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.
- Main article: IC design flow
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.
On a high level, the microarchitecture of a machine is often represented as a diagram or set of diagrams (typically in the form of block diagrams) that describes the relations and interconnections of the various microarchitectural elements. These elements can range from individual electronic components such as transistors and resistors to more complex units such as register files and multipliers to complete elements such as arithmetic logic units (ALUs) and floating point units (FPUs). Each of those microarchitectural elements are in turn represented by detailed schematics describing the interconnections of the logic gates. Finally, circuit diagrams are used to describe the connections of the transistors used to represent the logic gates schematic.
Modern microarchitectures are described using synthesizable HDLs such as Verilog or VHDL. The description of the circuit is known as RTL design. Register Transfer Level (RTL) can be efficiently described using HDL. Final RTL designs are then passed over for verification and then synthesis - converting the RTL into optimized gate level netlist. Those optimized netlists are then either mapped onto programmable devices such as FPGAs or get converted into geometric representations in what's known as the physical design stage.
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