Microarchitecture (µarch) is the underlying implementation of an instruction set architecture (ISA) - I.E. it is the physical hardware organization (on the transistor level) of an architecture. Multiple microarchitectures may and often do get designed for any one ISA - sometimes by different companies with different design goals (e.g. budget, thermal, power, and performance). Where the ISA describes what a certain operation does respect to correctness of program execution, the microarchitecture describes how exactly it is done.

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