COMPUTER

Introduction

In this article we will study about the structure of a computer system, characteristics of computer systems and types of computer systems. We will study some most commonly used auxiliary storage devices, such as tapes, disks, etc.

What is a Computer System?

The word computer means a programmable electronic device that can store, retrieve and process data. Therefore a computer is normally considered to be a calculating device that takes in facts known as data and, following instructions, it processes these facts to produce information. This data or information can be letters, numbers, pictures and symbols.

In order to process this data computer needs two things – hardware and software. By hardware it means the physical components of a computer system that we can see, touch and feel. By software it means the instruction or the set of instructions that a computer receives in order to carry out its tasks. These sets of instructions are called as computer programs. Hardware is usually fixed in design, but software can be easily changed. The ease with which software can be manipulated is what makes the computer such a versatile and powerful tool. Here we will discuss the computer hardware components and their functions.

Computer Hardware Components

As stated earlier, hardware refers to the physical pieces of a computer system that is used to store, process and retrieve data or information. The verb store, retrieve, and process imply three basic components of most computers:

• Memory Unit
• Central Processing Unit
• Input/Output Unit
• Auxiliary (Secondary) Storage Devices

Figure (a)

However a computer system has also auxiliary storage units to store the data or information permanently. Figure–a shows the block diagram of a typical computer system having these hardware components. In this section we will discuss these three computer hardware components as well as auxiliary storage units.

Memory Unit

The term memory unit holds both data and instructions for immediate access and use by the computer’s central processing unit during processing. In other words, the memory unit stores data or program currently in use. Actually the memory unit is an ordered sequence of storage cell, each capable of containing a piece of data. These memory storage cells are known variously as memory cells, memory locations, or places in memory. Each memory cell has a distinct address to which one refers in order to store or retrieved information. The memory unit holds data (input data or results of computation) and instructions (programs) as shown in figure-b.

Figure (b)

The memory unit consists of two important registers – memory address register (MAR) and memory buffer register (MBR). The memory unit communicates with the outside word through these two registers only. If we want to read the contents of a memory location then its address is placed in MAR and a read cycle is executed. And if we want to write something into a location then the location address is placed into MAR and the data to be stored is placed in the MBR before executing a write cycle.

This memory unit provides short-term, high-speed storage linked to the central processing unit for fast retrieval. However most memory requires a constant supply of electricity to hold data because they are typically consists of either magnetic cores or semiconductor circuits. The earliest form of main memory was based on vacuum tubes. Although the technology is changing, most memory units today is considered volatile, meaning that when the computer’s power goes out, intentionally or accidentally, the memory unit drops its data. This type of memory unit is called as main memory or primary memory unit because it is internal data storage of a computer.

The main memory is random access memory (RAM). The name drives from the fact that data or instruction can be stored in and retrieved at random, from anywhere, in the electronic main memory chips in the approximately same amount of time, no matter where the data is.

Read Only Memory (ROM)

Figure (c)

The very first question that come in your mind is – How does your computer know what to do when you turn it on and how does it know to check out your hardware components to see whether they have been connected correctly? The instructions that perform these critical operations can not be stored on random access memory as it is a volatile memory. Therefore these instructions are stored on a non-volatile memory, known as read only memory (ROM). This ROM is installed by the vendor inside the computer system. This ROM is also called as firmware, because it retains instructions permanently even when the power is off.

In earlier days, the cost of recording of data in ROM was very high, therefore it retained only those instructions that were very crucial to the operation of the computer system. But in recent years, due to technology advancement it may contain some additional software instructions.

In addition to ROMs, three additional categories of non-volatile memory are used, such as PROMs, EPROMs and EEPROMs.

PROM, stands for Programmable Read Only Memory, works similar to ROM, additionally the user can determine what data and instructions are recorded on them. But like ROM, the user can not modify the contents the PROM, once the data is recorded on them.

EPROM, stands for Erasable Programmable Read Only Memory, is one step after PROM. In this the data can be erased by ultra-violet light and new data can be recorded on it. To change instructions on the EPROM chip, the chip must be taken out of the system and then put back when changes have been made.

EEPROM, stands for Electrically Erasable Programmable Read Only Memory, is the latest addition to this family. In this changes can be made electrically under software control, rather than having to take chips out of the computer to change data. The only disadvantage of EEPROM is that they currently cost substantially more than regular ROM chips.

