The basic components and block diagram of a typical computer system was briefly described in previous article of "Basic Information Technology" however the study about how a computer operates is still incomplete. The four main components and all of the other elements of a typical computer system will studied in depth in this article so that one could be able to understand what role does these components and their sub-components play during the time when computer is operating. In other words, we can say that this article is focused over the microscopic study of a computer system that will cover every minute detail in addition to describing the well-known fundamental concepts.
As we have studied earlier that a computer system is an integration of hardware and software, the main hardware components like Input/Output peripherals, secondary storage devices, primary memory and its types and processor along with its components like ALU, CPU and set of registers will be discussed with full detail.Then come the interconnection elements like buses and ports that provide the way of communication or transfer of data/information within or out of the Central Processing Unit (CPU). The way simple machine instruction are organised and their Fetch-Decode-Execute cycle will also be discussed in detail. After the complete understanding of these components and criterion, the types and functionalities of different software (both system and application ) given at the end of this article. Thus we can have a full coverage of studying how a computer works as a full fledge data/information-processing system
Computer Architecture:
Computer architecture means the combination of different hardware components / parts within the computer system. In other words, we can say that computer architecture is the way to organise the four main components and connect them the obtain a complete computer system which are given one by one below:
- Processing Unit
- Primary / Internal / Main Memory
- Secondary / External / Auxiliary Memory
- Input / Output Peripherals
In addition to these basic components of a working computer system, we have buses, ports and cables that provide interconnections among the above main components. The block diagram of a typical computer system can be given as below
In this block diagram, the main hardware components of a typical computer system are connected together alongwith their subparts in such a way that the internal memory is directly connected to the processor and due to the reason internal memory especially RAM is called online to the processor. The other components are called peripherals that include Input / Output and secondary storage devices. These peripherals are indirectly connected through RAM and are also called offline to the processor. The different components of this block diagram can be explained one by one below:
- Processor:
This part of the system unit is also called CPU, which stand for Central Processing Unit is a complete box containing processor or CPU, mother / main board, main memory, hard disk within hard disk drive, floppy disk drive and buses. The system unit also contains ports on its rear-side for getting connected to the external Input / Output devices.
A processor or CPU by itself may be combination of different components. Its main parts include Arithmetic and Logic Unit (abbreviated as ALU), Control Unit (Abbreviated as CU) and a combination of different general - purpose and special - purpose storage locations called registers. ALU is responsible for perform ordinary arithmetic operation (like addition, subtraction, multiplication, division and modulus etc) along with different logical or comparison operations (like Less Then, Greater Then, Equal To, Not Equal To, Less or Equal and Greater or Equal, And, Or & Not). The arithmetic and logical operators concerned with the arithmetic and logic are applied during these operation.
The Control Unit (CU) acts in the way similar to the human nervous system to monitor and control the different components of the computer system during their functions whereas different registers of the processor provide storage space to hold the data temporarily in order to be processed. A special type of processor called Co-processor performs the different scientific computations and complex mathematical functions. Now a days a special hardware scheme called cache memory is introduced that is a small amount of storage within the processor. This cache memory is faster than main memory and slower than the processor's registers. The purpose of the cache memory is to make the most frequently used data or information readily available to someone when it is in the pocket. In short, all of the computational operations are performed within the computer processor.
The speed of processing is usually measured in Mega Hertz (MHz) or Giga Hertz (GHz). Up to 3.3 GHz capacity of processor are readily available in the market for Pentium - 4 or the latest (HT) Hyper Thread computer technology. - Main Memory:
Main Memory is called primary memory or internal memory or real memory and it is electronic in nature. It stores the data electronically. It has its two well known parts i.e. RAM and ROM which are described one by one below: - Random Access Memory (RAM)
One distinguishing characteristic of RAM is that it is possible both to read data from the memory and to write the data into the memory easily and rapidly. Both these reading and writing processes are accomplished through the use of electronic signals. The other distinguishing characteristic of RAM is that it is volatile. A RAM must be provided with a constant power supply. If the power is interrupted, then the data are lost. Thus RAM can be used only as a temporary storage.
RAM technology has been divided into two technology i.e. static and dynamic RAM so that:
* A Dynamic RAM or DRAM is made with cells that store data as charge on capacitors. The presence or absence of charge in capacitor is interpreted as binary 1 or 0 because capacitors have a natural tendency to discharge DRAMs require periodic charge refreshing to maintain data storage.
