Halt and catch fire

In computer engineering, Halt and Catch Fire, known by the assembly mnemonic HCF, is an idiom referring to a computer machine code instruction that causes the computer's central processing unit (CPU) to cease meaningful operation, typically requiring a restart of the computer. It originally referred to a fictitious instruction in IBM System/360 computers, but later computer developers who saw the joke created real versions of this instruction for some machines. In the case of real instructions the implication of this expression is that, whereas in most cases in which a CPU executes an unintended instruction (a bug in the code) the computer may still be able to recover, but in the case of an HCF instruction there is, by definition, no way for the system to recover without a restart.

The expression "catch fire" in this context is normally facetious, rather than literal, referring to a total loss of CPU functionality during the current session. Mythically, the CPU chip would be switching some circuits so fast that it would cause them to overheat and burn.

In a computer's assembly language, mnemonics are used that are directly equivalent to machine code instructions. The mnemonics are frequently three letters long, such as ADD, CMP (to compare two numbers), and JMP (jump to a different location in the program). The HCF instruction was originally a fictitious assembly language instruction, said to be under development at IBM for use in their System/360 computers, along with many other amusing three-letter acronyms like XPR (Execute Programmer) and CAI (Corrupt Accounting Information), and similar to other joke mnemonics such as "SDI" for "Self Destruct Immediately" and "CRN" for Convert to Roman Numerals. A list of such mnemonics, including HCF, shows up as "Overextended Mnemonics" in the April 1980 Creative Computing flip-side parody issue.

The S/360 instruction set already included numerous non-obvious mnemonics like ZAP (Zero and Add Packed), EDMK (EDit and MarK), TRT (TRanslate and Test), and Read Backward (an I/O channel command), and programmers began creating similarly cryptic, but fictitious, instructions in a humorous vein.

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