The 'Halt and Catch Fire' (HCF) phrase originated as engineering humor using the three-letter assembly mnemonic convention, but became real on the Motorola 6800 processor. Two undocumented opcodes ($9D and $DD) caused the CPU to enter a runaway state where the program counter incremented endlessly, reading all memory sequentially at high speed, with no way to recover except a reset or power cycle. Gerry Wheeler named and documented this behavior in BYTE magazine in December 1977. Motorola engineers later discovered the behavior was intentionally preserved as a fast RAM-scanning tool during hardware bring-up. The post also covers similar phenomena on other processors including the 6502, the Pentium F00F bug, and modern x86 fuzzing research that still uncovers invalid processor states.
