On NMOS, NMI or IRQ that happens during specific moments of BRK execution can cause BRK to be effectively skipped. On CMOS, this situation is correctly handled by executing BRK and then servicing the interrupt.
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== Branch instructions and Interrupts ==
The branch instructions have subtle interrupt polling behavior. Interrupts are always polled before the second CPU cycle (the operand fetch), but not before the third CPU cycle on a taken branch.
Additionally, for taken branches that cross a page boundary, interrupts are polled before the PCH fixup cycle.
An interrupt being detected at either of these polling points (including only being detected at the first one) will trigger a CPU interrupt.
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