中断问题

在中断处理函数结束后，清楚中断标志位为什么是将SRCPND该位置1而不是置零？

芯片手册有写的：芯片手册379页：
The SRCPND and INTPND registers can be read and written, so the service routine must clear the
pending condition by writing a 1 to the corresponding bit in the SRCPND register first and then clear the pending
condition in the INTPND registers by using the same method.