int Time(int argc, char *argv[])
{
    time_t tt;
    struct tm *t;
    tt = time(NULL);
    t = localtime(&tt);
    printf("time:%d:%d:%d:%d:%d:%d\n", t->tm_year + 1900, t->tm_mon, t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec);
    return 0;
}

int TimeAsm(int argc, char *argv[])
{
    time_t tt;
    struct tm *t;
    asm volatile(
        "mov $0,%%ebx\n\t"
        "mov $0xd,%%eax\n\t"
        "int $0x80\n\t"
        "mov %%eax,%0\n\t"
        : "=m" (tt)
    );
    t = localtime(&tt);
    printf("time:%d:%d:%d:%d:%d:%d\n", t->tm_year + 1900, t->tm_mon, t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec);
    return 0;
}

/* 代码文件路径：/linux-3.18.6/init/main.c */
asmlinkage __visible void __init start_kernel(void)
{
    ......
    trap_init();
    ......
}

/* 代码文件路径：/linux-3.18.6/arch/x86/kernel/traps.c */
void __init trap_init(void)
{   
    ......
    #ifdef CONFIG_X86_32
    set_system_trap_gate(SYSCALL_VECTOR, &system_call);
    set_bit(SYSCALL_VECTOR, used_vectors);
    #endif
    ......
}

/* 代码文件路径：/linux-3.18.6/arch/x86/include/asm/irq_vectors.h */
......
#ifdef CONFIG_X86_32
# define SYSCALL_VECTOR         0x80
#endif
......

/* 代码文件路径：/linux-3.18.6/arch/x86/kernel/entry_32.S */
# system call handler stub
# 系统调用汇编代码的起点
ENTRY(system_call)
    # step 1: 
    SAVE_ALL                # 进入中断处理程序前，保存内核态下的现场
    ......
    cmpl $(NR_syscalls), %eax # 比较传入的系统调用号是否大于最大的系统调用号
    jae syscall_badsys  # 如果传入的系统调用号太大，则跳转到处理无效系统调用号的处理程序
# step 2: 
syscall_call:
    call *sys_call_table(,%eax,4) # 根据 eax 中传入的系统调用号来调用对应的系统调用服务程序，在我们的例子中就是调用 sys_time
# step 3: 
syscall_after_call:
    movl %eax,PT_EAX(%esp)      # store the return value, 保存系统调用后返回的值到 eax 中
# step 4: 
syscall_exit: # 会检测要不要执行syscall_exit_work，正常情况会有一些需要处理的工作，比如当前进程有一些信号要处理，系统需要进行调度
    ......
    movl TI_flags(%ebp), %ecx
    testl $_TIF_ALLWORK_MASK, %ecx  # current->work
    jne syscall_exit_work

# step 5: 
restore_all:
restore_nocheck:
    RESTORE_REGS 4          # skip orig_eax/error_code
irq_return:
    INTERRUPT_RETURN    # 这是一个宏定义，本质上是一条 iret 指令
# system_call 结束位置
......
ENDPROC(system_call)

/* 代码文件路径：/linux-3.18.6/arch/x86/kernel/entry_32.S */
# 在进入中断处理程序前，保存相关寄存器的值
.macro SAVE_ALL 
    cld
    PUSH_GS
    pushl_cfi %fs
    /*CFI_REL_OFFSET fs, 0;*/
    pushl_cfi %es
    /*CFI_REL_OFFSET es, 0;*/
    pushl_cfi %ds
    /*CFI_REL_OFFSET ds, 0;*/
    pushl_cfi %eax
    CFI_REL_OFFSET eax, 0
    pushl_cfi %ebp
    CFI_REL_OFFSET ebp, 0
    pushl_cfi %edi
    CFI_REL_OFFSET edi, 0
    pushl_cfi %esi
    CFI_REL_OFFSET esi, 0
    pushl_cfi %edx
    CFI_REL_OFFSET edx, 0
    pushl_cfi %ecx
    CFI_REL_OFFSET ecx, 0
    pushl_cfi %ebx
    CFI_REL_OFFSET ebx, 0
    movl $(__USER_DS), %edx
    movl %edx, %ds
    movl %edx, %es
    movl $(__KERNEL_PERCPU), %edx
    movl %edx, %fs
    SET_KERNEL_GS %edx
.endm

call *sys_call_table(,%eax,4) # 根据 eax 中传入的系统调用号来调用对应的系统调用服务程序，在我们的例子中就是调用 sys_time

