All computers, whatever their size or function, have certain basic components. They have input devices for reading data into main memory, a central processing unit (CPU) for processing the data, output devices for printing, displaying or ouputting information, and auxiliary storage devices for permanent storage of programs and data.
The Processor
The processor has the following functions:
--> fetches the next instruction;
--> decodes the instruction,
--> executes the decoded instruction.
Most computers use integrated circuits, or chips, for their processors and main memory. A chip is about 1cm square and can hold millions of electronic components such as transistors, and resistors. The CPU of a microcomputer is called as a microprocessor. The processor and main memory of a PC are commonly held on a single board called a motherboard.
Main Memory
The program currently being executed and the data used by this program are held in main memory, which is divided into millions of individually addressable storage units called bytes. One byte can hold one character, or it can be used to hold a code representing, for example, a tiny part of a picture, a sound, or part of a computer program instruction. The total number of bytes in main memory is reffered to as the computer's memory size.
Computer mamory sizes are measured as follows:
1 Kilobyte (Kb) = 1024 bytes
1 Megabyte (Mb) = 1024 Kb = 1, 048, 576 bytes (about 1 million)
1 Gigabyte (Gb) = 1024 Mb = 1, 073,741,842 bytes (about 1 billion)
1 Terabyte (Tb) = 1024 Gb = 1, 099,511,627,776 bytes (about 1 triillion)
RAM and ROM
There are basically two kinds of main memory: Random Access Memory (RAM) which is the ordinary kind of main memory referred to above, used for storing programs which are currently running and data which is being processed. This type of memory is volatile which means that it loses all its contents as soon as the machine is switched off.
Read Only Memory (ROM) is the other type of main memory, and this is non-volatile, with its contents permanently etched into the memory chip at the manufacturing stage. It is used for example to hold the bootstrap loader, the program which runs as soon as the computer is switched on and instructs it to load the operating system from disk into main memory (RAM). It may also store fixed data associated with the computer system. In special purpose computers used in video recorders, washing machines and cars, the program instructions are stored in ROM.
Cache Memory
Cache memory is a type of very fast memory that is used to improve the speed of a computer, doubling it some cases. It acts as an intermediate store between the CPU and main memory, and works by storing the most frequently or recently used intructions and data so that it will be very fast to retrieve them again. Thus when an item of data is required, a whole block of data will be read into cache in the expectation that the next piece of data required is likely to be in the same block. The amount of cache memory is generally between 1Kb and 512Kb or more.
Disk Storage
The most common form of auxiliary storage (also known as external or secondary memory or backing store) is disk. All standalone PCs come equipped with an in-built hard disk, the capacity of which is also measured in bytes. A typical hard disk for a PC stores gigabytes, and is used for storing software including the operating system, other systems software, application programs and data for long term storage.
Floppy disks consist of a thin sheet of mylar plasticencased in a hard 3 1/2 "casing. The standard type of disk in use today has a capacity of 1.44Mb. Flash memory cards or sticks are rapidly replacing floppy disks. These can hold from 32Mb to 1Gb.
Primary and Secondary Storage
A computer's main memory (RAM) is known as primary storage. In order to execute a program, the program instructions and the data on which it is to operate .have to be loaded into main memory. Primary storage, however is volatile; when the computer is switched off, all the contents of memory are lost. This is one good reason to perform frequent saves to disk when working on, for example, a word processed document.
A magnetic disk consists of two surfaces, each of which contains concentric circles called tracks. Each track is divided into sectors. If you reformat a disk that already has data on it, all the data will be erased (although you can also do a 'quick format' which erases only the file directory).
Floppy Disks
The standard 3 1/2 “floppy disk is a thin, flexible plastic disk coated in metal oxide, enclosed in a rigid plastic casing. A standard high density disk has a storage capacity of 1.44 Megabytes. The disk can be removed from the drive unit and is highly portable. Floppy disks are inexpensive but easily damaged.
External hard drives, which can be plugged into a microcomputer, are available as extra storage.
For large-scale applications storing huge amounts of data, several hard disk units will be required. The disks may be either fixed {sealed inside the unit) or removable. Fixed disks are faster, more reliable, and have a greater storage capacity.
As with other types of disk, data is stored on concentric tracks, with tracks being divided into sectors. All the tracks that are accessible from one position of the read-write heads form a cylinder; data is recorded cylinder by cylinder to minimise movement of the read-write heads, thereby minimising access time.
Random access time is longer than that of a hard disk, but some optical storage is writeable or even re- writeable, CD-ROM, DVD-ROM (Read-only), CD-R, DVD-R (Writeable), CD-RW, DVD-RW (Re- Writeable).
CD-ROM
CD-ROMs can store around 680Mb of data, equivalent to hundreds of floppy disks. The data may be in text form, or may be in the form of graphics, photographic images, video clips or sound files. Although they do not transfer data as fast as a hard disk drive, their speed in increasing every year and is acceptable for most applications.
As the name suggests, the disks are read-only. When the master disk is created, a laser beam burns tiny pits in the surface of the disk, which (unlike a magnetic disk) has a single spiral track divided into sectors. To read data from the disk, a laser beam is reflected off the surface of the disk, detecting the presence or absence of pits which represent binary digits. CD- ROMs are widely used for distribution of software, multimedia files, catalogues and technical manuals.
Write Once, Read Many optical laser disks (WORM disks) look similar to CD-ROM disks, but they are often gold rather than silver in colours. An end-user company can use these disks to write their own material, typically for archiving or storing, say, graphic or photographic images which will not be changed.
CD-RW Disks
These are re-writeable disks and are more expensive than CD-R. These can be used for backing up where they may need to be overwritten.
Digital Versatile Disk Read-Only Memory. These disks are the same size as CD-ROMs and are made using similar materials and manufacturing techniques. They store about seven times as much data as a CD- ROM, because the track spacing and pit dimensions are smaller.
Flash Memory
Flash is electrically erasable programmable read-only memory (EEPROM). It is used in memory cards for peripheral devices such as digital cameras, mobile telephones, PDAs and MP3 players. It is also available as USB memory sticks and, when plugged into a computer's USB port, behaves like an external disk drive. Flash memory is inexpensive, high-capacity storage and is rapidly replacing floppy disks as portable secondary storage.
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