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patch space: n.

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insertion of machine-language instructions there (typically, the patch space is modified to contain new code, and the superseded code is patched to contain a jump or call to the patch space).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 near-universal use of compilers and interpreters has made this term rare; it is now primarily historical outside IBM shops. See patch (sense 4), zap (sense 4), hook.