* In a Static RAM, binary values are stored using traditional flip-flops logic-gate configuration. A Static RAM or SRAM will hold its data as long as power is supplied to it.
Both the Static and Dynamic RAMs are volatile. A Dynamic memory cell is simpler and smaller than Static memory cell. Thus a Dynamic RAM is denser because of smaller cells there are more cells per unit area and less expensive than a corresponding Static RAM. On the other hand, a Dynamic RAM requires the supporting refresh circuitry. For large memories, the fixed cost of refresh circuitry is more than compensated for by the smaller variable cost of Dynamic RAM cells. Thus Dynamic RAMs tend to be favored for large memory requirements. Finally, Static RAMs are generally somewhat faster than Dynamic RAMs. - Read Only Memory (ROM):
In sharp contrast to RAM is the Read Only Memory (ROM). A ROM contains permanent pattern of data that can not be changed. While it is possible to read a ROM, it is not possible to write new data into it. An advantage of ROM is that the data or program is permanently in main memory and doesn't need to be loaded from the secondary storage devices. A ROM is created like any other integrated circuit chip as part of the fabrication process. This presents two problems:
* The data insertion step includes a relatively large fixed cost whether one or thousands of copies of a particular ROM are fabricated.
* There is no room for error. If one bit is wrong, the whole batch of ROMs must be thrown below: - Programmable ROM (PROM):
When only a small number of ROM with particular memory content is needed, a less expensive alternative is the Programmable ROM called PROM. Like the ROM, the PROM is non-volatile and may be rewritten only once. For PROM the writing process is performed electrically and may be performed by a supplier or customer at a time later than the original chip fabrication. Special equipments are required for the writing or programming process. PROM provides flexibility and convenience.
Another variation on Read Only Memory is the Read Mostly Memory, which is useful for applications in which read operations are far more frequent than write operation but for which non-volatile storage is required. There are three common forms of Read Mostly memory: EPROM, EEPROM and Flash Memory. These are described one by one below: - Erasable Programmable Read Only Memory (EPROM):
The optically Erasable Programmable Read Only Memory (EPROM) is read and written electrically, as with PROM. However, before a write operation, all the storage cells must be erased to the same initial state by exposing the packaged chip to ultraviolet radiation. This erasure process can be performed repeatedly so that each erasure can take as much as 20 minutes to perform. Thus the EPROM can be altered multiple times and like the ROM and PROM holds its data virtually indefinitely. For comparable amounts of storage, the EPROM is more expensive than EPROM but it has the advantage of multiple update capability. - Electrically Erasable Programmable Read Only Memory (EEPROM):
A more attractive form of read-mostly memory is called Electrically Erasable Programmable Read Only Memory (EEPROM). This read -mostly memory that can be written into at any time without erasing prior contents i.e. only the byte or bytes, which are addressed are updated. The write operation takes considerably longer than the read operation, that is several hundreds microseconds per byte. The EEPROM combines the advantage of non-volatility with the flexibility of being update-able in place, using ordinary bus control, address and data lines EEPROM is more expensive than EPROM and is also less dense, supporting fewer bits per chip. - Flash Memory:
Another form of semiconductor memory is Flash Memory, which is also called due to the speed with which it can be reprogrammed. Flash memory is intermediate between EPROM and EEPROM in both cost and functionality. Like EEPROM , Flash memory uses an electrical erasing technology. An entire Flash memory can be erased in one or few seconds, which is much fasten than EPROM. In addition, it is possible to erase just blocks of memory rather than an entire chip. However, Flash memory doesn't provide Byte level erasure. Like EPROM, Flash memory uses only one transistor per bit and so achieves the high density of EPROM as compared to EEPROM.
ROM plays a very important role in booting the computer system. Booting is actually the phenomenon in which the computer system is properly checked by the ROM's routines to coordinate the loading of operating system from secondary storage to RAM. It is performed during the period of switching on the computer to the display of specific user interface screen such as Windows Desktop or DOS command prompt. - Secondary Memory:
Mass storage, external auxiliary memory are the other names of secondary memory. This memory is slower than the processor and main memory as it involves mechanical motion techniques during the storage and retrieval of data. This memory is larger in size as compared to main memory however the processor is unable to access this memory directly due to its offline loaded into RAM before the processor starts processing it. The reason that it is offline to the processor is that its speed is considerably slower than the processor and that's why RAM must play its role to link this memory to the processor.
