A deep dive on 6502's jump indirect bug and how you can handle it when emulating the 6502.

Two Stop Bits

The MOS 6502 processor has a well-known hardware bug in its JMP indirect instruction: when the pointer address falls on a page boundary (e.g., $70FF), the CPU fails to carry the low-order byte when fetching the high byte of the program counter, reading from $XX00 instead of the next page. This causes incorrect jumps. The fix in emulation is to detect when the pointer's low byte equals $FF and explicitly read PCH from $XX00 rather than pointer+1. Historical compilers like fig-Forth worked around this by padding code to avoid addresses ending in $XXFF.

6502 Emulation - The Jump Indirect Bug