Halt and Catch Fire

<humour, processor>

(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 Motorola 6800 microprocessor was the first for which an HCF opcode became widely known. This instruction caused the processor to read every memory location sequentially until reset.

[Gerry Wheeler, Byte, December 1977, p46, "Undocumented M6800 Instructions"].

Last updated: 2014-09-20

halting problem

The problem of determining in advance whether a particular program or algorithm will terminate or run forever. The halting problem is the canonical example of a provably unsolvable problem. Obviously any attempt to answer the question by actually executing the algorithm or simulating each step of its execution will only give an answer if the algorithm under consideration does terminate, otherwise the algorithm attempting to answer the question will itself run forever.

Some special cases of the halting problem are partially solvable given sufficient resources. For example, if it is possible to record the complete state of the execution of the algorithm at each step and the current state is ever identical to some previous state then the algorithm is in a loop. This might require an arbitrary amount of storage however. Alternatively, if there are at most N possible different states then the algorithm can run for at most N steps without looping.

A program analysis called termination analysis attempts to answer this question for limited kinds of input algorithm.

Last updated: 1994-10-20

Nearby terms:

HALMATHal/SHalt and Catch Firehalting problemHamiltonHamiltonian cycle

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