Central Processing Unit (CPU)

Central processing unit is the part of the computer system that executes the instructions of a program stored in memory unit. Central processing unit is the brain of the computer system because all the operations (storing/processing/retrieving) are performed by this unit. Central processing unit is the most complex computer system component, responsible for directing most of the computer system activities based on the instruction provided. The central processing unit circuitry of a microcomputer, called a microprocessor, fits on a chip about the size of your thumbnail.

Central processing unit has two important functional units – arithmetic/logic unit and control unit, and a set of control registers collectively known as Program Status Registers (PSR) that contains all the information defining the status of CPU at any instant of time. The parts of the computer system are usually connected by an electronic component called as a bus.

Arithmetic/Logic Unit (ALU)

Figure (e)

The arithmetic/logic unit performs all arithmetic operations (addition, subtraction, multiplication, division) and logical operations (comparisons of two values). Once the data is stored into primary storage unit from any input device, they are brought in and transferred as needed to the arithmetic/logic unit where processing takes place. You will be surprised to know that data is moved from primary storage unit to arithmetic/logic unit or vice versa many times before the processing is completed. And after the completion of processing, the result is stored in an output storage section and from there to an output device. However the intermediate results generated in the arithmetic/logic unit are temporarily placed in a designated working storage area until needed at a later time.

The arithmetic/logic unit controls the speed of calculations and therefore receives a great deal of attention from computer or IT engineers trying to meet the needs of the fast-paced business world.

Control Unit (CU)

The control unit controls the actions of the other components of the computer system in order to execute instructions (the program) in sequence. In other words, the control unit is responsible for directing and coordinating most of the computer system activities. It is true that the control unit does not process data but it definitely acts as a central nervous system for other data manipulating components. Now let us see how arithmetic/logic unit and control unit collectively perform the processing of the execution of an instruction.

It is already stated that PSR contains all the information defining the status of CPU at any instant of time. There is a field in PSR, known as instruction address register (IAR) that contains the address of next instruction to be executed. It is the control unit that initiates the processing of this instruction by sending this address to the memory unit for obtaining the information. This is called instruction fetch. After this, the instruction obtained is decoded and its operands are also fetched from the memory unit. This is called instruction decoding. Finally the actual instruction is passed to the arithmetic/logic unit for appropriate execution. This is called instruction execution. These three actions are collectively called as fetch-decode-and-execute.

The arithmetic/logic unit also contains a set of fast registers popularly known as general purpose registers (GPRs). These registers play an active part in the addressing, arithmetic and logical operations.

Bus

As stated earlier the term bus refers to an electrical pathway through which data are transmitted between the various computer system components. There are several types of buses in a typical computer system. For the user, the most important one is the data bus, which carries data through the CPU.

Traps/Interrupts

It is also the duty of the central processing unit to constantly monitor the occurrences of certain exceptional or special situations within and outside itself which would require immediate attention, during the execution of designated instructions.

Consider a situation where an arithmetic overflow/underflow occurs during the execution of an instruction. In this case the machine hardware raises a trap which is sensed by the central processing unit at the end of the current execution. Now it is the central processing unit which can divert from the execution of the current instruction sequence and performs some special actions to recover from the situation. On the other hand if an interrupt is raised by a special condition outside the central processing unit, such as pressing of a key by a programmer during the execution of an instruction, then the central processing unit can execute a special sequence of actions to appropriately handle the situation.

Thus like trap, the purpose of interrupt is also to attract the attention of central processing unit towards it. That’s why both traps and interrupts are generally termed as interrupts alone.

Input/Output Unit

The input unit is that unit which provides user interaction with the central processing unit and the output unit is that unit which provides a central processing unit interaction with user. The input/output unit consists of various input/output devices. Let us discuss various types of input and output devices.

Input Devices

The parts of the computer system that accept data to be processed from the user are called input devices. Computer uses a variety of input devices for input purpose. These input devices are broadly divided into two categories – direct input devices that allow direct human machine communication and indirect input devices that require data to be recorded on an input medium, such as magnetizeable medium.

The keyboard and mouse are typical examples of direct input devices and auxiliary storage devices are  examples of indirect input devices. Here we will discuss only the typical direct input devices.

