Self-modifying code

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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 for.

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 parameters (when the process is more commonly described as software 'configuration' and is somewhat analogous, in hardware terms, to setting jumpers for printed circuit boards). Alteration of program entry pointers is an equivalent indirect method of self-modification, but requiring the co-existence of one or more alternative instruction paths, increasing the 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 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 overlays its target instruction (in its 2nd byte) with the lowest order 8 bits of register 1, as a standard, legitimate method of (temporary) instruction modification.


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