Halt and Catch Fire
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Halt and Catch Fire, known by its mnemonic HCF, denotes any of several undocumented and semi-mythical computer machine code instructions with destructive side-effects, supposedly included for test purposes on several well-known architectures going as far back as IBM System/360.
The Motorola 6800 microprocessor was the first for which an HCF opcode became widely known. The instruction makes the processor enter a mode intended for manufacturing testing, in which it continuously performs memory read cycles from successive addresses, with no intervening instruction fetches. Effectively the address bus becomes a counter, allowing the operation of all address lines to be quickly verified. It has been reported that in some system configurations this could damage the bus drivers due to contention, though there is no documentation of any such occurrence.