Microarchitecture

In electronics engineering and computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as µarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular processor. A given ISA may be implemented with different microarchitectures; implementations may vary due to different goals of a given design or due to shifts in technology.

Computer architecture is the combination of microarchitecture and instruction set.

The ISA is roughly the same as the programming model of a processor as seen by an assembly language programmer or compiler writer. The ISA includes the execution model, processor registers, address and data formats among other things. The microarchitecture includes the constituent parts of the processor and how these interconnect and interoperate to implement the ISA.

The microarchitecture of a machine is usually represented as (more or less detailed) diagrams that describe the interconnections of the various microarchitectural elements of the machine, which may be anything from single gates and registers, to complete arithmetic logic units (ALUs) and even larger elements. These diagrams generally separate the datapath (where data is placed) and the control path (which can be said to steer the data).

The person designing a system usually draws the specific microarchitecture as a kind of data flow diagram. Like a block diagram, the microarchitecture diagram shows microarchitectural elements such as the arithmetic and logic unit and the register file as a single schematic symbol. Typically, the diagram connects those elements with arrows, thick lines and thin lines to distinguish between three-state buses (which require a three-state buffer for each device that drives the bus), unidirectional buses (always driven by a single source, such as the way the address bus on simpler computers is always driven by the memory address register), and individual control lines. Very simple computers have a single data bus organization – they have a single three-state bus. The diagram of more complex computers usually shows multiple three-state buses, which help the machine do more operations simultaneously.

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