Polling Sequence

The 8051 automatically evaluates whether an interrupt should occur after every instruction. When checking for interrupt conditions, it checks them in the following order : -

 1.  External 0 Interrupt
2.  Timer 0 Interrupt
3.  External 1 Interrupt
4.  Timer 1 Interrupt
5.  Serial Interrupt

 This means that if a Serial Interrupt occurs at the exact same instant that an External 0 Interrupt occurs, the External 0 Interrupt will be executed first and the Serial Interrupt will be executed once the External 0 Interrupt has completed.

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