Computer Knowledge Basics, Operations, Devices

Computer Knowledge Basics notes

The heart of the computer is called CPU or Processor. The CPU performs:
  • Arithmetic Operations
  • Logical Operations
  • Input-Output Operations
  • Internal data movements (moving data between various parts of storage)
  • Data manipulation


a. Arithmetic-Logic Unit (ALU): It performs the actual calculations (addition, subtraction, division, multiplication) and comparison (equal to, greater, lesser, positive, negative, zero etc).
b. Control Unit (CU): It coordinates the operation of the hardware, the flow and executionof data and instructions that are fed into the memory or main storage via the CPU.
c. Memory Unit (Main Store): It holds data instructions (that are being interpreted and executed), intermediate results and final results ready for output.

A computer has four basic types of registers:
1) Accumulator register (ACC)
2) Store operand register (SOR)
3) Control register (CR)
4) Instruction address register (IAR).

Buses: The control unit and internal storage are linked together by sets of parallel electrical conducting lines called buses. The buses that carry data are called data buses. The retrieval of data from memory is carried by the memory address bus.

Peripheral Devices

Input devices allow the user to get data into the machine. A large range of input, devices are available, e.g. keyboard, mouse.
Output devices are peripherals used to output results to the user. They include printers, plotters and VDUs (Visual Display Units).
Backing storage devices or the secondary storage devices are mainly hard disk, CDS, disk packs, floppy disks, and tapes used to store data and programmes permanently.

On the basis of their size computers can be classified as:
i) Micro Computers: It is the smallest type of computer. Inside a Micro computer, the ALU and CU are combined on a single chip called microprocessor. Micro computers contain two types of memory:
A. Random Access Memory (RAM): Where programmes and data are held temporarily during processing. Data stored in RAM is lost when the machine is switched off.
B. Read Only Memory (ROM): Where permanent instructions of data are held. The ROM does not require a continuous supply of power to retain its contents.

Micro computers are used as home computer, or by small businessmen or individual professional where volumes of data processing and speed requirements are small. These are also extensively used for computer graphics, Computer Aided Design (CAD) and Computer Assisted Manufacturing (CAM) applications.

Personal Computers (PC): These have slightly better capabilities and are used generally for business applications. These are single user system. Currently IBM PC in, various configurations and IBM - PC compatibles are the most popular small computers.

Mini Computers: Mini computers are more powerful than the micro computers and can support several users. They have larger RAM and backing storage capacity and can process data more quickly. They are basically used for process control systems.

Main frame computers: Main frames are very large computers with very high capacity of main store. They can process large amounts of data very quickly. So they are used by banks, big companies and Govt. departments as their main computer. These can be linked into a network with smaller departmental computers, micro computers or with each other.

Super Computers: Complex scientific applications like weather forecasting require a large amount of data to be manipulated within a very short time.

Input Devices:

a) Keyboards: The key board resembles a typewriter. But there are additional keys that handle control functions. The computer keyboard has three categories of keys.
  • Alphanumeric keys 
  • Special key
  • Function keys    

The Alphanumeric Keys comprises of alphabets (A-Z or a-z), numbers (0-9) and other characters, like space, ./ × ' ; : — ~ ! @ # $ % ^ & * ( ) - + = | \ { } [ ].

The Special Keys perform specific tasks. Some of the special keys are, Enter or the Return key, Backspace key, Del key, the Ins key, Shift keys, e Caps lock key, Num Lock key, Ctrl key and the Alt key.

The Function Keys are used to perform a set of operations by a single keystroke.

Mouse: A mouse enables the user to manipulate a pointer or an arrow on a terminal or micro computer screen.

Joystick: Joysticks are designed in the shape of handles that swivel in 360 degree arcs, enabling their users to control screen figures.

Barcode reader: Data can be coded in the form of light and dark bars with coded spacings and thickness. These are called barcodes which are commonly used to identify items. Each item is labelled with a Universal Product Code (UPC).

The Optical Mark Reader (OMR) is a device which can detect the presence or absence of a mark on a paper. OMR is used in reading answer sheets, questionnaires.

Optical Character Reader (OCR) is an improvement over OMR. This can not. only detect a mark but can also recognize its shape and identify characters directly from source documents.

Magnetic Ink Character Recognition (MICR) systems use special ink which can be magnetised, to print characters that can then be read and decoded by special magnetic devices. This system is widely used by banks for processing cheques. The cheque number, the bank and branch code and the account number are printed with ink containing magnetisable particles of iron oxide.

