All I/O operations are memory-mapped. There are no port-based I/O instructions.
=== IRQ / NMI / BRK / RESET ===
On a RESET, the CPU loads the vector from $FFFC/$FFFD into the program counter and continues fetching instructions from there.
On NMOS, the simultaneous assertion of a hardware interrupt line and execution of BRK was not accounted for in the design — the BRK instruction will be ignored in such a case. On CMOS, this situation is correctly handled by executing BRK and then servicing the interrupt.
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== Half Cycles ==