Computer architecture is a specification detailing how a set of software and hardware technology standards interact to form a computer system or platform. In short, computer architecture refers to how a computer system is designed and what technologies it is compatible with.As with other contexts and meanings of the word architecture, computer architecture is likened to the art of determining the needs of the user/system/technology, and creating a logical design and standards based on those requirements.most types of computers today. This was proposed by the mathematician John von Neumann in 1945. It describes the design of an electronic computer with its CPU, which includes the arithmetic logic unit, control unit, registers, memory for data and instructions, an input/output interface and external storage functions.There are three categories of computer architecture:

System Design: This includes all hardware components in the system, including data processors aside from the CPU, such as the graphics processing unit and direct memory access. It also includes memory controllers, data paths and miscellaneous things like multiprocessing and virtualization.

Instruction Set Architecture (ISA): This is the embedded programming language of the central processing unit. It defines the CPU's functions and capabilities based on what programming it can perform or process. This includes the word size, processor register types, memory addressing modes, data formats and the instruction set that programmers use.

Microarchitecture: Otherwise known as computer organization, this type of architecture defines the data paths, data processing and storage elements, as well as how they should be implemented in the ISA.

During normal operations it feeds instructions and data to the processor, and at other times it is the source or destination of data transferred by I/O devices. Information in a memory is accessed by itsaddress. In programming language terms, one can view memory as a one-dimensional arrayM. A processor's request to the memory might be ``send the instruction at locationM[1000]'' or a disk controller's request might be ``store the following block of data in locationsM[0]throughM[255].''

Input/output (I/O) devices transfer information without altering it between the external world and one or more internal components. I/O devices can be secondary memories, for example disks and tapes, or devices used to communicate directly with users, such as video displays, keyboards, and mouses.

The communication channels that tie the system together can either be simple links that connect two devices or more complexswitchesthat interconnect several components and allow any two of them to communicate at a given point in time. When a switch is configured to allow two devices to exchange information, all other devices that rely on the switch areblocked, i.e. they must wait until the switch can be reconfigured.

A common convention used in drawing simple ``stick figures'' of computer systems is the PMS notation[32]. In a PMS diagram each major component is represented by a single letter, e.g.Pfor processor,Mfor memory, orSfor switch. A subscript on a letter distinguished different types of components, e.g.for primary memory andfor cache memory. Lines connecting two components represent links, and lines connecting more than two components represent a switch. Although they are primitive and might appear at first glance to be too simple, PMS diagrams convey quite a bit of information and have several advantages, not the least of which is they are independent of any particular manufacturer's notations.

As an example of a PMS diagram and a relatively simple computer architecture, Figure 1 shows the major components of the original Apple Macintosh personal computer. The first thing one notices is a single communication channel, known as thebus, that connects all the other major components. Since the bus is a switch, only two of these components can communicate at any time. When the switch is configured for an I/O transfer, for example from main memory () to the disk (via K), the processor is unable to fetch data or instructions and remains idle. This organization is typical of personal computers and low end workstations; mainframes, supercomputers, and other high performance systems have much richer (and thus more expensive) structures for connecting I/O devices to internal main memory that allow the processor to keep working at full speed during I/O operations.