进出异常

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进入异常

当异常出现时，异常模式分组的R14、SPSR用于保存状态。

ARM7TDMI（自动）处理异常的步骤：

R14_<exception_mode > = return link              ；保护返回地址

SPSR_<exception_mode > = CPSR                ；保护状态标志

CPSR[4:0]= exception_mode number                ；模式切换

CPSR[5] =0                                     ；ARM状态

If <exception_mode > == Reset or FIQ then        

CPSR[6] =1                                     ；禁止快中断

/* 否则CPSR[6] 不变*/

CPSR[7] =1                                     ；禁止正常中断

PC =exception vector address                      ；异常向量地址到PC

 

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退出异常

R14_<exception_mode >→PC

SPSR_<exception_mode >→CPSR

在恢复CPSR时，T、F、I自动恢复为异常发生前的值。

例：各种异常返回指令操作：

①、复位：在禁止中断的模式下，从地址0x00000000/0xffff0000开始执行程序。

②、数据中止：SUBS PC，R14，#4

③、FIQ：SUBS PC，R14，#4

④、IRQ：SUBS PC，R14，#4

⑤、预取中止：SUBS PC，R14，#4

⑥、未定义指令：MOVS PC，R14

⑦、SWI：MOVS PC，R14