Computer architecture

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Computer architecture includes at least three main subcategories:^[1]

Instruction set architecture, or ISA, is the abstract model of a computing system that is seen by a machine language (or assembly language) programmer, including the instruction set, memory address modes, processor registers, and address and data formats.
Microarchitecture, also known as Computer organization is a lower level, a detailed description of the system that is sufficient for completely describing the operation of all parts of the computing system, and how they are inter-connected and inter-operate in order to implement the ISA.^[2] The size of a computer's cache for instance, is an organizational issue that generally has nothing to do with the ISA.
System Design which includes all of the other hardware components within a computing system such as:

Once both ISA and microarchitecture has been specified, the actual computing system needs to be designed into hardware. This design process is called implementation. Implementation is usually a hardware engineering design process.

Implementation can be further broken down into three but not fully separate pieces:

Logic Implementation: Design of blocks defined in the microarchitecture, mainly, at the register-transfer and gate levels.
Circuit Implementation: Transistor-level design of basic elements (gates, multiplexers, flip-flops, etc.) as well as of some larger blocks (ALUs, caches etc.) that may be implemented at this level, or even at a lower physical level, for performance reasons.
Physical Implementation: Physical circuits are drawn out, the different circuit components are placed in a chip floor-plan or on a board and the wires connecting them are routed.

For CPUs, the entire implementation process is often called CPU design; it can also be a family of related CPU designs, such as RISC and CISC.

More sub-definitions

Some practitioners of computer architecture use more fine subcategories:

Macroarchitecture: An architectural layers that are more abstract than microarchitecture, e.g. ISA.
Instruction Set Architecture (ISA): As defined above.
UISA (Microcode Instruction Set Architecture): A family of machines with different hardware level microarchitectures may share a common microcode architecture, and hence called a UISA.
Assembly ISA: A smart assembler may convert an abstract assembly language common to a group of CPUs into slightly different machine language for different CPU implementations.
Programmer Visible Macroarchitecture: Higher level language tools such as compilers may define a definite interface to programmers using them, abstracting differences between underlying ISA, UISA, and microarchitectures; for example the C, C++, or Java standards define three different definite programming interfaces.
Pin Architecture: The set of functions that a microprocessor is expected to provide, from the point of view of a hardware platform. E.g. signals that the processor is expected to emit during executing an instruction.

↑ John L. Hennessy and David A. Patterson (2003). Computer Architecture: A Quantitative Approach (Third Edition ed.). Morgan Kaufmann Publishers, Inc. ISBN 1558605967. {{cite book}}: |edition= has extra text (help)
↑ Phillip A. Laplante (2001). Dictionary of Computer Science, Engineering, and Technology. CRC Press. pp. 94–95. ISBN 0849326915.