Some of the distinguishing features of secondary storage are: - Secondary storage is magnetic in nature and therefore magnetic mechanisms are used to store the data permanently.
- Data or information is stored in the form of files so that a file may be defined as "an area of the secondary memory where data or information is permanently stored". Each file may have its unique file name or identification through which it can be accessed.
- The storage of data in secondary memory may follow certain file organisation technique such as Sequential, Indexed Sequential and Random/Direct access file organisation. The choice of a particular file organisation highly depends upon the nature of secondary storage device e.g. Sequential access file organisation is adopted for Magnetic Tape whereas Direct/Random access organisation of file is the most suitable for floppy or hard disk.
Some of the advantages of secondary memory are: - Data remains permanently stored even when the computer is switched off. This data or information remains in the secondary memory till its deletion by the computer user.
- Very high volumes of data or information could be recorded for longer periods and may be updated and retrieved efficiently when needed.
- Transfer of data or information can also performed with the help of this memory e.g. floppy disk and CDs are especially suitable to promote data from one computer to another.
- A very major benefit is that the system files associated with any operating system are permanently resident within this memory. These files are loaded into RAM at the time of booting the computer system.
- In this case of any damage to the computer system or unpleasant loss of data, the backup and recovery procedures and facilitated through this memory.
Some of the most commonly used secondary devices and storage devices and their important characteristics are described one by one below in detail: - Magnetic Disks:
A disk is a circular platter that consists of metal or plastic coated with certain magnetizable material such as Ferric Oxide. A conducting coil named as head is used to record data on the disk surface and retrieve it later on. This head is in touch with the disk surface during a read or write operation while the platter is continuously rotating beneath it.
The write mechanism is based on the flow of electricity through the coil that produces a magnetic field due to which magnetic patterns are recorded on the surface below with difference for positive and negative currents. In the reverse, the read mechanism is based on the motion of magnetic field relative to coil that produces an electrical current in it. When the surface of the disk passes under the head, it generates a current of same polarity as already recorded. This is how electronic signals from RAM comprising the data are interpreted into their magnetic counterparts and vice versa during the read and write operations. respectively.
Disk Formatting: In order to make the magnetic disks usable for data storage, they need to be properly formatted according to the formatting capability of a particular operating system the computer is using. Concentric set of circles or rings called tracks are formed during the process of formatting so that each track is similar in width to head. Typically 500 to 2000 tracks per surface are possible. Gaps are used to separate adjacent tracks in order to prevent or at least minimize the chance of errors due to misalignment of the head or simply due to the interference of magnetic fields. The same number of bits is typically stored on each track due to which the density i.e. bits per linear inch increases in moving from outermost to innermost track.
The data is transferred to/from the disk collectively in the form of blocks and these blocks are usually smaller than the capacities of their respective tracks. The data is stored in the block-sized regions called sectors so that there are typically 10 to 100 sectors per track. The length of these sectors may be fixed or variable. Adjacent sectors are separated by inter-sectors gaps to avoid any sort of errors caused by their mutual interference.
Access Time: The time required to access and retrieve the data may depend on three factors that are stated one by one below: - Seek time i.,e. the time during which read/write head is placed over the exact track to pick up data from.
- Latency time i.e. the time during which the disk is rotated continuously till coming of correct sector under the read/write head.
- Data Transfer Rate i.e. the time during which the data is transferred from the disk surface to the main memory i.e. RAM.
The Access Time can be calculated by using the relation:
Access Times = Seek Time + Latency Time + Data Transfer Rate
Typs: Two main types of magnetic disks that are most commonly used are: - Diskette or floppy contains a thin circular piece of plastic coated with Ferric Oxide that is enclosed in a rigid plastic jacket for protection. There is a special opening in the jacket through which the disk is read. In standard 3.5 inch disk, this opening is protected by metallic covering untill the diskette is inserted into its drive for accessing data from it. There is a write protect procedure that prevents a disk from any sort of overwriting. There is a small hole in the corner of the diskette that is to be opened for keeping the diskette write protected. These floppies are also called move-able disks, as they are capable to be be transferred from one computer to another one easily.
