syscallarg_t	sr_retval[2];	/* Only valid if sr_error == 0. */
 	int		sr_error;
 };
 
diff --git a/sys/sys/types.h b/sys/sys/types.h
--- a/sys/sys/types.h
+++ b/sys/sys/types.h
@@ -270,6 +270,8 @@
 
 typedef __rman_res_t    rman_res_t;
 
+typedef __register_t	syscallarg_t;
+
 #ifdef _KERNEL
 typedef	int		boolean_t;
 typedef	struct _device	*device_t;
diff --git a/sys/tools/makesyscalls.lua b/sys/tools/makesyscalls.lua
--- a/sys/tools/makesyscalls.lua
+++ b/sys/tools/makesyscalls.lua
@@ -860,7 +860,7 @@
 			write_line("sysarg", "};\n")
 		else
 			write_line("sysarg", string.format(
-			    "struct %s {\n\tregister_t dummy;\n};\n", argalias))
+			    "struct %s {\n\tsyscallarg_t dummy;\n};\n", argalias))
 		end
 	end
 
@@ -986,7 +986,7 @@
 		write_line(out, "};\n")
 	elseif flags & nargflags == 0 then
 		write_line("sysarg", string.format(
-		    "struct %s {\n\tregister_t dummy;\n};\n", argalias))
+		    "struct %s {\n\tsyscallarg_t dummy;\n};\n", argalias))
 	end
 	if flags & dprotoflags == 0 then
 		write_line(outdcl, string.format(
@@ -1439,8 +1439,8 @@
 
 struct thread;
 
-#define	PAD_(t)	(sizeof(register_t) <= sizeof(t) ? \
-		0 : sizeof(register_t) - sizeof(t))
+#define	PAD_(t)	(sizeof(syscallarg_t) <= sizeof(t) ? \
+		0 : sizeof(syscallarg_t) - sizeof(t))
 
 #if BYTE_ORDER == LITTLE_ENDIAN
 #define	PADL_(t)	0
@@ -1530,7 +1530,7 @@
 process_sysfile(sysfile)
 
 write_line("sysinc",
-    "\n#define AS(name) (sizeof(struct name) / sizeof(register_t))\n")
+    "\n#define AS(name) (sizeof(struct name) / sizeof(syscallarg_t))\n")
 
 for _, v in pairs(compat_options) do
 	if v["count"] > 0 then
diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c
--- a/sys/vm/vm_mmap.c
+++ b/sys/vm/vm_mmap.c
@@ -420,7 +420,7 @@
 	}
 
 	if (error == 0)
-		td->td_retval[0] = (register_t) (addr + pageoff);
+		td->td_retval[0] = addr + pageoff;
 done:
 	if (fp)
 		fdrop(fp, td);
diff --git a/usr.bin/truss/syscall.h b/usr.bin/truss/syscall.h
--- a/usr.bin/truss/syscall.h
+++ b/usr.bin/truss/syscall.h
@@ -227,7 +227,7 @@
 };
 
 struct syscall *get_syscall(struct threadinfo *, u_int, u_int);
-char *print_arg(struct syscall_arg *, unsigned long *, register_t *,
+char *print_arg(struct syscall_arg *, unsigned long *, syscallarg_t *,
     struct trussinfo *);
 
 /*
@@ -251,8 +251,8 @@
 #define LINUX_SENDMSG		16
 #define LINUX_RECVMSG		17
 
-#define PAD_(t) (sizeof(register_t) <= sizeof(t) ? \
-    0 : sizeof(register_t) - sizeof(t))
+#define PAD_(t) (sizeof(syscallarg_t) <= sizeof(t) ? \
+    0 : sizeof(syscallarg_t) - sizeof(t))
 
 #if BYTE_ORDER == LITTLE_ENDIAN
 #define PADL_(t)	0
@@ -271,5 +271,5 @@
 };
 
 void print_syscall(struct trussinfo *);
-void print_syscall_ret(struct trussinfo *, int, register_t *);
+void print_syscall_ret(struct trussinfo *, int, syscallarg_t *);
 void print_summary(struct trussinfo *trussinfo);
diff --git a/usr.bin/truss/syscalls.c b/usr.bin/truss/syscalls.c
--- a/usr.bin/truss/syscalls.c
+++ b/usr.bin/truss/syscalls.c
@@ -1556,7 +1556,7 @@
  * an array of all of the system call arguments.
  */
 char *
-print_arg(struct syscall_arg *sc, unsigned long *args, register_t *retval,
+print_arg(struct syscall_arg *sc, unsigned long *args, syscallarg_t *retval,
     struct trussinfo *trussinfo)
 {
 	FILE *fp;
@@ -2729,7 +2729,7 @@
 }
 
 void
-print_syscall_ret(struct trussinfo *trussinfo, int error, register_t *retval)
+print_syscall_ret(struct trussinfo *trussinfo, int error, syscallarg_t *retval)
 {
 	struct timespec timediff;
 	struct threadinfo *t;
diff --git a/usr.bin/truss/truss.h b/usr.bin/truss/truss.h
--- a/usr.bin/truss/truss.h
+++ b/usr.bin/truss/truss.h
@@ -81,7 +81,7 @@
 	struct syscall *sc;
 	unsigned int number;
 	unsigned int nargs;
-	unsigned long args[10];
+	syscallarg_t args[10];
 	char *s_args[10];	/* the printable arguments */
 };