Halt and Catch Fire
(HCF) Any of several undocumented and semi-mythical machine instructions with destructive side-effects, supposedly included for test purposes on several well-known architectures going as far back as the IBM 360.
The Motorola 6800 microprocessor was the first for which an HCF opcode became widely known. This instruction caused the processor to read every memory location sequentially until reset.
[Gerry Wheeler, Byte, December 1977, p46, "Undocumented M6800 Instructions"].