The Keyboard

The keyboard of the computer is the most important device for communication between the computer operator and the computer. Figure-g shows a typical diagram of the keyboard. The keyboard used for computers is similar to that of typewriter. The data can be entered directly into the computer through the keyboard by pressing keys. When any of the keys is pressed, the keyboard transmits the information to the computer. A typical keyboard has following set of keys:

• Full set of alphabetic characters and numbers
• Special characters like @, #, $ and so on
• Function keys F1 to F10 (or F12) which are used by different software according to their needs
• Shift and Caps Lock keys which affect the case of the alphabetic characters
• The Enter key used to submit command
• Backspace key to erase the last character
• Esc (Escape) key used to cancel some program or action
• Ins (Insert) and Del (Delete) keys to insert and delete characters
• Tab key similar to the Tab of the Typewriter
• Cursor keys (arrow keys) to move the cursor one character left, one character right, one line up and one line down, and the associated Home, PgDn, PgUp and End keys
• Some special purpose keys, such as PrintScreen (prints whatever on the screen, except when it operates in graphics mode), Scroll Lock and Pause/Break

Most modern computers have a keypad with either 101 or 102 keys (the original keyboards had only 84 keys). The newer type of keyboards has an extra set of cursor movement keys.

The Mouse

Figure (h)

A mouse is a small pointing object, containing one to three buttons, that moves a pointer on the video monitor when it is scrolled on a flat surface (usually a mouse pad). Figure-h shows a typical diagram of a mouse. It has rollers on its undersurface area on the desk, which senses the magnitude and direction of the movement using a system of wheels and axles. This information is passed as a signal to the connected computer. The keyboard interacts with all programs, but graphical programs, such as Windows, also work with a mouse. Actually a mouse accomplishes function visually instead of verbally. You point to the thing you want to do and click a button. It is often faster and easier to accomplish task using a mouse.

A mouse pointer can have more than one shape depending upon the user choice. Whenever you want to point at any object, move the mouse pointer close to or on top of the object. If you want to click, you press and releases a mouse button without moving the mouse. However if you want to double click then you click twice in rapid succession without moving the mouse, finally if you want to drag an object, you point at the object and press and hold down the appropriate mouse button while you move the mouse in the direction in which you want the object to go. After reaching at the desired location you release the mouse button to drop it here.

Now a days most software use mouse as an input device in addition to the keyboard and you can select menu options, navigate screen, etc. very effectively, using a mouse.

Output Devices

The parts of the computer system that is used to display or print the results are called as output devices. Computer uses a variety of output devices, such as Visual Display Unit, printers, plotters, photographic output devices, punched card/paper tape and so on. However Liquid Crystal Display (LCDs) are also becoming more commonplace as output device. Here we will study the most frequently used output devices, that is Visual Display Unit and printers.

Visual Display Unit (VDU)

A Visual Display Unit (VDU) looks like a TV screen, which uses a cathode ray tube (CRT) to display information, as shown in figure-i. When the data input is typed through the keyboard, electric signals are generated. The CPU converts these signals into alphanumeric or graphics display. Thus the output is obtained as optical display. The Visual Display Unit (VDU) is also called as monitor or screen.

Using Visual Display Unit, one can also see the information typed at his/her keyboard. In this way the Visual Display Unit enables you to have a visual check of the information whatever you are typing on your keyboard. When you press a key, the corresponding character is displayed on the Visual Display Unit. If you type wrong data accidentally (before its processing) then you can easily correct it.

On monitor everything, if it is in text mode, will be displayed in 24 lines and 80 columns. However the mode of operations of some computers can also be changed. In such case, the Visual Display Unit works with high resolution. The mode of dividing the screen into small square grid areas for visual display is known as resolution. One square area is meant for displaying one dot and a group of such dots makes up a pixel. Thus every character is made up of several pixel. In other words you can say that a pixel is the smallest individual entity on the screen. The most common technique for forming characters on video screen is the dot matrix consisting of 35 pixels (5*7). The more is the number of dots or pixels in a screen, the better will be the capability of computer to display information.

Some new monitors also provide monitor control buttons which help you to adjust the image displaying on the screen, such as Horizontal position, Vertical position, Horizontal size, Vertical size, Brightness, contrast and so on.

The Printers

Printers are output devices which are used to get the “hard copy” of the output on paper (and some other media like transparency slides also). Printers are primarily used to prepare documents, reports, data dumps and document that include graphics, in human readable form on paper.

Printers are broadly divided in two categories:

• Impact Printers
• Non-impact Printers

In an impact printer, the print head strikes the ribbon to deposit ink or carbon on the paper. They use electromechanical mechanism for the printing. As impact printers strike against the paper, they can be used for printing multiple copies by using paper with carbon but due to mechanical movement, they are slow. On the other hand, non-impact printers do not use a striking mechanism, rather they make use of thermal, chemical, inkjet technologies to print the characters. Non-impact printers can not be used for multiple copies but are usually faster.

