系統調用是os操作系統提供的服務,用戶程序通過各種系統調用,來引用內核提供的各種服務,系統調用的執行讓用戶程序陷入內核,該陷入動作由swi軟中斷完成.

#if defined(__thumb__)                             //thumb模式
#define __syscall(name)                          /
    "push    {r7}/n/t"                           /
    "mov    r7, #" __sys1(__NR_##name) "/n/t"    /
    "swi    0/n/t"                               /
    "pop    {r7}"
#else                                              //arm模式
#define __syscall(name) "swi/t" __sys1(__NR_##name) "/n/t"
#endif
#define __sys2(x) #x
#define __sys1(x) __sys2(x)
#define __NR_SYSCALL_BASE    0x900000       //此為OS_NUMBER << 20運算值
            #define __NR_open            (__NR_SYSCALL_BASE+ 5) //0x900005

　　舉一個例子來說:open系統調用,庫函數最終會調用__syscall(open),宏展開之后為swi #__NR_open,即,swi #0x900005觸發中斷,中斷號0x900005存放在[lr,#-4]地址中,處理器跳轉到arch/arm/kernel/entry-common.S中vector_swi讀取[lr,#-4]地址中的中斷號,之后查詢arch/arm/kernel/entry-common.S中的sys_call_table系統調用表,該表內容在arch/arm/kernel/calls.S中定義,__NR_open在表中對應的順序號為

...
.long    SYMBOL_NAME(sys_open)                     //第5個
...
將sys_call_table[5]中內容傳給pc,系統進入sys_open函數,處理實質的open動作
注:用到的一些函數數據所在文件,如下所示
arch/arm/kernel/calls.S聲明了系統調用函數
include/asm-arm/unistd.h定義了系統調用的調用號規則
vector_swi定義在arch/arm/kernel/entry-common.S
vector_IRQ定義在arch/arm/kernel/entry-armv.S
vector_FIQ定義在arch/arm/kernel/entry-armv.S
arch/arm/kernel/entry-common.S中對sys_call_table進行了定義:
    .type    sys_call_table, #object
ENTRY(sys_call_table)
            #include "calls.S"                   //將calls.S中的內容順序鏈接到這里

　　__syscall_start:

　　源程序：

ENTRY(vector_swi)
    save_user_regs
    zero_fp
    get_scno                               //將[lr,#-4]中的中斷號轉儲到scno(r7)
    arm710_bug_check scno, ip
#ifdef CONFIG_ALIGNMENT_TRAP
    ldr    ip, __cr_alignment
    ldr    ip, [ip]
    mcr    p15, 0, ip, c1, c0                   @ update control register
#endif
    enable_irq ip
    str    r4, [sp, #-S_OFF]!                   @ push fifth arg
    get_current_task tsk
    ldr    ip, [tsk, #TSK_PTRACE]               @ check for syscall tracing
    bic    scno, scno, #0xff000000              @ mask off SWI op-code
//#define OS_NUMBER    9[entry-header.S]
//所以對于上面示例中open系統調用號scno=0x900005
//eor scno,scno,#0x900000
//之后scno=0x05
    eor    scno, scno, #OS_NUMBER << 20         @ check OS number
//sys_call_table項為calls.S的內容
    adr    tbl, sys_call_table                  @ load syscall table pointer
    tst    ip, #PT_TRACESYS                     @ are we tracing syscalls?
    bne    __sys_trace
    adrsvc    al, lr, ret_fast_syscall          @ return address
    cmp    scno, #NR_syscalls                   @ check upper syscall limit
//執行sys_open函數
    ldrcc    pc, [tbl, scno, lsl #2]             @ call sys_* routine
    add    r1, sp, #S_OFF
2:  mov    why, #0                              @ no longer a real syscall
    cmp    scno, #ARMSWI_OFFSET
    eor    r0, scno, #OS_NUMBER << 20           @ put OS number back
    bcs    SYMBOL_NAME(arm_syscall)    
    b    SYMBOL_NAME(sys_ni_syscall)             @ not private func
    /*
     * This is the really slow path. We're going to be doing
     * context switches, and waiting for our parent to respond.
     */
__sys_trace:
    add    r1, sp, #S_OFF
    mov    r0, #0                                @ trace entry [IP = 0]
    bl    SYMBOL_NAME(syscall_trace)
/*
//2007-07-01 gliethttp [entry-header.S]
//Like adr, but force SVC mode (if required)
  .macro adrsvc, cond, reg, label
     adr/cond /reg, /label
  .endm
//對應反匯編:
//add lr, pc, #16 ; lr = __sys_trace_return
*/
    adrsvc    al, lr, __sys_trace_return        @ return address
    add    r1, sp, #S_R0 + S_OFF                @ pointer to regs
    cmp    scno, #NR_syscalls                   @ check upper syscall limit
    ldmccia    r1, {r0 - r3}                    @ have to reload r0 - r3
    ldrcc    pc, [tbl, scno, lsl #2]            @ call sys_* routine
    b    2b
__sys_trace_return:
    str    r0, [sp, #S_R0 + S_OFF]!             @ save returned r0
    mov    r1, sp
    mov    r0, #1                               @ trace exit [IP = 1]
    bl    SYMBOL_NAME(syscall_trace)
    b    ret_disable_irq
    .align    5
#ifdef CONFIG_ALIGNMENT_TRAP
    .type    __cr_alignment, #object
__cr_alignment:
    .word    SYMBOL_NAME(cr_alignment)
#endif
    .type    sys_call_table, #object
ENTRY(sys_call_table)
            #include "calls.S"