February 21st, 2010 
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A stripped ATX case lying on its side.
A computer case is the enclosure that contains the main components of a computer. Cases are usually constructed from steel, aluminium, or plastic, although other materials such as wood, plexiglas or fans[11] have also been used in case designs. Cases can come in many different sizes, or form factors. The size and shape of a computer case is usually determined by the form factor of the motherboard that it is designed to accommodate, since this is the largest and most central component of most computers. Consequently, personal computer form factors typically specify only the internal dimensions and layout of the case. Form factors for rack-mounted and blade servers may include precise external dimensions as well, since these cases must themselves fit in specific enclosures.
Currently, the most popular form factor for desktop computers is ATX, although microATX and small form factors have become very popular for a variety of uses. Companies like Shuttle Inc. and AOpen have popularized small cases, for which FlexATX is the most common motherboard size.
Central processing unit
Central processing unit
The central processing unit, or CPU, is that part of a computer which executes software program instructions. In older computers this circuitry was formerly on several printed circuit boards, but in PCs is a single integrated circuit. Nearly all PCs contain a type of CPU known as a microprocessor. The microprocessor often plugs into the motherboard using one of many different types of sockets. IBM PC compatible computers use an x86-compatible processor, usually made by Intel, AMD, VIA Technologies or Transmeta. Apple Macintosh computers were initially built with the Motorola 680×0 family of processors, then switched to the PowerPC series (a RISC architecture jointly developed by Apple Computer, IBM and Motorola), but as of 2006, Apple switched again, this time to x86-compatible processors by Intel. Modern CPUs are equipped with a fan attached via heat sink.
Motherboard
The motherboard, also referred to as systemboard or mainboard, is the primary circuit board within a personal computer. Many other components connect directly or indirectly to the motherboard. Motherboards usually contain one or more CPUs, supporting circuitry – usually integrated circuits (ICs) – providing the interface between the CPU memory and input/output peripheral circuits, main memory, and facilities for initial setup of the computer immediately after power-on (often called boot firmware or, in IBM PC compatible computers, a BIOS). In many portable and embedded personal computers, the motherboard houses nearly all of the PC’s core components. Often a motherboard will also contain one or more peripheral buses and physical connectors for expansion purposes. Sometimes a secondary daughter board is connected to the motherboard to provide further expandability or to satisfy space constraints.
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