n. Mnemonic for 'Halt and Catch Fire', 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 MC6800 microprocessor was the first for which the HCF opcode
became widely known. This instruction caused the processor to
a subset of the bus lines as rapidly as it could; in
some configurations this can actually cause lines to burn