Usage
Self-modifying code can be used for various purposes:
- Semi-automatic optimizing of a state dependent loop.
- Run-time code generation, or specialization of an algorithm in runtime or loadtime (which is popular, for example, in the domain of real-time graphics) such as a general sort utility – preparing code to perform the key comparison described in a specific invocation.
- Altering of inlined state of an object, or simulating the high-level construction of closures.
- Patching of subroutine (pointer) address calling, usually as performed at load/initialization time of dynamic libraries, or else on each invocation, patching the subroutine's internal references to its parameters so as to use actual addresses of specific routines. (i.e. Indirect 'self-modification').
- Evolutionary computing systems such as genetic programming.
- Hiding of code to prevent reverse engineering (by use of a disassembler or debugger) or to evade detection by virus/spyware scanning software and the like.
- Filling 100% of memory (in some architectures) with a rolling pattern of repeating opcodes, to erase all programs and data, or to burn-in hardware.
- Compressing code to be decompressed and executed at runtime, e.g., when memory or disk space is limited.
- Some very limited instruction sets leave no option but to use self-modifying code to perform certain functions. For example, a one instruction set computer (OISC) machine that uses only the subtract-and-branch-if-negative "instruction" cannot do an indirect copy (something like the equivalent of "*a = **b" in the C language) without using self-modifying code.
- Altering instructions for fault-tolerance.
Read more about this topic: Self-modifying Code
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