The basic components of a microcomputer system are:
1. A central processing unit (CPU).
2. A memory, comprising both ‘read/write’ and ‘read-only’ devices (commonly called RAM and ROM respectively).
3. A mass storage device for programs andjor data (e.g. a floppy and/or hard disk drive).
4.  A means of providing user input and output (via a keyboard and display interface).
5. Interface circuits for external input and output (I/O). These circuits (commonly called ‘ports’) simplify the connection of peripheral devices such as printers, modems, mice, and joysticks.

In a microcomputer (as distinct from a mini or mainframe machine) the functions of the CPU are provided by a single VLSI microprocessor chip (e.g. an Intel 8086, 8088, 80286, 80386, 80486, or Pentium). The microprocessor is crucial to the overall performance of the system.

Indeed, successive generations of PC are normally categorized by reference to the type of chip used. The ‘original’ PC used an 8088, AT systems are based on an 80286, ’386 machines use an 80386, and so on.

Semiconductor devices are also used for the fast redd/write and readonly memory. Strictly speaking, both types of memory permit ‘random access’ since any item of data can be retrieved with equal ease regardless of its actual location within the memory. Despite this, the term ‘RAM’ has become synonymous with semiconductor read/write memory. (VLSI means very large scale integration, i.e. a complex chip.) 

The semiconductor ROM provides non-volatile storage for part of the operating system code (this ‘BIOS’ code remains intact when the power supply is disconnected). The semiconductor RAM provides storage for the remainder of the operating system code (the ‘DOS’), applications programs and transient data (including that which corresponds to the screen display).

It is important to note that any program or data stored in RAM will be lost when the power supply is switched off or disconnected. The only exception to this is a small amount of ‘CMOS memory’ kept alive by means of a battery.

This ‘battery-backed’ memory is used to retain important configuration data, such as the type of hard and floppy disk fitted to the system and the amount of RAM present.

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