As far as printing characters is concerned, printers can also be classified into three categories:

Character Printers – Character printers print a character at a time. Such printers are also called as serial printers. Examples of character printers are dot matrix printers (DMP), daisy wheel, thermal wheel, and inkjet printers.

Line Printers – Line printers print a line at a time. Examples of line printers are – Wheel printers, Chain printers and Drum printers.

Page Printers – Page printers print an entire page at a time. Laser printers are typical example of page printers. Laser printers use the laser technology, that is by means of a laser beam directed on a photo sensitive, which is made visible by a toner and transferred and fixed on paper. The laser printers are capable of producing very high speed quality products at a very high speed at the rate of 4 -12 standard pages per minute.

Printers vary greatly in price and capability by considering the various factors, such as type of printing, speed  and volume of printing the printer will have to support etc. Printers are usually connected to the parallel port of a computer. A parallel interface transmits 8 or more bits at the same time.

Auxiliary Storage Devices

An auxiliary storage device (sometimes called as secondary storage device) is used hold coded data, ready for use by the computer, in between the times when we actually want to use the data. Auxiliary storage devices are installed into the system cabinet of the personal computer (PC), but these devices are considered external to the computer’s main (primary) memory. Auxiliary storage devices hold data and program permanently even when the power supply to the computer system is off. Whenever the data, which is stored on an auxiliary storage device, is needed, we tell the computer to transfer the data from the auxiliary storage device to its memory.

Typical auxiliary storage devices are magnetic tape drives, magnetic disk drives, magnetic drums, Winchester disks and optical disks. Here we will discuss some important ones.

Magnetic Tape Drives

A magnetic tape drive is a unit in which the data or information is stored and retrieved in a strict sequence manner. Figure-j shows the diagram of a typical magnetic tape drive. It consists of magnetic tape which is half inch wide 2400 feet long plastic coated with a magnetic material wound on “spools” of 10.5” diameter that have metallic cores and plastic sides. It is similar to the tape on tape recorder except that it is of a higher quality and more durable. Data or information is recorded on the magnetic tape in the form of magnetized spots with storage density varying from 200 to 1600 characters per inch.

In magnetic tape drive, the magnetic tape is wound on one “spool” and another “spool” is needed for a read or write operation. As tape is unwound from one spool it gets wound on another spool. The tape is moved and brought to a constant speed by the device before a read or write operation can be performed. And after the tape is finished it is rewound on the original spool for other read/write operation. Between these two spools, there is a read/write mechanism, which is used to access information or to place information on a tape.

Data is recorded on the magnetic tape as a series of blocks and each block is distinguished by a gap called inter record gaps (IRGs) . These inter record gaps provide the required time for acceleration and deacceleration of tape during reading/writing operation. Here one should remember that a reading/writing operation always transfers one block of data.

Though the information can be accessed serially it is most widely used storage device where information is needed in a sequence. And it is also a convenient way of carrying information from one place to another. Another plus point of using magnetic tapes is that they are reusable. Additionally a magnetic tape acts as an input/output device. Information from the magnetic tape is fed to the computer and the output is transferred to the magnetic tape and kept for future use. The main limitation with magnetic tape is that it moves in one direction only, that’s why only reading/writing operation can be performed at a time.

Magnetic Disks

Magnetic disks are another most frequently used auxiliary storage device. A magnetic disk drive may be thought of as a cross between a record player and a tape recorder because it uses a thin platter made out of a magnetic material. Like record/playback head in a tape recorder, it has a read/write head that moves in and out in the radial direction on the top of magnetic surface and reads or writes on the magnetic surface. A spindle is connected to a rotating mechanism that enables the disks to spin at a high speed around its axis.

Though the length of the track may vary, each contains the same number of characters. It means that the outer track have less density of character than inner one. In other words you can say that the recording density of data on the tracks nearer to the center of the disk is same as those nearer the edge of the disk. That’s why the sector on an outer track is physically longer, whereas a sector on an inner track is physically shorter.

Many such magnetic disk plates form a disk pack. In a disk pack the same number of track on all the disks together form a cylinder. The magnetic disk drive is connected directly to the computer through a disk controller. A disk controller can control more than one disk drive. The disk controller is a piece of electronic circuit, which sends command to the disk drive.