- Hard Disk is composed of one or more platter that are permanently sealed within a hard metallic casing. Now a days up to 1 Tera Byte hard disk are available in the market. These hard disks are fixed in computer CPU and are seldom transferred from one computer to another. For the better use of the disk space, a hard disk can be divided into any number of partitions like C: D: E: etc however making too many partitions is not a good management practice for the memory of hard disk
- Magnetic Taps:
Magnetic tape and the tape drives are analogous to a home tape recorder system. It uses the same reading and recording techniques as that of the magnetic disk as the medium used in it is a flexible tape that is coated with magnetic oxide. The tape medium is composed of a particular number of parallel tracks. The number of these tracks was 9 in earlier taps due to which one byte data could be stored at a time while the 9th bit was used for an additional parity bit. However, never tape systems use either 18 or 36 tracks that correspond to a Word or Double Word respectively. As the data from a magnetic disk is read and written in contiguous blocks called Physical records, blocks on the tape are separated by gaps that are called inter-record gaps. The data is written in "frames" across the tape surface so that one Byte may be represented with the help of a single frame.
A magnetic disk drive is sequential access which means that for n records, when n=0,1,2,3, ............ if the tape head is positioned at record number 1 then in order to read the nth record, it is necessary to read all the physical records from 1st to nth records one at a time. IF the head position is beyond the desired record, it is necessary to rewind the tape for a specific distance and begin reading forward. Unlike a magnetic disk, the tape is in motion only during a read or write operation. As with a disk, the tape is formatted to assist in locating the physical records.
In contrast to the magnetic disk, which is a direct-access device, a tape is sequential in nature. A disk drive doesn't read all the sectors on a disk sequentially to get to the desired record whereas a magnetic tape drive reads all the sectors between the starting and the desired location of data. Magnetic tape was the first kind of secondary memory and is still widely used for its lowest cost however it is very slow in speed than all of the secondary storage devices. - Optical Memory:
Both the magnetic disk and tape use magnetic mechanisms to permanently store the data however, the amount of data stored by such techniques is often limited or it will need hard disk that is not easily transferred from one computer to another one. Another method of storing voluminous data like one including sound, moving picture video, animated sequences, text and graphics etc is provided by the optical memory that uses LASER beam technology for burning tiny holes in the surface of a disk. Such a disk is digital optical storage medium usually made of plastic. The presence of absence of a hole corresponds to a digital 0 or 1 and a LASER beam in a optical disk reader can read the information contained in it. Some of the Optical memory devices are - Compact Disk (CD) is a non-erasable disk that stores the digitised audio information. The standard system uses 12cm disks and they can record more than 60 minutes of playing time without any interruption.
- Compact Disk Read Only Memory (CD-ROM) is non-erasable dis for storing the computer data. The standard system uses 12cm disks and can hold more than 600 MB data.
- Digital Versatile Disk (DVD) is a technology for producing digitised, compressed representation of video information as well as large volumes of other digital data.
- Write Once Read Many (WORM) is a disk that is more easily written than CD-ROM thus making single copy disks commercially feasible. As with CD-ROM, after performing the write operation the disk is read-only. The most popular size 5.25 inch that can hold data from 200 to 800 MB.
- Erasable Optical Disk is a disk that uses optical technology but that can be easily erased and rewritten. Both 3.5 inch and 5.25 inch are in use. Typically they have data storage capacity of 650 MB.
- Magneto-Optical Disk is a disk that uses optical technology for read and magnetic recording techniques assisted by optical focusing. Both 3.5 inch and 5.25 inch disks are in used. Typically they have the data storage capacity above 1 GB,
A very common application optical memory especially CD-ROM is that it can store Software toolworks Multimedia Encyclopedia that contains all 21 volumes of the academic American Encyclopedia. This encyclopedia comprises full text of about 33000 articles as well as comprehensive index of titles, words, pictures and maps. In addition, there are thousands of pictures, hundreds of sounds and digitised animations along with dozens of video clips. - Input / Output Devices:
These devices are also called input/output peripherals or simply peripherals as these devices are connected to the processor through RAM. Their speed for data input and output is the slowest. The reason of too much slow speed of input/output devices as compared to the other components of computer system is that mechanical motions are highly involved in their working. These devices can be catagorised into two main sections i.e. input section and output section and are described one by one below: - Input Section:
As the name suggests, this is the receiving section for a computer system where from the data is entered for processing. This section is composed of a variety of input devices like keyboard, mouse, microphone, digital camera and so on. Some of the commercially used input devices are described one by one below: - Optical Character Reader (OCR):
This is an input device that is capable to read or input the textual data on a very large scale. An OCR takes the textual data in typed or printed or hand written from on documents. It uses the Optical Character Recognition process in which the characters that are read are matched against their prestored patterns present in OCR's memory. If a match is found, the Optical Character Recognition is successful and vice versa.
