Self-modifying code: Difference between revisions

Revision as of 02:00, 11 November 2003

Revision as of 12:55, 14 November 2003

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==Assembly style self-modifying code:==

The kinds of self-modifying code that are used in assembly, can be for various purpuses:

# Optimisation of a state dependant loop.

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}

Self -modifying code in this case would simply be a matter of rewriting the loop like this:

repeat N times {

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}

Note that 2-state replacement of the opcode, can be easly written as

'xor var at address with the value "opcodeOf(Inc) xor opcodeOf(dec)"'

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In some cases self-modifying code executes slower on modern processors. This is because a modern processor will usually try to keep blocks of code in its [[cache]] memory. Each time the program rewrites a part of itself, the rewritten part must be loaded into the cache again, which results in a slight delay.

The cache invalidation issue on modern processors, usualyusually means, that self modifing-modifying code would still be faster, only when the modification will occureoccur seldomlyrarely. Such as in the case of a state switching in an inner loop.

This concideration is not unique to processors with code cache, since on any processor, rewriting the code never does come for free.

Self-modifying code was used in the early days of computers in order to save memory space, which was limited. It was also used to implement [[subroutine]] calls and returns when the instruction set only provided simple branching or skipping instructions to vary the flow of control (this is still relevant in certain ultra-[[RISC]] architectures, at least theoretically, e.g. one such system has a sole branching instruction with three operands: subtract-and-branch-if-negative).

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==TODO==

#an example and discussion of 'high-level' self-modifying code such as in LISP.

# examples and discussion of traditional uses of self-modifying code, such as in graphic Blitting units, specialisation of algorithms (like sort with embed'ingembedding a cmp), and in interpeterinterpreter kernalskernels.