(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 Motorola6800microprocessor was the first for which an HCF opcode became widely known.This instruction caused the processor to toggle a subset of the bus lines as rapidly as it couldin some configurations this could actually cause lines to burn up.[Confirm?](1995-12-14)