OCRs are most commonly used in commercial scale billing system such as gas, electricity bills and credit card reading system. In case of Optical Character Recognition, the text read is stored in the RAM, In case of failure to the interpretation of characters, the operator is capable to enter the character manually. - Page and Hand Scanners:
A page scanner is capable of converting a whole page of printed materials into a form that can be manipulated by the computer system. A complete OCR page consists of a scanner and software such as Word Scan Plus, which runs under Windows operating system. This particular package is responsible for scanning of both text and graphics from the same page.
A hand scanner is that one which the user passes over a page for scanning. These scanners are available for both monochrome and coloured application. However its image if the scanner is not passed over the page at an even speed.
Working of a Scanner:
During the scanning process, the scanner shines a bright light onto the image. In order to light over each portion of the image, the scan head moves from top to bottom of the document or picture. During its motion over each line of the image, the scan head collects the data by measuring the intensity of light reflected back from the image. Thus each scanned line results into a stream of data which is then converted into digital information with certain number of bits representing each tiny area in the scanned picture. For example black and white scanning, only one bit is required, whereas for 256 shades of gray 8 bits are required. All of the information is then stored into the computer memory and can also be saved on the hard disk.
The resolution of scanner is measured in Dots Per Inch (Abbreviation as DPI) along x-axis and y-axis. The higher the resolution of scanner, the sharper will be the image but the scanned image will need more memory. For coloured images, three passes of the scan head are required which are red, green and blue. The filter of each colour eliminates except the one that matches the specific colour and the three resulting images are then combined into one complete full colour image. Scanners are usually used to input steady or dead images. - Magnetic In Character Reader (MICR):
This device is used to recognise the character encoded using special magnetised ink that contains Ferric Oxide. Such characters are mostly written along the bottom of bank's cheques and are used to identify the bank number and the customer account number. Similarly the amount of money is also encoded on the cheque by the bank operator for processing. These devices can process 1000 cheques per minute. Certain utility companies like gas, electricity and credit card companies may also use this method of data capture to input and process large volumes of data. - Optical Mark Reader (OMR):
An OMR is a scanning device that is used to detect the marks made in the preset positions on a special data entry form. The computer system may use an OMR in order to score marks for MCQs type tests, and other questionnaires such as National Identity Card form etc. The specific marked options of these documents with the help of an optical marker is read and manipulated into the computer using an OMR. - Magnetic Strip Card Reader:
A magnetic strip card is a plastic card with a magnetic strip. These cards are commonly used and known as Cash card. The magnetic strip typically holds about 72 characters of information. In most of the banks, money can be withdrawn from the customer's accounts through Cash machines also called Automated Teller Machines (ATMs). These ATMs use a special keypad and magnetic strip card reader. The customer's card is encoded with:
* Customer's account number.
* Customer's personal identification number in encoded form.
* The bank sort code i.e. bank branch number.
* The withdrawal limit of the customer.
* The amount withdrawn in the time period e.g. day.
In order to use the Automated Teller Machine (ATM) the customer inserts his / her magnetic strip card. The machine asks for the personal identification number of the customer. When the customer enters the required identification number, the machine checks it against the encoded personal identification number on the card and also with that present in the customer's account information. If the personal identification number is found correct, the customer is asked about the kind of transaction i.e. whether the customer wants to deposit the money into his / her account or withdraw it. After selecting the specific transaction, the customer is asked about the amount. In case of withdrawal, the amount is checked against the total amount in the customer account and if everything is satisfactory, the specific transaction is processed and the desired amount of money in the form of currency notes are issued to the customer. - Smart Cards:
A smart card is a plastic card similar to a credit card but instead of magnetic strip, it uses a microchip and memory. It can be used to make purchases, transfer the memory in and out of bank accounts and even make transactions over the telephone. An amount of money is stored on the microchip that can be spent like cash.
Some people use the smart cards containing their names and address to pay for electricity charges, at supermarkets and petrol stations. Similarly college student and office workers use smart cards to register their presence, pay for their food and drinks, gain access to the computer rooms or borrow books from the libraries. Smart cards are capable of holding equivalent of 3 pages of typewritten data. - Electronic Point of Sale System (EPOS):
These input devices are also called bar code readers. Different products have their bar codes that are usually 13 digits long. The left most digit represents the country of origin such as 5 for UK, the next 6 digits represents the manufacturer's code and the rest 5 are the product number and check digits.
