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HandWiki. Halt and Catch Fire. Encyclopedia. Available online: https://encyclopedia.pub/entry/33038 (accessed on 28 September 2026).
HandWiki. Halt and Catch Fire. Encyclopedia. Available at: https://encyclopedia.pub/entry/33038. Accessed September 28, 2026.
HandWiki. "Halt and Catch Fire" Encyclopedia, https://encyclopedia.pub/entry/33038 (accessed September 28, 2026).
HandWiki. (2022, November 04). Halt and Catch Fire. In Encyclopedia. https://encyclopedia.pub/entry/33038
HandWiki. "Halt and Catch Fire." Encyclopedia. Web. 04 November, 2022.
Halt and Catch Fire
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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, making a joke about its numerous non-obvious instruction mnemonics. With the advent of the MC6800, a design flaw was discovered by the programmers. Due to incomplete opcode decoding, two illegal opcodes, 0x9D and 0xDD, will cause the program counter on the processor to increment endlessly, which renders the processor useless. Those codes have been unofficially named HCF. During the design process of MC6802, engineers originally planned to remove this instruction, but kept it as-is for testing purposes. As a result, HCF was officially recognized as a real instruction. Later, HCF became a humorous catch-all term for instructions that may freeze a processor, including intentional instructions for testing purposes, and unintentional illegal instructions. Some are considered hardware defects, and if the system is shared, a malicious user can execute it to launch a denial-of-service attack. 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, 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" is a facetious exaggeration of the speed with which the CPU chip would be switching some bus circuits, causing them to overheat and burn.

denial-of-service malicious user central processing unit

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