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