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 (e.g. CPU, GPU, FPU, DSP, Coprocessor, ASCI, etc..). 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). The exact design of the microarchitecture ultimately determines its capabilities with respect to those design goals.

Overview

The instruction set architecture (ISA) can be seen as a high-level contract between the architect and the programmer. It sets out to define how the machine behaves with respect to correctness of program execution. The ISA, however, does not concern itself with the intimate details of how the machine gets it done. To some degree it can be seen as black box or a virtual machine. This is where the microarchitecture fills in the details. The microarchitecture describes exactly how the behavior described by the ISA is done. The microarchitecture defines how every single digital signal is routed around and manipulated to achieve the desired result. The design of a microarchitecture can range from a very simple to highly complex depending on the outcome the engineers hope to achieve.

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