系统调用是os操作系统提供的服务,用户程序通过各种系统调用,来引用内核提供的各种服务,系统调用的执行让用户程序陷入内核,该陷入动作由swi软中断完成. At91rm9200处理器对应的linux2.4.19内核系统调用对应的软中断定义如下: #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 //此为OS_NUMBER << 20运算值 #define __NR_open (__NR_SYSCALL_BASE+ 5) //0x900005 举一个例子来说:open系统调用,库函数最终会调用__syscall (open),宏展开之后为swi #__NR_open,即,swi #0x900005触发中断,中断号0x900005存放在[lr,#-4]地址中,处理器跳转到arch/arm/kernel/entry- common.S中vector_swi读取[lr,#-4]地址中的中断号,之后查询arch/arm/kernel/entry-common.S中的sys_call_table系统调用表,该表内容在arch/arm/kernel/calls.S中定义,__NR_open在表中对应的顺序号为 __syscall_start: ... .long SYMBOL_NAME(sys_open) //第5个 ... 将sys_call_table[5]中内容传给pc,系统进入sys_open函数,处理实质的open动作 注:用到的一些函数数据所在文件,如下所示 arch/arm/kernel/calls.S声明了系统调用函数 include/asm-arm/unistd.h定义了系统调用的调用号规则 vector_swi定义在arch/arm/kernel/entry-common.S vector_IRQ定义在arch/arm/kernel/entry-armv.S vector_FIQ定义在arch/arm/kernel/entry-armv.S arch/arm/kernel/entry-common.S中对sys_call_table进行了定义: .type sys_call_table, #object ENTRY(sys_call_table) #include "calls.S" //将calls.S中的内容顺序链接到这里 源程序： ENTRY(vector_swi) save_user_regs zero_fp get_scno //将[lr,#-4]中的中断号转储到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] //所以对于上面示例中open系统调用号scno=0x900005 //eor scno,scno,#0x900000 //之后scno=0x05 eor scno, scno, #OS_NUMBER << 20 @ check OS number //sys_call_table项为calls.S的内容 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 //执行sys_open函数 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 //对应反汇编: //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"