An unused block of bits left in a binary so that it can later be modified by insertion of machine-language instructions there (typically, the patch space is modified to contain new code, and the superseded code is patched to contain a jump or call to the patch space). The widening use of HLLs has made this term rare; it is now primarily historical outside IBM shops. See patch, zap, hook.
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Nearby terms: PATA « patch « patch pumpkin « patch space » PATCHY » path » path coverage testing