/* Legacy ABI only, possibly thumb mode. */
tst r8, #PSR_T_BIT @ this is SPSR from save_user_regs
addne scno, r7, #__NR_SYSCALL_BASE @ put OS number in
ldreq scno, [lr, #-4]
#else
/* Legacy ABI only. */
ldr scno, [lr, #-4] @ get SWI instruction
A710( and ip, scno, #0x0f000000 @ check for SWI )
A710( teq ip, #0x0f000000 )
A710( bne .Larm710bug )
#endif
#ifdef CONFIG_ALIGNMENT_TRAP
ldr ip, __cr_alignment
ldr ip, [ip]
mcr p15, 0, ip, c1, c0 @ update control register
#endif
enable_irq
get_thread_info tsk
adr tbl, sys_call_table @ load syscall table pointer #获取系统调用表的基地址
ldr ip, [tsk, #TI_FLAGS] @ check for syscall tracing
#if defined(CONFIG_OABI_COMPAT)
/*
* If the swi argument is zero, this is an EABI call and we do nothing.
*
* If this is an old ABI call, get the syscall number into scno and
* get the old ABI syscall table address.
*/
bics r10, r10, #0xff000000
eorne scno, r10, #__NR_OABI_SYSCALL_BASE
ldrne tbl, =sys_oabi_call_table
#elif !defined(CONFIG_AEABI)
bic scno, scno, #0xff000000 @ mask off SWI op-code
eor scno, scno, #__NR_SYSCALL_BASE @ check OS number
#endif
stmdb sp!, {r4, r5} @ push fifth and sixth args
tst ip, #_TIF_SYSCALL_TRACE @ are we tracing syscalls?
bne __sys_trace
cmp scno, #NR_syscalls @ check upper syscall limit
adr lr, BSYM(ret_fast_syscall) @ return address
ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine #跳到系统调用函数
add r1, sp, #S_OFF
2: mov why, #0 @ no longer a real syscall
cmp scno, #(__ARM_NR_BASE - __NR_SYSCALL_BASE)
eor r0, scno, #__NR_SYSCALL_BASE @ put OS number back
bcs arm_syscall
b sys_ni_syscall @ not private func
从上面可以看出，当CPU从中断向量表转到vector_swi 之后，完成了几件事情：1.取出系统调用号 2.根据系统调用号取出系统调用函数在系统调用表的基地址，得到一个系统调用函数的函数指针 3. 根据系统调用表的基地址和系统调用号，得到这个系统调用表里的项，每一个表项都是一个函数指针，把这个函数指针赋给PC , 则实现了跳转到系统调用函数。
系统调用表定义在：arch/arm/kernel/Calls.S
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*
* This file is included thrice in entry-common.S
*/
/* 0 */ CALL(sys_restart_syscall)
CALL(sys_exit)
CALL(sys_fork_wrapper)
CALL(sys_read)
CALL(sys_write)
/* 5 */ CALL(sys_open)
CALL(sys_close)
CALL(sys_ni_syscall) /* was sys_waitpid */
CALL(sys_creat)
CALL(sys_link)
/* 10 */ CALL(sys_unlink)
CALL(sys_execve_wrapper)
CALL(sys_chdir)
CALL(OBSOLETE(sys_time)) /* used by libc4 */
CALL(sys_mknod)
/* 15 */ CALL(sys_chmod)
CALL(sys_lchown16)
CALL(sys_ni_syscall) /* was sys_break */
CALL(sys_ni_syscall) /* was sys_stat */
CALL(sys_lseek)
/* 20 */ CALL(sys_getpid)
CALL(sys_mount)
CALL(OBSOLETE(sys_oldumount)) /* used by libc4 */
CALL(sys_setuid16)
CALL(sys_getuid16)
/* 25 */ CALL(OBSOLETE(sys_stime))
CALL(sys_ptrace)
CALL(OBSOLETE(sys_alarm)) /* used by libc4 */
CALL(sys_ni_syscall) /* was sys_fstat */
CALL(sys_pause)
/* 30 */ CALL(OBSOLETE(sys_utime)) /* used by libc4 */
CALL(sys_ni_syscall) /* was sys_stty */
CALL(sys_ni_syscall) /* was sys_getty */
CALL(sys_access)
CALL(sys_nice)
/* 35 */ CALL(sys_ni_syscall) /* was sys_ftime */
CALL(sys_sync)
CALL(sys_kill)
CALL(sys_rename)
CALL(sys_mkdir)
/* 40 */ CALL(sys_rmdir)
CALL(sys_dup)
CALL(sys_pipe)
CALL(sys_times)
CALL(sys_ni_syscall) /* was sys_prof */
/* 45 */ CALL(sys_brk)
CALL(sys_setgid16)
CALL(sys_getgid16)
CALL(sys_ni_syscall) /* was sys_signal */
CALL(sys_geteuid16)
/* 50 */ CALL(sys_getegid16)
CALL(sys_acct)
CALL(sys_umount)
CALL(sys_ni_syscall) /* was sys_lock */
CALL(sys_ioctl)
/* 55 */ CALL(sys_fcntl)
.......