A computer system consists of mainly four basic units namely:
1. input unit
2. storage unit
3. central processing unit 4. output unit
A computer performs five major operations or functions irrespective of its size and make. These are
1. it accepts data or instructions as input,
2. it stores data and instruction
3. it processes data as per the instructions,
4. it controls all operations inside a computer, and
5. it gives results in the form of output.
1. Input Unit:
This unit is used for entering data and programs into the computer system by the user for processing.
2. Storage Unit:
The storage unit is used for storing data and instructions before and after processing. Also called internal storage main memory or random access memory (RAM).
3. Processing:
The task of performing operations like arithmetic and logical operations is called processing.
4. Output Unit:
The output unit is used for storing the result as output produced by the computer after processing.
Central Processing Unit
Central Processing Unit (CPU) takes data and instructions from the storage unit and make all sorts of calculations based on the instructions given and the type of data provided. It then sent back to the storage unit. CPU includes Arithmetic logic unit (ALU) and contro unit (CU).
1. Arithmetic Logic Unit:
All calculations and comparisons, based on the instructions provided, are carried out within the ALU. It performs arithmetid functions like addition, subtraction, multiplication, division and also logical operations like greater than, less than and equal to etc.
2. Control Unit:
Controlling of all operations like input, processing and output are performed by control unit. It takes care of step by step processing of all operation in side the computer.
Input/Output Devices
These devices are used to enter information and instructions into a computer for storag or processing and to deliver the processed data to a user. These input/output devices are also known as peripherals since they surround the CPU and memory of a compute system.
1. Keyboard:
The keyboard is very much like a standard typewriter keyboard wit a few additional keys. The basic QWERTY Layout of characters is maintained make it easy to use the system. The additional keys are included to perford certain special functions. These are known as function keys that vary in numbe from keyboard to keyboard.
2. Mouse:
A device that controls the movement of the cursor or pointer on display screen. A mouse is a small object you can roll along a hard and flat surface. Its name is derived from its shape, which looks a bit like mouse. As you move the mouse, the pointer on the display screen moves in the same direction.
3. VDU
Visual Display Unit is often used synonymously with 'computer screen' or "display." Monitor is an output device that resembles the television screen. The monitor is associated with a keyboard for manual input characters and displays the information as it is keyed in. It also displays the program or application output. Like the television, monitors are also available in different sizes.
Other Input Devices
Different types of scanners are:
1. OCR
These are scanner devices that are capable of detecting alphabetic and numeric characters by comparing the shapes with internally stored patterns. These are expensive and are used only for large-volume processing applications eg. by credit-card companies
2. OMR
These scanners are capable of recognizing a pre-specified type of mark made by a pencil. These are normally used for validation of input documents, evaluating answer-sheets in objective-type tests e.g. GRE, GMAT:
3. Bar Code Reader
Data coded in the form of light and dark lines or bars are known as bar code, used particularly by the retail trade. Bar code reader is a device used for reading bar code data which is performed by laser beam scanner which is linked computer.
4.Desk scanning
The scanning process involves application of the light source to the image. The light is reflected back from the image into the scanner optics where the varying levels of light are interpreted. The image is then reconstructed digitally and displayed on the screen.
5. Touch Screen
It is a computer screen designed or modified to recognize the location of a touch on its surface. By touching the screen, the user can make a selection or move a cursor. This device is generally used in ATM Machine.
6. Light Pen
A light pen is a pointing device. It is used to select a displayed menu option on the CRT, a photosensitive pen like device. It is capable of sensing a position on the CRT screen when its tip touches the screen.
Other Output Devices
1. Printers
Printers are primary output devices used to prepare permanent documents for human use. Printers are classified as:
1. Impact printers
2. Non-Impact printers
Impact Printers
These operate like a typewriter, pressing a typeface against paper and inked ribbon. They are of three types.
1. Dot Matrix PrinterThese are serial printers, i.e., they print one character at a time. Each character is printed as a pattern of dots.
2. Daisy Wheel Printer These printers have a print wheel with a set of print characters on the outside tips of flat spokes. You can print a specific character through spinning the wheel.
2. Line printers These are known as Line printers because they use impact method to produce one line at-a-time printed output. E.g. chain printer, band printer drum printer.
2. Non-Impact Printers
These printers use thermal, electrostatic, chemical and inkjet technologies.
1. Thermal Printers These printers use heat to produce an image on special papet It designed to heat the surface of chemically treated paper so that a dot is produced based on the reaction of the chemical to the heat .No ribbon or ink is involved.
2. Ink Jet printer
The ink jet printer ejects a steady stream of ink drop towards the printed page. The drops are selectively discarded by electrostatic attraction to leave only those that are needed to form the desired symbol. Those that are not needed are captured in a tiny gutter & filtered to remove impurities. They are then recirculated through the drop-generating mechanism.
3. Laser Printer
The process of printing of laser printer resembles the operation of a photocopy machine. The major advantages of laser printers are, very high speed, Low noise level, low maintenance requirements and very high image quality.
Display Devices
This is one of the most important peripheral devices in the computer system, The following different kinds of display devices are available:
1.Liquid crystal Earlier LCDS were used commonly in watches and clocks. But nowadays LCD is applied to display devices of a computer. The major advantage of LCD is the low energy consumption. They also have color capability but the image quality is relatively poor.
