PC Components

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PC Components - A Technical Insight

The following pages contain technical information about the main components found in a standard personal computer.

Since the release of the original IBM PC new and enhanced components have been added to the basic design. The rapid advances of the personal computer were partly because of software demands placed on hardware, such as graphical user interfaces (GUI's) or memory hungry applications and new technologies and design techniques processor, motherboard, component and peripheral manufacturers standardized.

Overview

The components I am referring to are usually integrated on the motherboard. The processor and support circuits, memory (RAM and ROM), the input/output interface (including the serial and parallel ports, keyboard interface, disk interface, graphics interface, etc) and buses (the data bus, the address and control bus). There was no real time clock on the original PC and had to be manufactured on an expansion card and placed in one of the motherboard expansion slots. (One of the early upgrades available for PC compatibles).
A computer carries out a series of commands called a program. The processor is designed and organised with the ability to do this. The processor can also read and write to memory because instructions and data used by programs is temporarily held there, secondly the processor is capable of understanding and carry out the series of instructions. The processor is also able to instruct other components on what to do and therefore manage the computers operation.
Although the processor makes a distinction between instructions and data , memory does not (apart from cache). Memory is temporary (unlike ours) and needs constant reminding by dedicated circuitry (except for static ROM). Most memory needs a constant power and hence is referred to as volatile, memory that does not need power to retain data is known as non-volatile (straight forward when there was only one main type of ROM and RAM). Memory is the workspace that programs use when running. The size of workspace is equivalent to the amount of memory installed. The memory can be written to, read from and over written (there is no need for the memory to be cleared first), by both the processor and the input / output devices.
The input/output devices form a window between the system and the outside world allowing communication with the memory and processor. A bridge between the user and the computer. I/O devices (peripherals) include the keyboard, the monitor, mouse, printer, modem, network and disk storage devices.
PC components are connected together by the bus architecture which can be thought of as a highway along which devices communicate. The original PC bus architecture was 8 bits wide and could transfer about 1MB per second compared to a modern 64 bit bus which can transfer data in excess of 500 MB per second. Modern applications place a heavy demand on the bus architecture. New bus designs made applications not previously possible, such as multimedia available on the PC. Pentium processors are able to support multiprocessor buses and many motherboard designs are able to accommodate two or more processors. When systems have multiple bus types in a single design there are devices that bridge between the bus types. New buses will appear that support new designs such as SIMPL, HydraXS and VBus.
The computer case or box houses the motherboard, devices, drives and a power supply (PSU). There are several popular types of case available, such as desktop, mini-desktop, slimline desktop, tower, mini tower and midi tower. All the components need power to function. AC current is converted to DC and output at voltages of 5, 12 and 3.3 and a fixed polarity of either positive or negative. Power supply units are rated in watts.

To better understand how the industry arrived at its present state of supporting multiple processor types and speeds, multiple physical packaging styles and bus architectures read The Personal Computer guide. Below are block diagrams illustrating the layout of the different bus architectures.

The PC and PC XT block diagram :

A typical system used four expansion slots
1. Floppy Disk Controller
2. Hard Disk Drive controller
3. The Display adaptor
4. Combined Serial and Parallel Port Adaptor

The PC AT ISA block diagram :

8 slots with two usually used
1. EGA or VGA display controller adaptor
2. Multi I/O Port adaptor with
- 2 serial ports
- a parallel port
- floppy disk drive interface
- IDE interface for hard disk drives
- a Game Port

VESA Local Bus system block diagram :

A typical system may use 2 VL expansion slots
1. Multi I/O with EIDE
2. SVGA (Super-Video Graphics Array) Display Adaptor.

PCI ISA Bus Block diagram :

One of the PCI slots is placed close to one PCI slot and share the (but not at the same time) a back-plate.
Some motherboards only offer two ISA slots and as many as five PCI slots.