手册中
When a pending bit is set, the interrupt service routine starts
whenever the I-flag or F-flag is cleared to 0.
If the corresponding mask bit is 1 and the interrupt is generated, the
pending bit will be set. If the global mask bit is set to 1, the interrupt pending bit will be set but all interrupts will not
be serviced.
既然非全局mask屏蔽不了pending位
而pending位一旦置1就允许ISR(假设FI置0)
那mask还有什么作用呀