/* 代码文件路径：/linux-3.18.6/arch/x86/kernel/syscall_32.c */
#define __SYSCALL_I386(nr, sym, compat) [nr] = sym,

typedef asmlinkage void (*sys_call_ptr_t)(void);

extern asmlinkage void sys_ni_syscall(void);

__visible const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = {
    /*
     * Smells like a compiler bug -- it doesn't work
     * when the & below is removed.
     */
    [0 ... __NR_syscall_max] = &sys_ni_syscall,
#include <asm/syscalls_32.h>
};

#define __SYSCALL_I386(nr, sym, compat) [nr] = sym,

[1] = sys_exit

/* 代码文件路径：/linux-3.18.6/arch/x86/kernel/entry_32.S */
syscall_exit_work:      # 退出系统调用后所做的一些工作
    testl $_TIF_WORK_SYSCALL_EXIT, %ecx
    jz work_pending # 跳转到 work_pending 去处理一些信号相关的处理程序
    TRACE_IRQS_ON
    ENABLE_INTERRUPTS(CLBR_ANY) # could let syscall_trace_leave() call
                    # schedule() instead
    movl %esp, %eax
    call syscall_trace_leave
    jmp resume_userspace
END(syscall_exit_work)

/* 代码文件路径：/linux-3.18.6/arch/x86/kernel/entry_32.S */
# 执行完中断处理程序后，返回时恢复原来保存在栈中的寄存器的值
.macro RESTORE_INT_REGS
    popl_cfi %ebx
    CFI_RESTORE ebx
    popl_cfi %ecx
    CFI_RESTORE ecx
    popl_cfi %edx
    CFI_RESTORE edx
    popl_cfi %esi
    CFI_RESTORE esi
    popl_cfi %edi
    CFI_RESTORE edi
    popl_cfi %ebp
    CFI_RESTORE ebp
    popl_cfi %eax
    CFI_RESTORE eax
.endm

.macro RESTORE_REGS pop=0
    RESTORE_INT_REGS
1:  popl_cfi %ds
    /*CFI_RESTORE ds;*/
2:  popl_cfi %es
    /*CFI_RESTORE es;*/
3:  popl_cfi %fs
    /*CFI_RESTORE fs;*/
    POP_GS \pop
.pushsection .fixup, "ax"
4:  movl $0, (%esp)
    jmp 1b
5:  movl $0, (%esp)
    jmp 2b
6:  movl $0, (%esp)
    jmp 3b
.popsection
    _ASM_EXTABLE(1b,4b)
    _ASM_EXTABLE(2b,5b)
    _ASM_EXTABLE(3b,6b)
    POP_GS_EX
.endm

/* 代码文件路径：/linux-3.18.6/arch/x86/include/asm/syscalls_32.h */
__SYSCALL_I386(13, sys_time, compat_sys_time)

/* 代码文件路径：/linux-3.18.6/kernel/time/time.c */
SYSCALL_DEFINE1(time, time_t __user *, tloc)
{
    time_t i = get_seconds();

    if (tloc) {
        if (put_user(i,tloc))
            return -EFAULT;
    }
    force_successful_syscall_return();
    return i;
}

/* 代码文件路径：/linux-3.18.6/include/linux/syscalls.h */
#define SYSCALL_METADATA(sname, nb, ...)
#endif

#define SYSCALL_DEFINE0(sname)                  \
    SYSCALL_METADATA(_##sname, 0);              \
    asmlinkage long sys_##sname(void)

#define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE4(name, ...) SYSCALL_DEFINEx(4, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)

#define SYSCALL_DEFINEx(x, sname, ...)              \
    SYSCALL_METADATA(sname, x, __VA_ARGS__)         \
    __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

#define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
#define __SYSCALL_DEFINEx(x, name, ...)                 \
    asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))   \
        __attribute__((alias(__stringify(SyS##name))));     \

asmlinkage long sys_time(time_t __user *tloc);