Topic 3: Hardware

All Components of a Computer

All computers has its basic components: input devices, central processing unit (CPU) output devices and auxiliary storage for permanent storage programs and data.

The processor

A processor is the logic circuitry that responds to and processes the basic instructions that drive a computer.

Its functions are to fetched the next instruction, decodes the instruction and executes the decoded instruction.




Most computers use integrated circuits for their processors and main memory. Microprocessor is the CPU of a microcomputer. The processor and main memory of a PC are commonly held on single board called motherboard.

Main memory

A storage device used by the computer to hold the currently executing program and its working data.

Computer memory size:
1 Kilobytes (Kb) : 1024 bytes
1 Megabytes (Mb) : 1024 Kb :1 048 576 bytes ( about 1 million)
1 Gigabytes (Gb) : 1240 Mg : 1 073 741 842 (about 1 billion)
1 Terabyte (Tb) : 1024 Gb : 1 099 511 627 776 bytes (about 1 trillion)

RAM and ROM

RAM and ROM are the two basic kinds of memory. RAM, Read Access Memory, the type of memory that stored programs and data which is currently in use and is volatile which means that all data is lose when the computer is shut down. ROM, Read Only Memory, the type of memory which is manufactured with fixed contents and is non-volatile, data is not lose when computer is shut down.



Cache Memory

A fast memory holding recently accessed data. It is designed to speed up subsequent access to the same data. Act as intermediate store between CPU and main memory.

As the microprocessor processes data, it looks first in the cache memory and if it finds the data there (from a previous reading of data), thus it does not have to do the more time-consuming reading of data from larger memory. The amount of cache memory generally between 1Kb and 512Kb.



Disk Storage

Disk is the most common form of auxiliary storage (external/secondary/backing). All PCs are equipped with in-built hard disk.

Primary and Secondary storage

Primary storage is data in random access memory (RAM) and other "built-in" devices.
It holds memory for a short period of time (volatile), data is lost when switch off. In order to execute the a program, the program instruction and the data on which it is operate have to be loaded into main memory.

For secondary storage, sometimes called auxiliary storage, is all data storage that is not currently in a computer's primary storage memory for example magnetic tape, hard disk, CD-ROM and other external devices.

Magnetic Disk

A flat rotating disc covered on one or both sides with some magnetisable material. The two main types are the hard disk and the floppy disk.

Data is stored on either or both surfaces of discs in concentric rings called "tracks". Each track is divided into a whole number of "sectors". Data is read and written by a disk drive which rotates the discs and positions the read/write "heads" over the desired track(s). If you reformat a disk that already has that has data on it, all data will be erased.

Floppy disk

A flexible plastic disk coated with magnetic material and covered by a protective jacket, used primarily by computers to store data magnetically. Also called diskette.
The standard floppy disk is 3 1/2 . A standard high density disk has a storage capacity of 1.44 Mb.

Although they cannot store as much data as convential disks and the data cannot be retrieved as easily, floppy disks have become extremely popular in situations where flexibility, low cost, and easy use are important. but it is easily damaged.

Hard disks for microcomputers

Hard disk used with PCs consists of one or more disk platters permanently sealed inside a casing. Hard disk typically have a capacity of between 40Gb and 160Gb or more. There is also external hard disk which can be plugged into the USB port of the microcomputer as extra storage.







Hard disks for minis and mainframes

For large scale applications (mainframe) storing huge amounts of data, several hard disks 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 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.


Magnetic tape

A storage medium used for data collection, backup and archiving.

Narrow plastic ribbon coated with an easily magnetizable material on which data can be recorded.

Data is recorded in 'frames' accross the tape with one frame representing one byte. The frames form tracks along the length of the tape, with 9 tracks being common, giving 8 data tracks and one parity track.

For mass storage on commercial mainframe computers, large reel-to-reel tapes are still used, but cartridges are becoming popular. Various types of cartridge are now standard on minicomputers and PCs, while audio cassettes were used with early home computers.

Uses of magnetic tape

Tape is cheap and convenient medium for backup and is also used for archiving past transactions or other data that may be needed again. Cartridge tape drivers are in common use for backing up the hard disk of personal computer being much more convenient than dozens of floppy disks. A cartridge tape can store several Gigabytes or data.

Optical disk

A plastic-coated disk that stores digital data, such as music or text, as tiny pits etched into the surface and is read with a laser scanning the surface. Also called laser disk.

CD, CD-ROM, DVD-ROM and DVD-Video are read-only optical disks that are recorded at the time of manufacture and cannot be erased. CD-R, DVD-R, WORM and magneto-optic (in WORM mode) disks are write-once. They are recorded in the user's environment, but cannot be erased. CR-RW, DVD-RAM, DVD-RW and MO disks are rewritable.

CD-ROM

Compact Disc Read Only Memory) A compact disc format used to store programs and data files. Holding either 650MB or 700MB. CD-ROMs are capable of delivering digitized sounds, images, and multimedia presentations.

CD-ROMs are made by "burning" a blank "CD-R" disc and sending it to a media manufacturer, which creates a master disc that is used to stamp out any required quantity. Users can also burn blank CD-Rs one at a time, using a PC or stand-alone duplicator.


CD-R disks

(CD-Recordable) A writable CD technology using a type of compact disc that can be recorded, but not erased (CD-Rs are "write once" discs). CD-R discs are used to master CD-ROMs, to back up data and to make copies of data for distribution.

CD-RW disks

These are rewritable disks and more expensive than CD-R. Can be used for backing up where they may need to be overwritten.




DVD-ROM

Digital Versatile Disk Read Only-Memory. These disks are the same size as CD-ROM and are made up using similar materials and manufacturing techniques. However it can store data about seven times more than CD-ROM because the tracks spacing and pit dimension are smaller.

Flash Memory

Flash memory is non-volatile, which means that it does not need power to maintain the information stored in the chip. In addition, flash memory offers fast read access time. It use in memory cards for peripheral devices such as digital camera, mobile phones, 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. It is inexpensive and has high capacity.