A LESER beam is used to read the bar code on the item that is to be purchased. The computer to which this scanning device is connected, look up the price of item and description, prints all this information on the customer's receipt and adds the amount to the total amount earned by purchases. These bar code readers are also used to borrow books from computerised libraries. Whenever a book is borrowed, its ISBN (International Standard Book Number) is read along with the book title book number and author name and then all of these information are stored into the library account of borrower. - Graphic Table:
A graphic tablet is a square shaped graphical input device of 35cm side lengths. This device has a very high resolution and is used to trace out drawings. There is a stylus whose position can be detected by computer and the x and y coordinates are transmitted to the system. The drawing that is to be read is placed over the flat surface of tablet and the stylus is positioned over the specific location of the graphic to input. - Mouse:
Mouse is a well-known input device, It has various buttons that are clicked to display menu, select specific menu options and to fill certain area with a specific colour. It may also be used to draw geometrical shapes. During the motion of mouse along the flat surface of mouse pad, a ball bearing underneath the mouse transmits direction of movement to sensors continuously. The sensors produce analogue signals that are converted to binary signals by using analogue to digital converters and are input through specific mouse port to the computer CPU. - Keyboard:
A keyboard is most commonly used input device that is homogeneous to a typewriter. It may be found with every computer system. The user may enter the data through the keyboard by pressing the different keys on it. Currently there are keyboards available for a variety of languages spoken in the world like Urdu, Arabic French, German and so on along with ordinary English keyboard. These keyboards are much more helpful in giving typewritten commands, programs data and other inputs to the computer. - Light Pens:
It is a pen shaped device that incorporates a light sensor so that when it is closely held to a screen or paper containing text or image, the object is detected. This device can be moved over the required graphics object to create or modify it. It must be noted that a light pen is completely different from a Bar Code Reader even when some of the Bar Code Reader are also Pen shaped. - Digital Cameras:
The use of digital cameras is so common now a days that is most of the situation like online video chatting or video conferencing, recording images into the computers and to capture live imaged of objects for various purposes, we take their presence for granted. Digital cameras are input devices that feed the live images of objects into the computer system in the form of videos. An important difference between scanners and digital cameras is that scanners need image on some hard surface (like photograph) for its input while a digital camera does not care about any hard surface on which the image is contained rather it will input any image whether it is real world or in the form of a photograph. - Jay Sticks:
A jay stick is an input device that is most commonly used as a good controlling device than anyone else for moving graphical objects. It contains a stick shaped handle that provides a good control to its user during their activities. It is mostly used in playing computer games or running simulation packages that involve moving objects. - Track Balls:
A track ball is a pointing device similar to a mouse however it contains a ball that directs movement on the screen. Track balls are mostly used to make images more accurate than mouse or any other pointing device. The user continuously rotates the ball in the desired direction for drawing the specific objects. - Microphone:
A microphone is an input device that input sound data into the computer. In case when the user wants to record or send his/her speech especially during voice chatting, microphone plays an important role. A microphone may also be used in capturing the sound for speech recognition purpose, security and so on. A wide variety of microphones are readily available. - Touch Screens:
These are special screens that contain the capability of input using their sense of touch mechanisms. A touch screen allows its user to enter the data just by touching a specific area of the screen and is therefore highly applicable in industrial environments such as manufacturing and security systems. In such situations the users are capable to move and enter their commands or inputs easily and quickly just by touching the screen surface however in normal office activities these touch screens are not appropriate to be used as it is highly a tiring job to continually reach a screen to touch it. - Output Sections:
This section is also called presentation section of computer system as it displays the output results of the computer system. There are two main forms of output. These are: - Hardcopy:
An output is said to be in its hard copy for when it is contained on some hard surface like paper, glass or plastic. In this form an output can be seen as well as touched. Hard copies of output are usually generated with the help of printers and plotters. Hard copy exists whether the computer is on or off. - Softcopy:
Another form of output that can be only be seen but can not be touched is its soft copy. In this form the output exits till computer is on. If the computer is switched off then the soft copy of out disappears. Soft copies of output are generated through computer monitors, speakers and other output devices.
