What is a computer?

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A computer is an electronic device that accepts data and instructions, processes the data according to the instructions provided, returns the results of processing to the user and stores it until the information is needed.

Functions of a Computer

Input The transmission of data from the user to the computer is called input.

Processing The computer has an electronic brain called the Central Processing Unit, which is responsible for processing all data and instructions given to the computer. Processing is the manipulation of data based on the instructions given by the user.

Storage Storage is the holding of information until it is ready for use. The computer has memory chips, which are designed to hold information temporarily while it is needed and secondary storage devices which stores data permanently for future use.

Output The computer returns the processed information to the user. This is called output.

The basic components of all computer system are:

  • The Central Processing Unit (CPU)

  • Main Memory/ immediate access storage

  • Backing storage/disk storage

  • Peripheral devices

  • CPU - The CPU is a microprocessor chip made up of integrated circuits. It is called the brains of the computer as this is where all the processing takes place. It is made up of two main components that enable the CPU to complete its job; the ALU and the CU.


Control Unit

  • Interprets stored instructions in sequence

  • Issues commands to all elements of the computer

  • Controls the input and output devices

  • Controls the flow of data and instructions in and out of memory.

Arithmetic Logic Unit:

  • Performs fundamental arithmetic operation (+,-,/,*)

  • Performs logic operations. Example (OR, AND, NOT, =,<, <=, >=, >) on data values sent to the memory.

Main Memory (RAM & ROM)

Main Memory or Ram is memory chip that is volatile meaning data is lost when the power goes. It is used to store data and instruction that is being processed and the results of processing.


  • Holds data, programs and instructions temporarily during processing

  • Holds the results of processing

Backing Storage: This is memory external to the computer (except or the internal hard disk) that is used for storage of large quantities of data or large programs. It is also known as secondary store, auxiliary store or external store E.g. (Hard Disk, diskette, zip disk, compact disk, and microfilm).

Peripheral Devices:

These are detachable instruments that enhances the computer capabilities (e.g. input and output devices, printer, scanner, speakers, microphone, digital camera, keyboard)

Bus- This is an electrical path along which data travels as bits and bytes.


Word: The largest amount of data that can be accessed together.

Word size: The number of bits the CPU processes at one time. One aspect of the CPU that affects the speed of a computer is the word size.

Address: The identification of a particular location in memory where a data item or instruction is stored.

Location: Is the address where data is stored in a memory


Address Content: The information stored in a location in

The purpose of memory is to provide storage of data and instructions during processing and temporary storage for the results of processing.

Read Only Memory (ROM)

Random Access Memory (RAM)

Programmable Read Only Memory (PROM)

Erasable Programmable Read Only Memory (EPROM)

Electrically Erasable Programmable Read Only Memory (EEPROM)
Bistable Device: This is a device, which can exist in two discrete stable states on or off(E.g. Computer switch, light switch, remote control). On is 1 and off is 0.
Random Access Memory (RAM)

RAM is the name given to chips that are used for main memory. Main memory or primary storage:

  • Stores data and the application program instructions that direct the work to be done temporarily, during processing.

  • Temporarily stores the results of processing.

RAM memory is said to be volatile because the programs and data stored in RAM are erased when the power to the computer is turned off.

Read Only Memory (ROM)

ROM is used to store the start up instructions and data used when a computer is first turned on (BIOS). With ROM data is permanently recorded in the memory when it is manufactured. This memory is described as nonvolatile because it retains its contents even when the power is turned off. The data or programs that are stored in ROM can be read and used, but cannot be altered, hence the name Read Only.

Eg. of ROM are (computer used in automobiles, appliances).
Programmable Read Only Memory (PROM)

This is a blank ROM chip, on which users can store information permanently. These PROM chips can be programmed only once.

Erasable Programmable Read Only Memory (EPROM)

This type of memory chip can be erased by removing it from the computer and exposing it to ultraviolet light. This chip is can be programmed and erased repeatedly.

