Corrected Spectrum ROM

This ROM corrects a well-known bug in the Spectrum’s NMI handler — one so notorious that the ROM disassembly flags it with an inline comment: “Note: This should have been ‘JR Z’!”

The NMI routine (at 0066h) is designed to allow a controlled system reset: if the system variable NMIADD holds a non-zero value when the NMI line fires, the CPU is supposed to jump to that address and reset. But in the original Spectrum ROM, a single wrong opcode — JR NZ (20h) instead of JR Z (28h) at 006Dh — inverts the logic entirely, making the reset trigger when it shouldn’t and suppressing it when it should fire. The mechanism is effectively dead in every unmodified Spectrum.

This ROM replaces 20h with 28h, applying the canonical one-byte correction. With JR Z in place, the routine behaves as intended: NMIADD = 0000h safely bypasses the reset, while any non-zero value causes the jump — giving NMI-capable hardware a working reset hook for the first time.