arch/i386/kernel/entry.S:







ENTRY(system_call)



	pushl ％eax			# save orig_eax



	SAVE_ALL



	GET_CURRENT(％ebx)



	cmpl $(NR_syscalls),％eax



	jae badsys



	testb $0x20,flags(％ebx)		# PF_TRACESYS



	jne tracesys



	call *SYMBOL_NAME(sys_call_table)(,％eax,4)



	movl ％eax,EAX(％esp)		# save the return value



	ALIGN



	.globl ret_from_sys_call



	.globl ret_from_intr



ret_from_sys_call:



	movl SYMBOL_NAME(bh_mask),％eax



	andl SYMBOL_NAME(bh_active),％eax



	jne handle_bottom_half



ret_with_reschedule:



	cmpl $0,need_resched(％ebx)	# need_resched可被pdate_process_times()所置位



	jne reschedule




	cmpl $0,sigpending(％ebx)



	jne signal_return



restore_all:



	RESTORE_ALL







	ALIGN



signal_return:



	sti				# we can get here from an interrupt handler



	testl $(VM_MASK),EFLAGS(％esp)



	movl ％esp,％eax



	jne v86_signal_return



	xorl ％edx,％edx



	call SYMBOL_NAME(do_signal)



	jmp restore_all







	ALIGN



v86_signal_return:



	call SYMBOL_NAME(save_v86_state)



	movl ％eax,％esp



	xorl ％edx,％edx



	call SYMBOL_NAME(do_signal)



	jmp restore_all







	ALIGN



tracesys:



	movl $-ENOSYS,EAX(％esp)



	call SYMBOL_NAME(syscall_trace)



	movl ORIG_EAX(％esp),％eax



        cmpl $(NR_syscalls),％eax 



        jae 1f 



	call *SYMBOL_NAME(sys_call_table)(,％eax,4)



	movl ％eax,EAX(％esp)		# save the return value



1:	call SYMBOL_NAME(syscall_trace)



	jmp ret_from_sys_call



badsys:



	movl $-ENOSYS,EAX(％esp)



	jmp ret_from_sys_call







	ALIGN



ret_from_exception:



	movl SYMBOL_NAME(bh_mask),％eax



	andl SYMBOL_NAME(bh_active),％eax



	jne handle_bottom_half



	ALIGN







ret_from_intr:				# 硬件中断的返回点



	GET_CURRENT(％ebx)



	movl EFLAGS(％esp),％eax		# mix EFLAGS and CS



	movb CS(％esp),％al



	testl $(VM_MASK | 3),％eax	# return to VM86 mode or non-supervisor?



	jne ret_with_reschedule		# 如果是VM86态或用户态就转ret_with_reschedule



	jmp restore_all








	ALIGN



handle_bottom_half:



	call SYMBOL_NAME(do_bottom_half)



	jmp ret_from_intr







	ALIGN



reschedule:



	call SYMBOL_NAME(schedule)    # test



	jmp ret_from_sys_call




---------------------------------------



kernel/sched.c:







update_process_times()被Timer Bottom Half所调用







static void update_process_times(unsigned long ticks, unsigned long system)



{



/*



 * SMP does this on a per-CPU basis elsewhere



 */



#ifndef  __SMP__



	struct task_struct * p = current;



	unsigned long user = ticks - system;



	if (p->pid) {



		p->counter -= ticks;



		if (p->counter < 0) {



			p->counter = 0;



			p->need_resched = 1;




		}



		if (p->priority < DEF_PRIORITY)



			kstat.cpu_nice += user;



		else



			kstat.cpu_user += user;



		kstat.cpu_system += system;



	}



	update_one_process(p, ticks, user, system, 0);



#endif



}