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Interrupt Handler
From SnesLab
An Interrupt Handler is a subroutine which the CPU automatically runs when an interrupt occurs.
ISR stands for "interrupt service routine." Interrupts are never serviced when an instruction is only partially finished running (except if you count MVP/MVN which can be interrupted once every 7 cycles).
Use BRK to force a software interrupt.
Return control back with RTI or RETI.