首先在unistd.h中有系统调用编号，然后再linux1.0版本中在kernel/sys_call.S中有如下代码：
_system_call:
pushl %eax # save orig_eax
SAVE_ALL
movl $-ENOSYS,EAX(%esp)
cmpl _NR_syscalls,%eax
jae ret_from_sys_call
movl _current,%ebx
andl $~CF_MASK,EFLAGS(%esp) # clear carry - assume no errors
movl $0,errno(%ebx)
movl %db6,%edx
movl %edx,dbgreg6(%ebx) # save current hardware debugging status
testb $0x20,flags(%ebx) # PF_TRACESYS
jne 1f
call _sys_call_table(,%eax,4)
movl %eax,EAX(%esp) # save the return value
movl errno(%ebx),%edx
negl %edx
je ret_from_sys_call
movl %edx,EAX(%esp)
orl $(CF_MASK),EFLAGS(%esp) # set carry to indicate error
jmp ret_from_sys_call
这段代码在linux2.6.12中是
# system call handler stub
ENTRY(system_call)
pushl %eax # save orig_eax
SAVE_ALL
GET_THREAD_INFO(%ebp)
# system call tracing in operation
/* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */
testw $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP),TI_flags(%ebp)
jnz syscall_trace_entry
cmpl $(nr_syscalls), %eax
jae syscall_badsys
syscall_call:
call *sys_call_table(,%eax,4)
movl %eax,EAX(%esp) # store the return value
syscall_exit:
cli # make sure we don't miss an interrupt
# setting need_resched or sigpending
# between sampling and the iret
movl TI_flags(%ebp), %ecx
testw $_TIF_ALLWORK_MASK, %cx # current->work
jne syscall_exit_work
这个是系统调用的入口点，其中sys_call_table在linux1.0中的kernel/sched.c中定义了一个函数表。