In computer engineering, Halt and Catch Fire, known by the assembly language 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 (introduced in 1964), making a joke about its numerous non-obvious instruction mnemonics.
After the introduction of the MC6800 processor in 1974, a design flaw was discovered by programmers. Due to incomplete opcode decoding, two illegal opcodes, 0x9D and 0xDD, will cause the program counter on the processor to increment endlessly, which locks the processor until reset. Those codes have been unofficially named HCF. During the design process of the 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. Hence, the address bus effectively becomes a counter, allowing the operation of all address lines to be quickly verified. Once the processor entered this mode, it is not responsive to interrupts, so normal operation can only be restored by a reset (hence the "Drop Dead" and "Halt and Catch Fire" monikers). These references are thus to the unresponsive behavior of the CPU in this state, and not to any form of erratic behavior. Motorola kept the HCF behavior in the 6802 variant of the processor (which released in 1977) as an intentional self-test for the 6802's 128 bytes of onboard RAM.
Other HCF-like instructions were found later on the Motorola 6800 when executing undocumented opcodes FD (cycling twice slower than 9D/DD) or CD/ED (cycling at a human-readable very low frequency on a limited number of high-address lines).
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