Editing Machine code (section)

==Overlapping instructions==
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On processor architectures with [[variable-length instruction set]]s<ref name="Jacob-Jakubowski-Venkatesan_2007"/> (such as [[Intel]]'s [[x86]] processor family) it is, within the limits of the control-flow [[self-synchronizing code|resynchronizing]] phenomenon known as the [[Kruskal count]],<ref name="Lagarias-Rains-Vanderbei_2001"/><ref name="Jacob-Jakubowski-Venkatesan_2007"/><ref name="Andriesse-Bos_2014"/><ref name="Jakubowski_2016"/><ref name="Jämthagen_2016"/> sometimes possible through opcode-level programming to deliberately arrange the resulting code so that two code paths share a common fragment of opcode sequences.<ref group="nb" name="NB_Merging_or_branching"/> These are called ''overlapping instructions'', ''overlapping opcodes'', ''overlapping code'', ''overlapped code'', ''instruction scission'', or ''jump into the middle of an instruction''.<ref name="HN_2021"/><ref name="Kinder_2010"/><ref name="RE_2013"/>

In the 1970s and 1980s, overlapping instructions were sometimes used to preserve memory space. One example were in the implementation of error tables in [[Microsoft]]'s [[Altair BASIC]], where ''interleaved instructions'' mutually shared their instruction bytes.<ref name="Gates"/><ref name="Jacob-Jakubowski-Venkatesan_2007"/><ref name="HN_2021"/> The technique is rarely used today, but might still be necessary to resort to in areas where extreme optimization for size is necessary on byte-level such as in the implementation of [[boot loader]]s which have to fit into [[boot sector]]s.<ref group="nb" name="NB_DR-DOS_707"/>

It is also sometimes used as a [[code obfuscation]] technique as a measure against [[disassembly]] and tampering.<ref name="Jacob-Jakubowski-Venkatesan_2007"/><ref name="Jakubowski_2016"/>

The principle is also used in shared code sequences of [[fat binaries]] which must run on multiple instruction-set-incompatible processor platforms.<ref group="nb" name="NB_Merging_or_branching"/>

This property is also used to find [[unintended instruction]]s called [[gadget (machine instruction sequence)|gadget]]s in existing code repositories and is used in [[return-oriented programming]] as alternative to [[code injection]] for exploits such as [[return-to-libc attack]]s.<ref name="Shacham_2007"/><ref name="Jacob-Jakubowski-Venkatesan_2007"/>