The phrase HCF originated as engineering humor, became real on the Motorola 6800 via opcodes $9D/$DD, and traces back to a 1977 BYTE article by Gerry Wheeler.
Key Takeaways
Wheeler coined the HCF mnemonic himself in BYTE Dec 1977; the 6800 had 59 undocumented opcodes, two of which turned the address bus into a free-running 16-bit counter.
The only recovery from HCF on the 6800 is reset or power cycle; interrupts cannot break the loop.
Motorola engineers internally called it HACOF, and product engineering kept the behavior because it was useful for fast RAM scanning during bring-up.
The IBM System/360 actually could catch fire from sustained core memory access on an invalid opcode, making the “catch fire” joke partially literal.
The pattern extends beyond Motorola: illegal 6502 opcodes, the Pentium F00F bug, and modern x86 fuzzing still surface invalid processor states.
Hacker News Comment Review
Minimal technical discussion; comments are brief and do not add hardware detail or correction beyond the article itself.
No substantive debate on the 6800 silicon behavior, F00F bug, or fuzzing angle appeared in the thread.
Notable Comments
@thisisauserid: “This article is deadbeef on arrival” – sharp hex-culture wordplay, no technical content.