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Comparison of instruction set architectures
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<p> An <a href="/facts/Instruction_set_architecture/8p8vwUeE">instruction set architecture</a> (ISA) is an abstract model of a <a href="/facts/Computer/SQe1aK05">computer</a>, also referred to as computer architecture. A realization of an ISA is called an <i>implementation</i>. An ISA permits multiple implementations that may vary in <a href="/facts/Computer_performance/td1qKox4">performance</a>, physical size, and monetary cost (among other things); because the ISA serves as the <a href="/facts/Interface_(computing)/zdfp8unT">interface</a> between <a href="/facts/Software/RP79lVSW">software</a> and <a href="/facts/Computer_hardware/8jSjsCNv">hardware</a>, software that has been written for an ISA can run on different implementations of the same ISA. This has enabled <a href="/facts/Binary_compatibility/7lkFAUfv">binary compatibility</a> between different generations of computers to be easily achieved, and the development of computer families. Both of these developments have helped to lower the cost of computers and to increase their applicability. For these reasons, the ISA is one of the most important abstractions in <a href="/facts/Computing/Kghtm9fX">computing</a> today. </p><p>An ISA defines everything a <a href="/facts/Machine_language/bgdQ1Fxf">machine language</a> <a href="/facts/Programmer/WGTS1Jv9">programmer</a> needs to know in order to program a computer. What an ISA defines differs between ISAs; in general, ISAs define the supported <a href="/facts/Data_type/fcx5hgVL">data types</a>, what state there is (such as the <a href="/facts/Main_memory/wPHKFpdx">main memory</a> and <a href="/facts/Processor_register/32RvbUdz">registers</a>) and their semantics (such as the <a href="/facts/Memory_consistency/a8Xphrs7">memory consistency</a> and <a href="/facts/Addressing_mode/g78wINgK">addressing modes</a>), the <i>instruction set</i> (the set of <a href="/facts/Machine_instruction/bgdQ1Fxf">machine instructions</a> that comprises a computer's machine language), and the <a href="/facts/Input%2foutput/WQTE8Jas">input/output</a> model. </p>