Self-modifying code

In computer science, self-modifying code is code that alters its own instructions while it is executing - usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, thus simplifying maintenance. Self-modification is an alternative to the method of "flag setting" and conditional program branching, used primarily to reduce the number of times a condition needs to be tested. The term is usually only applied to code where the self-modification is intentional, not in situations where code accidentally modifies itself due to an error such as a buffer overflow.

The method is frequently used for conditionally invoking test/debugging code without requiring additional computational overhead for every input/output cycle.

The modifications may be performed:

In either case, the modifications may be performed directly to the machine code instructions themselves, by overlaying new instructions over the existing ones (for example: altering a compare and branch to an unconditional branch or alternatively a 'NOP').

In the IBM/360 and Z/Architecture instruction set, an EXECUTE (EX) instruction logically overlays the second byte of its target instruction with the low-order 8 bits of register 1. This provides the effect of self-modification although the actual instruction in storage is not altered.

Self-modification can be accomplished in a variety of ways depending upon the programming language and its support for pointers and/or access to dynamic compiler or interpreter 'engines':

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