Electrically Erasable Programmable Read Only Memory (EEPROM) This type of memory chip can be erased electrically without being removed from the computer. This chip is can be programmed and erased repeatedly. Eg.Thumb Drives are EEPROM chips.
Data Representation
Bit: Acronym for binary digit. It can have a value of 1 or 0.
Byte: A group of 8 bits, usually representing a single character
Kilobyte (K or KB): Equal to 1024 bytes or 1000 (thousand) bytes
Megabyte (MB): Represents 1,000,000 (million) bytes
Gigabyte (GB): Represent 1 000,000,000 (billion) bytes
Terabyte(TB): Represents one Trillion bytes
Buffer is used to temporarily store data during input and output operations. It is used to compensate for the difference in speed between the processor and other devices.

Cache is used to temporarily store most frequently used data and information. So it is can be quickly accessed.
Secondary Storage, also called auxiliary storage/backing storage, is used for long term storage of programs and data. Most secondary storage devices provide a more permanent form of storage than main memory because they are nonvolatile. Due to the expense and limitation of Random Access Memory (RAM) secondary storage is needed.
Categories of Secondary Devices and Media
Secondary storage devices can be categorized based on the technology used to create the device or based on the method of accessing data.
Secondary storage device categorized by how data is accessed.
Direct Access/Immediate Access /Random Access– Any Piece of data can be accessed immediately. Examples: Floppy disk, Hard Disk (Fixed, moveable, exchangeable)
Sequential Access – Data can only be accessed by the read/write head passing over all the data until the required data item is found, Eg. Magnetic Tape:

Floppy disk:

Hard Disk (Fixed, moveable, exchangeable):
For some devices data can be accessed both sequentially and randomly. Eg. Magneto Optical Disk, Compact Disk (CD), Digital Versatile Display (DVD)
Secondary storage device categorised by technology used to create disks.
Magnetic- a special types of magnetized plastic is used to create disk surfaces. Eg. Floppy Disk, Hard Disk, Magnetic Tape

Optical To record data on the disks tiny lasers create peaks and valleys in the plastic layer of the circular disk, which are read as 0s and 1s respectively. Eg.Compact Disk (CD), Digital Video Display (DVD).
Magnetic Tape
Consist of a thin ribbon of plastic. The tape is coated on one side with a material that can be magnetized to record the bit patterns that represents data. Prior to Compact disks, magnetic tapes were the primary method of storing large amounts of data. The most common types of magnetic tape devices are cartridge and reel-to-reel.
Characteristics of Magnetic Tapes

  • Fairly cheap

  • Not easily damaged

  • Main use for backup and archiving

Floppy Disk

Floppies, are made up of the same material as magnetic tapes, except they are thicker. Information can be stored on one side (SINGLE DENSITY) or on both sides (DOUBLE DENSITY). They come in different sizes and colours:

The circular piece of plastic is enclosed in a ridgid plastic shell and a piece of metal called the shutter covers the reading and writing surface. Paper liners help keep the recording surfaces clean.

Taking Care of diskettes

  • Keep away from dirt, water an magnetic fields

  • Do not write on diskettes rather than on the labels before applying to diskettes

  • Avoid exposure to heat and food

  • Do not touch internal parts of the disk

Hard Disk

A Hard Disk provides larger and faster secondary storage capabilities than diskettes. Hard disks consist of one or more rigid platters that allow data to be magnetically recorded on the surface of the platter.

The platters are permanently mounted inside the computer and are not removable like diskettes.

Fixed head:

Some hard disks are sometimes called (fixed disks) because they cannot be removed from the computer like diskettes. They are permanently installed in the computer.

Moving Head

Removable hard disk - A type of magnetic disk, which is not permanently attached to the disk drive (not a fixed disk) but which can be taken out (portable) and replaced, allowing many disks to be used in the same drive.
How data is stored on Magnetic Disks

Magnetic disks must be formatted or divided into tracks and sectors (storage areas) before data can be written to it (stored) or read from it (accessed).

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