swi (#num | 0x900000) (0x900000是个magic值)

也就是说原来的调用方式(Old ABI)是通过跟随在swi指令中的调用号来进行的，现在的是根据r7中的值。

看两个宏，一个是

CONFIG_OABI_COMPAT 意思是说和old ABI兼容

另一个是

CONFIG_AEABI 意思是说指定方式为EABI

这两个宏可以同时配置，也可以都不配，也可以配置任何一种。

我说一下内核是怎么处理这一问题的。

我们知道，sys_call_table 在内核中是个跳转表，这个表中存储的是一系列的函数指针，这些指针就是系统调用函数的指针，如(sys_open).系统调用是根据一个调用号（通常就是表的索引）找到实际该调用内核哪个函数，然后运行该函数完成的。

首先，对于old ABI,内核给出的处理是给它建立一个单独的system calltable,叫sys_oabi_call_table，这样，兼容方式下就会有两个system call table, 以oldABI方式的系统调用会执行old_syscall_table表中的系统调用函数，EABI方式的系统调用会用sys_call_table中的函数指针。

配置无外乎以下4中

第一 两个宏都配置 行为就是上面说的那样

第二 只配置CONFIG_OABI_COMPAT ， 那么以old ABI方式调用的会用sys_oabi_call_table，以EABI方式调用的 用sys_call_table，和1实质相同，只是情况1更加明确。

第三 只配置CONFIG_AEABI 系统中不存在 sys_oabi_call_table， 对old ABI方式调用不兼容。只能 以EABI方式调用，用sys_call_table

第四 两个都没有配置 系统默认会只允许old ABI方式，但是不存在old_syscall_table，最终会通过sys_call_table 完成函数调用

可以参考下面的代码

对我们的项目比较有用。

.align 5

ENTRY(vector_swi)

sub sp, sp, #S_FRAME_SIZE

stmia sp, {r0 - r12} @ Calling r0 - r12

add r8, sp, #S_PC

stmdb r8, {sp, lr}^ @ Calling sp, lr

mrs r8, spsr @ called from non-FIQ mode, so ok.

str lr, [sp, #S_PC] @ Save calling PC

str r8, [sp, #S_PSR] @ Save CPSR

str r0, [sp, #S_OLD_R0] @ Save OLD_R0

zero_fp

/*

* Get the system call number.

*/

#if defined(CONFIG_OABI_COMPAT)

/*

* If we have CONFIG_OABI_COMPAT then we need to look at the swi

* value to determine if it is an EABI or an old ABI call.

*/

#ifdef CONFIG_ARM_THUMB

tst r8, #PSR_T_BIT

movne r10, #0 @ no thumb OABI emulation

ldreq r10, [lr, #-4] @ get SWI instruction

#else

ldr r10, [lr, #-4] @ get SWI instruction

A710( and ip, r10, #0x0f000000 @ check for SWI )

A710( teq ip, #0x0f000000 )

A710( bne .Larm710bug )

#endif

#elif defined(CONFIG_AEABI)

/*

* Pure EABI user space always put syscall number into scno (r7).

*/

A710( ldr ip, [lr, #-4] @ get SWI instruction )

A710( and ip, ip, #0x0f000000 @ check for SWI )

A710( teq ip, #0x0f000000 )

A710( bne .Larm710bug )

#elif defined(CONFIG_ARM_THUMB)

/* Legacy ABI only, possibly thumb mode. */

tst r8, #PSR_T_BIT @ this is SPSR from save_user_regs

addne scno, r7, #__NR_SYSCALL_BASE @ put OS number in

ldreq scno, [lr, #-4]

#else

/* Legacy ABI only. */

ldr scno, [lr, #-4] @ get SWI instruction

A710( and ip, scno, #0x0f000000 @ check for SWI )

A710( teq ip, #0x0f000000 )

A710( bne .Larm710bug )

#endif

#ifdef CONFIG_ALIGNMENT_TRAP

ldr ip, __cr_alignment

ldr ip, [ip]

mcr p15, 0, ip, c1, c0 @ update control register

#endif

enable_irq

get_thread_info tsk

adr tbl, sys_call_table @ load syscall table pointer

ldr ip, [tsk, #TI_FLAGS] @ check for syscall tracing

#if defined(CONFIG_OABI_COMPAT)

/*

* If the swi argument is zero, this is an EABI call and we do nothing.

*

* If this is an old ABI call, get the syscall number into scno and

* get the old ABI syscall table address.

*/

bics r10, r10, #0xff000000

eorne scno, r10, #__NR_OABI_SYSCALL_BASE

ldrne tbl, =sys_oabi_call_table

#elif !defined(CONFIG_AEABI)

bic scno, scno, #0xff000000 @ mask off SWI op-code

eor scno, scno, #__NR_SYSCALL_BASE @ check OS number

#endif

stmdb sp!, {r4, r5} @ push fifth and sixth args

tst ip, #_TIF_SYSCALL_TRACE @ are we tracing syscalls?

bne __sys_trace

cmp scno, #NR_syscalls @ check upper syscall limit

adr lr, ret_fast_syscall @ return address

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, #(__ARM_NR_BASE - __NR_SYSCALL_BASE)

eor r0, scno, #__NR_SYSCALL_BASE @ put OS number back

bcs arm_syscall

b 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:

mov r2, scno

add r1, sp, #S_OFF

mov r0, #0 @ trace entry [IP = 0]

bl syscall_trace

adr lr, __sys_trace_return @ return address

mov scno, r0 @ syscall number (possibly new)

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