WHAT HAPPENS WHEN AN INTERRUPT ENDS?

An interrupt ends when the program executes the RETI (Return from Interrupt) instruction. 

When the RETI instruction is executed the following actions are taken by the microcontroller:

•    Two bytes are popped off the stack into the Program Counter to restore normal program execution.

•    Interrupt status is restored to its pre-interrupt status.

WHAT HAPPENS WHEN AN INTERRUPT OCCURS?

When an interrupt is triggered, the following actions are taken automatically by the

•    The current Program Counter is saved on the stack, low-byte first.

•    Interrupts of the same and lower priority are blocked.

•    In the case of Timer and External interrupts, the corresponding interrupt flag is set.

•    Program execution transfers to the corresponding interrupt handler vector address.

•    The Interrupt Handler Routine executes.