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{{Short description|Set of abstract symbols that describe a computer program's operations to a processor}}
{{Short description|Set of abstract symbols that describe a computer program's operations to a processor}}In computer science, an '''instruction set architecture''' ('''ISA''') is an abstract model that generally defines how software controls the CPU in a computer or a family of computers.<ref>{{Cite web |title=GLOSSARY: Instruction Set Architecture (ISA) |url=https://www.arm.com/glossary/isa |archive-url=https://web.archive.org/web/20231111175250/https://www.arm.com/glossary/isa |archive-date=2023-11-11 |access-date=2024-02-03 |website=arm.com}}</ref> A device or program that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ''implementation'' of that ISA.▼
{{broader|Computer architecture}}
{{Machine code}}
▲
In general, an ISA defines the supported instructions, data types, registers, the hardware support for managing main memory, fundamental features (such as the memory consistency, addressing modes, virtual memory), and the input/output model of implementations of the ISA.▼
▲In general, an ISA defines the supported [[Machine code|instructions]], [[data
An ISA specifies the behavior of machine code running on implementations of that ISA in a fashion that does not depend on the characteristics of that implementation, providing binary compatibility between implementations. This enables multiple implementations of an ISA that differ in characteristics such as performance, physical size, and monetary cost (among other things), but that are capable of running the same machine code, so that a lower-performance, lower-cost machine can be replaced with a higher-cost, higher-performance machine without having to replace software. It also enables the evolution of the microarchitectures of the implementations of that ISA, so that a newer, higher-performance implementation of an ISA can run software that runs on previous generations of implementations.▼
▲An ISA specifies the behavior of [[machine code]] running on implementations of that ISA in a fashion that does not depend on the characteristics of that implementation, providing [[binary compatibility]] between implementations. This enables multiple implementations of an ISA that differ in characteristics such as [[Computer performance|performance]], physical size, and monetary cost (among other things), but that are capable of running the same machine code, so that a lower-performance, lower-cost machine can be replaced with a higher-cost, higher-performance machine without having to replace software. It also enables the evolution of the
If an operating system maintains a standard and compatible application binary interface (ABI) for a particular ISA, machine code will run on future implementations of that ISA and operating system. However, if an ISA supports running multiple operating systems, it does not guarantee that machine code for one operating system will run on another operating system, unless the first operating system supports running machine code built for the other operating system.▼
▲If an [[operating system]] maintains a standard and compatible [[application binary interface]] (ABI) for a particular ISA, machine code will run on future implementations of that ISA and operating system. However, if an ISA supports running multiple operating systems, it does not guarantee that machine code for one operating system will run on another operating system, unless the first operating system supports running machine code built for the other operating system.
An ISA can be extended by adding instructions or other capabilities, or adding support for larger addresses and data values; an implementation of the extended ISA will still be able to execute machine code for versions of the ISA without those extensions. Machine code using those extensions will only run on implementations that support those extensions.▼
▲An ISA can be extended by adding instructions or other capabilities, or adding support for larger addresses and data values; an implementation of the extended ISA will still be able to execute [[machine code]] for versions of the ISA without those extensions. Machine code using those extensions will only run on implementations that support those extensions.
The binary compatibility that they provide makes ISAs one of the most fundamental abstractions in computing.▼
▲The binary compatibility that they provide makes ISAs one of the most fundamental abstractions in [[computing]].
==Overview==
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==Instruction set implementation==
{{Main|Processor design|Semiconductor device fabrication|Silicon compiler}}
A given instruction set can be implemented in a variety of ways. All ways of implementing a particular instruction set provide the same [[programming model]], and all implementations of that instruction set are able to run the same executables. The various ways of implementing an instruction set give different tradeoffs between cost, performance, power consumption, size, etc.
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