When the computer needs to access some data, the disk controller performs some specified actions, such as moving the head assembly to the corresponding cylinder, activating the head on the related track when the target sector passes under it. Thus a typical the typical disk access time is defined as:

disk access time = seek time + rotational latency + transfer time

Seek time is the time taken by read/write head to reach the specified cylinder/track. Normally seek time constitutes a very high percentage of the disk access time. Rotational latency is the time spent in waiting for the correct sector to come under the read/write head as the disk rotates. This time is also called as latency time. Like seek time, the latency time is also variable because the read/write head may take a maximum of full rotation or a minimum of zero rotation. Transfer time is the time necessary to transfer the contents of a sector to the memory buffer of the disk controller. The transfer time is usually the smallest of the three and it is highly dependent on the sector size.

Unlike magnetic tapes, magnetic disks are random access devices in which information can be stored and retrieved randomly. Basically magnetic disks are of three types:

1. Hard Disks
2. Floppy Disks
3. Winchester Disks

Hard Disks

A hard disk consists of several hard, rigid magnetic plates that are held together on a common spindle. In such type of disks, information can be stored on both sides of all plates except the outer surface of the top and bottom plates. Figure-l shows a typical diagram of a hard disk.

The magnetic disk is divided into a fixed number of concentric circles, called tracks, which may vary from 100 to 1000. The tracks are normally numbered from 0 as the outermost track, with the number increasing inwards. All tracks are divided into sectors. A sector is the smallest unit of disk space that can be utilized, and all sectors on a magnetic disk have the same capacity in terms of number of bytes, say 512 bytes per sector

Hard disks can be further of two types – fixed hard disks or removable hard disks. Fixed hard disks can not be easily removable from their read/write mechanism. That’s why they rotate at very high speed. On the other head, in removable hard disks the read/write head mechanism can easily withdraw itself from the disk pack. Since the read/write head assembly and the disk packs are not tightly coupled, the speed of removable hard disks are lower than that of fixed heads.

Floppy Disks

A floppy disk is an electromechanical assembly, containing a flat magnetic disk coated with magnetic material on one or both sides. The coated disk is covered in a square jacket, typically made of plastic material. The inner side of this jacket is very smooth so that it can not hinder the rotation of the floppy while it is in the jacket. It is called floppy because when it swung like a hand fan it flops.

There is a hole in the center through which the spindle of drive unit rotates the disk. There is also a window that enables read/write head to make a close contact with storage surface of the disk. Additionally it has a third hole by which the beginning of a track is determined by optical means. It has a write protect slot, which can be used to prevent the accidental deletion/overwriting of data on the floppy.

Floppy disks are available in standard sizes of 3.5”, 5.25” and 8.0”. The 5.25” floppy disk is known as minifloppy disk and 3.5” floppy disk is known as microfloppy disk. Single-sided and double-sided disks with double or high densities are available. The capacity of 5.25” floppy disks vary from 360KB to 1.2MB and that of 3.5” floppy ranges to 1.44MB. Figure-m shows a 5.25” floppy disk and 3.5” floppy disk.

Figure m

The floppy disk is a low cost device, which can be erased and reused many times. The main characteristics of a floppy disk is its low cost, compact and highly reliable.

Winchester Disks

Winchester disks are hermetically sealed units, which contain disk drives along with the disks. The sealing of all the components of Winchester disk prevent contamination from dust and other small particles. In such type of disks, the read/write heads are designed to take off and land on the disk surface. To reduce the friction, the disk surface is coated with a special lubricant. Since its units are sealed, therefore its maintenance costs are completely eliminated.

This technology enables larger number of tracks, more compact encoding of data, higher speeds, lower costs and more reliability. It comes in sizes varying from 1.8” to 14”, with storage capacities varying from as little 10MB to GigaBytes.

Optical Disks (CD-ROM)

Compact Disc – Read Only Memory (CD-ROM) marks the beginning of a new era during which the emphasis likely to switch from magnetic storage to optical technology. CD-ROM was announced as an ‘information storage medium’ by Philips and Sony in October 1983, with the standard physical format issued in June 1985. The standard 12-cm diameter CD-ROM supports upto about 660Mbytes data capacity. You can say that approximately 400 1.44MB floppy disks is equal to a single CD-ROM disc. A CD-ROM can store upto 250,000 A4 pages of text or approximately 100,000,000 words.

Like audio CD, CD-ROM disc physically consists of a metallic disc bonded to a polycarbonate base. And it is coated with a transparent, protective lacquer. It is a reflective light system, where laser light is shone against turns of track CD-ROM which are encoded digital data using pits and areas of land. Pits scatter the laser light, while areas of land produce reflective light. After this the reflected light is diverted to a photodetector that produce a series of electrical pulses corresponding to encoded data. It rotates at a very high speed