Hi Martin.

If I understand correctly you are asking to which address is vector the program counter when an interrupt occurs.

If the interrupt is enable ( see IE register) when it occur the program is vector to the corresponding address ( that is firmware establish and cannot be change). For ex. the vector address for external interrupt 0 is 03H . Then when you finish serving the interrupt (ISR)the last instruction must be RETI that restore the program counter with the correct address so the program continuous the execution one instruction ahead from where it was before the interruption occurs.
Some ISR must clear the flag EX. serial port interrupt. Because if not you will execute only one instruction of the main program and jump again in the interrupt.

This is only a description without details. If you need more details see datasheets or let us know your problem that sure someone will answer you.

Good Luck
Daniel
This description is without details
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