Editing Self-modifying code (section)

{{Short description|Source code that alters its instructions to the hardware while executing}}
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{{Use dmy dates|date=January 2022|cs1-dates=y}}
{{Use list-defined references|date=December 2021}}
{{Use American English|date=January 2019}}

In [[computer science]], '''self-modifying code''' ('''SMC''' or '''SMoC''') is [[source code|code]] that alters its own [[instruction (computer science)|instruction]]s while it is [[execution (computing)|executing]] – usually to reduce the [[instruction path length]] and improve [[computer performance|performance]] or simply to reduce otherwise [[duplicate code|repetitively similar code]], thus simplifying [[software maintenance|maintenance]]. 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]].

Self-modifying code can involve overwriting existing instructions or generating new code at run time and transferring control to that code.

Self-modification can be used as 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 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:
* '''only during initialization''' – based on input [[Parameter#Computing|parameter]]s (when the process is more commonly described as software '[[computer configuration|configuration]]' and is somewhat analogous, in hardware terms, to setting [[jumper (computing)|jumper]]s for [[printed circuit board]]s). Alteration of program entry [[pointer (computer programming)|pointer]]s is an equivalent indirect method of self-modification, but requiring the co-existence of one or more alternative instruction paths, increasing the [[binary file|program size]].
* '''throughout execution''' ("on the fly") – based on particular program states that have been reached during the execution

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

In the [[IBM System/360 architecture]], and its successors up to [[z/Architecture]], an EXECUTE (EX) instruction ''logically'' overlays the second byte of its target instruction with the low-order 8 bits of [[general-purpose register|register]] 1. This provides the effect of self-modification although the actual instruction in storage is not altered.