Computer architecture

In computer science and computer engineering, computer architecture or digital computer organization is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit (CPU) performs internally and accesses addresses in memory.
It may also be defined as the science and art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals.
Computer architecture comprises at least three main subcategories:[1]
Instruction set architecture, or ISA, is the abstract image of a computing system that is seen by a machine language (or assembly language) programmer, including the instruction set, word size, memory address modes, processor registers, and address and data formats.
Microarchitecture, also known as Computer organization is a lower level, more concrete and detailed, description of the system that involves how the constituent parts of the system are interconnected and how they interoperate 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:
System interconnects such as computer buses and switches
Memory controllers and hierarchies
CPU off-load mechanisms such as direct memory access (DMA)
Issues like multiprocessing.
Once both ISA and microarchitecture have been specified, the actual device needs to be designed into hardware. This design process is called implementation. Implementation is usually not considered architectural definition, but rather hardware design engineering.
Implementation can be further broken down into three (not fully distinct) pieces:
Logic Implementation — design of blocks defined in the microarchitecture at (primarily) the register-transfer and gate levels.
Circuit Implementation — transistor-level design of basic elements (gates, multiplexers, latches etc) as well as of some larger blocks (ALUs, caches etc) that may be implemented at this level, or even (partly) at the physical level, for performance reasons.
Physical Implementation — physical circuits are drawn out, the different circuit components are placed in a chip floorplan or on a board and the wires connecting them are routed.