2. Projection Displays A large screen upon which images are projected replaces a personal size of the previous displays. These systems can be connected to computer and whatever appears on the computer terminal gets enlarged and projected on a large screen. Another meth LCD Flat screen and to project the LCD Image using Overhead projector. These are popularly used for seminars, classroom lectures and presentations etc. is to connect a computer to an presentation etc.
Computer Memory
Memory Location
The memory is divided into large number of small parts called cells. Each location or cell has a unique address which varies from zero to memory size minus one. For example if computer has 64k words, memory unit has 64 • 1024=65536 memory locations. The address of these locations varies from 0 to 65535. Location is further subdivided into bits.
1.Bit
A (short for binary digit) is the smallest unit of data in a computer. A bit has a single binary value, either 0 or I. 1.
2. Byte
Abbreviation for binary term, a unit of storage capable of holding a single character. On almost all modern computers, a byte is equal to 8 bits. Large amounts of memory are indicated in terms of kilobytes (1,024 bytes), megabytes (1,048,576 bytes), and gigabytes (1,073,741,824 bytes).
Representing Data in Computers
1 Byte = 8 Bits
1 Kilobyte =1024 Bytes
1 Megabyte = 1024 Kilobyte
1 Gigabyte = 1024
3. Megabyte Primary Storage /Internal Memory/Main Memory
Alternatively referred to as main memory, primary memory, or system memory, Random Access Memory (RAM) is a hardware device that allows information to be stored and retrieved on a computer. Because information is accessed randomly instead of sequentially computer can access the data much faster. RAM is a volatile memory i.e. if the computer is turned off, all data contained in RAM is lost. RAM can be classified as DRAM (Dynamic RAM) or SRAM (Static RAM).
4. Secondary Storage
Alternatively referred to as external memory, secondary memory, and auxiliary storage, a secondary storage device is a non-volatile device that holds data until it is deleted overwritten. Their storage capacity is much higher compared to main memory of the computer. Secondary Storage devices accept data or program instructions from the processor retain them and then write them back to the processor as and when needed. The examples secondary storage devices are Floppy disk, Hard Disk, Magnetic tapes, Pen or jump drives etc.
5.Floppy Diskette
The floppy diskette is a direct access storage device although its capacity is much less than a hard disk. The diskette is made out of a flexible plastic material. This base coated with an iron-oxide recording material. Data is recorded as tiny magnetic spots. The surface is divided into tracks and sectors. The size of each sector is fixed (512 bytes). Data is stored on both sides of the diskette. There is one head per surface in the floppy drive for reading/writing data on the diskette. The circular plastic disk is enclosed within a smoothly lined, protective square jacket to protect it from dust and scratches. The standard size of a floppy is 3.5 inch. This device is now obsolete.
6. Hard Disk
It is a magnetic disk on which you can store computer data. Hard disks hold more data and are faster than floppy disks. A hard disk, can store more than 1 terabytes. A Single hard disk usually consists of several platters. Each platter requires two read/write heads for each side. All the read/write heads are attached to a single access arm so that they cannot move independently. Each platter has the same number of tracks, and track location that cuts across all platters is called a cylinder.
7. Magnetic Tape
This is also a magnetic media which stores data sequentially in them. Though they provide unlimited storage capacity they suffer form a very serious problem. Since the data stored in them is sequential in nature, retrieval of data takes a very long time. These are used for archival backup storage.
8. CD-ROM
Compact disk, read-only memory optical disks have a very large storage density and the access time is relatively low.
9. Digital Versatile
Disc Digital Versatile Disk is in genre of optical discs with same overall dimension of CDs but much higher capacity. These can store at least
times more data than CD ROM.
10. Static Memory
and Memory Sticks It is also commonly known as pen drive or jump drive or flash drive. Static memory devices use memory chips to store information. This information is retained even after power is turned off. They connect to a USB port on the computer and offer capacities of 64 GB or more. Due to their size and shape, these devices have extensively used for transportation of files between systems. Many portable and hand-held devices rely entirely on static memory for storage.
11. Semiconductor Memory
These days, internal memory consists of extremely small bit storage circuits (flip-flops} etched on a silicon chip. All the electronic elements to store a bit are placed in such a small area of the chip that a single chip can store millions of bits. The individual chips are arranged in groups to form a memory module.
Types of Semiconductor Memory
1. Random Access Memory (RAM)
Any information can be read from and written AM. It is a read/write memory. It is volatile memory i.e. its contents are lost power supply is interrupted or turned off. The main memory of the computer is RAM.
2. Read Only Memory (ROM)
ROM permanently programmed with information during manufacture, by implementing the appropriate pattern of two state values, It cannot be changed subsequently by a normal write operation. It thus completely non-volatile. It is mainly used to hold those programs, which are required permanently.
3. Programmable Read Only Memory (PROM)
This can be 'programmed to record information using special electronic equipment known as a PROM programmer. However, it cannot be changed subsequently.
4. Erasable Programmable Read Only Memory (EPROM)
EPROM is a PROM, which can be reversed by exposing it to an ultraviolet light source. The device can be re-erased and re-programmed again and again.
5. Cache Memory
It is a small capacity high-speed memory used to make processing fastet. The main memory can process information very fast, but it takes much longer to transfer data to and from the input/output devices. The cache memory compensates for this mismatch in operating speeds. It holds those parts of data and the active program, which are most frequently used. Thus, the performance rate of the CPU improves
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