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===Assembly language===
Self-modifying code is quite straightforward to implement when using [[assembly language]]. Instructions can be dynamically created in [[computer memory|memory]] (or else overlaid over existing code in non-protected program storage), in a sequence equivalent to the ones that a standard compiler may generate as the [[object code]]. With modern processors, there can be unintended [[side effect (computer science)|side effect]]s on the [[CPU cache]] that must be considered. The method was frequently used for testing 'first time' conditions, as in this suitably commented [[IBM/360]] [[assembler (computer programming)|assembler]] example. It uses instruction overlay to reduce the [[instruction path length]] by (N×1)−1 where N is the number of records on the file (−1 being the [[computational overhead|overhead]] to perform the overlay).
SUBRTN NOP OPENED FIRST TIME HERE?
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