Output Devices

The Visual Display Unit (VDU) is a device used for interactive processing, i.e. data that is being keyed in, is displayed on the screen or monitor. Messages and processed information are also displayed on the screen.

The combination of keyboard and the VDU is usually referred to as Video Display Terminal (VDT), which is an input/ output (I/O) device.

The formation of images is controlled by the video controller. The video controller along with the memory is termed as the display adaptor. Display adaptors are of various types and are normally classified on the basis of resolution, colour and display mode:

Monochrome Graphics Adaptor (MGA): The term graphics is a misnomer since the device supports text only.

Colour Graphics Adaptor (CGA) supports both text and graphics mode.

Hercules Graphics Adaptor (HGA) is a monochrome adaptor with an additional graphics display mode which provides high resolution monochrome graphics.

Enhanced Graphics Adaptor (EGA)  was developed in an effort to unify the variety of display adaptors.
It functions in colour and monochrome modes in various resolutions.

Video Graphics Array (VGA) supports 256 colours at a time with a high resolution.

Super Video Graphics Adaptor (SVGA) supports 24-bit true colour upto 1024* 768 dots.

The results of processing could be written by the computer onto a tape or disk, to be used at a later time or to be given to another computer as input.

Character Printers print one character at a time and are used for low-volume printing jobs.

The dot matrix printer is a versatile low-cost device capable of printing in various languages, printing letters of various types, in bold, italics or underlined. It can also be used to print graphics.

Ink Jet Printers: Droplets of ink are electrically charged after leaving a nozzle. The droplets are guided to the proper positions on the paper by electrically charged deflection plates.

Laser printers are page printers. A page of text or pictures is composed at a time. A laser printer utilises a laser beam that sensitized selected areas on a print page.

Spool: Simultaneous Peripheral Operations On-line. Using the spooling technique, output is not sent directly to the printer. The output is first transferred to an intermediary storage medium, such as a disk file.

Printer buffer: The CPU transfers print output to temporary memory called a "buffer".

Data Storage Devices

Data processing system often requires access to very large quantities of data. The computer memory or Immediate Access Storage (IAS) is unsuitable for this task. The amount of data needed for any commercial system is too large to be held in the processor, and secondly, normal processor memory is volatile, i.e. it does not retain its contents once power is switched off or even if there is a temporary break in power. Some form of backing storage is therefore required.

Main memory
The memory unit of the CPU is a place where programmes or instructions and data are stored while processing. The device consists of a number of storage locations. Each storage location may be identified by a unique number which is called its address. During processing, data may be stored in any location which is identified by the address of the location.

RAM: It is possible to read from and write to any location within RAM by specifying its location or address. New data can be written onto any location. However, while doing so the previous existing data is erased. RAM chips may be classified as i) Dynamic RAM chips and ii) Static RAM chips.

In the event of power loss, dynamic RAM loses its contents. It is thus called volatile storage.

Static RAM: They are also volatile storage devices. However, as long as they are supplied with power, they do not require special regenerator circuits to retain the stored data.

Read-Only Memory chips (ROM)  Data is hardwired onto these chips at the time of manufacture. They cannot be changed by the user. ROM retains the data in it even in the absence of power and is thus nonvolatile storage.

There are some types of read only memory called Programmable Read-Only Memory (PROM). Critical or lengthy operations that are slowly carried out by software can be converted into micro programs and fused into a programmable read-only memory chip. PROM can be programmed only once.

The Erasable Programmable Read-Only Memory (EPROM) makes it possible for the user to repeatedly erase and reprogram the ROM. Erasing is done by exposing the EPROM to ultra-violet rays of a specific frequency.

An Electrically Erasable Programmable Read-Only Memory (EEPROM) can be programmed through the use of special electrical pulses.

CD-ROM (Compact Disk Read-Only Memory) is a non-erasable disk used for storing computer data.

Virtual Memory
The size of the memory unit in the CPU is often inadequate, when large programmes need to be stored or large amounts of data need to be processed. To overcome this drawback, the technique of virtual memory is used.

cache memory
A copy of portions of main memory is maintained in cache. When the CPU reads a word of memory, it first reads cache. If found, the word is delivered to the CPU.

Magnetic Tape
Magnetic tapes are convenient, inexpensive devices which can be used to store large volumes of data. The access time in the case of magnetic tapes is quite high.

  • Large volumes of data can be stored.
  • Data can be transported easily.
  • Cost effective.
  • Re-usable medium.
  • For data that has to be loaded sequentially.
The Write Once Read Many (WORM) disk is prepared in such a way that it can be written once with a laser beam of modest intensity. It is possible to read the disk a number of times.

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