The output section of a computer is composed of a number of devices that either generate soft copies or hard copies of output. These devices are discussed below: - Printer:
A printer is an output device that is used to generate a hard copy of output. Various types of printers are described below: - Character Printers:
These printers are capable to print a single character at a time. Their rate of printing varies from 20 to 600 character per second. These printers are most commonly used with PCs or minicomputers. Different types of Character printers are: - Dot Matrix Printer:
A Dot Matrix printer is an Impact printer that works in a fashion similar to that of a typewriter. The print head of a Dot Matrix printer is composed of a specific number of pins that ranges for 9 to 24 pins. This print head moves over a ribbon (containing ink) beneath which the paper is adjusted. During the printing operation the pins are hammered on the ribbon and make it to strike the paper surface. Thus dots are created on the paper surface. These dots are shaped together in the form of character. A considerable noise is generated during printing. The speed of printing is also very slow. Due to low SPOOL (Simultaneous Peripheral Operation Online) area, a Dot Matrix printer is usually not capable to print graphics.
The printing quality is highly dependent upon the number of pins in the print head so that higher the number of pins, more closer will be the dots and thus better will be the quality of print and vice versa. - Inkjet Printer:
Inkjet printers are very popular non-impact printers and are usually used to generate coloured output. In this printer, a droplet of ink is fired on the page with the help of boiling process that take place in a microscopic tube thus letting the steam to eject the droplet. Using the chemical process to get a specific colour from a mixture of 2 or more colours, the output is normally obtained in the form of coloured hard copy however the printing phenomena is slower and also the printer ink cartridge is considerably expensive in order to have output in all of the demanded colours such as Red, Green, Blue and Yellow etc. - Laser Printer:
LASER printer are very popular now a days and they work very similar to LASER photocopy machines. A toner containing powdered ink is used to transfer the ink particles onto the paper surface and then fuses them on the paper using the heat and pressure. The LASER light plays a very important role in this printing phenomenon. LASER printer are highly fast and silent during their printing operations. The only cost is that of its toner that needs to be replaced after printing a specified number of pages i.e. about 5000 pager per toner. - Line Printer:
These printers are used to print on commercial scale and are most commonly used with mini or mainframe computers. Unlike a character printer that prints just a single character at a time, these printers are used to print a complete line at a time.
A good example of line printers is a chain printer in which a chain moves between two wheels. It has slugs so that one slug contain 3 characters. The paper passes between the slugs on chain and a hummer shaped surface. In order to print a line of character, the page is hammered on the specific combination of characters on the slug. The mini or mainframe computer makes the adjustment of lines and characters so fast that about 3000 lines of text can be printed per minute using this printer. - Computer Output to Microfilm:
Computer Output to Microfilm can be abbreviated as COM devices that record images on photographic film. The recorded images are then read with the help of special reader devices called COM readers. The images are captured either through display on Visual Display Unit (VDU) from where it is photograph with a camera onto microfilm or using an electronic beam or LASER beam that writes directly onto the special film.
Microfilms are very fast usually having information retrieval speed of over 100000 Cycles Per Second (CPS). These microfilms are also cheaper as information output on thousand of papers can be recorded in a single role of microfilm of 16mm. Microfilms are also highly compact because a microfilm on only one inch can be capable to record a bulk amount of information. These COM devices are most commonly used within: - Customers Bank records.
- Library catalogues.
- Book catalogues in book sellers.
- Old invoices and receipts.
- Monitors:
Monitors are also called Visual Display Units (VDUs) or Cathode Rays Tubes (CRT) or most simply screens. Monitors can be found with almost all of computer systems and are used to display a soft copy of computer output. The screens of monitors contain tiny points or dots that are called pixels (stand for Picture Elements). When the cathode rays are fired on these pixels, they get flashed on hitting with a cathode ray of a specific intensity. In case of coloured monitors, the colour of a pixel may be any however in case of monochrome monitor, the colour is either black or white.
Screen pixels are organised into rows and columns so that rows are the horizontal grouping of pixels on the screen within an area of one inch whereas columns are vertical grouping of pixels within one-inch area of the screen. The number of rows and columns of pixels per inch area of the screen is called Screen Resolution. The quality of soft copy of output is totally dependent on screen resolution so that higher the screen resolution, sharper will be the display and vice versa. For example, a screen resolution of 10 x 10 will generate sharper image than 12 x 8 of pixels per inch area of the screen surface. - Printer:
A plotter generates hard copies of drawings such as lines, plants, machines, engineering maps, building designs and graphs etc in a very accurate manner. Plotter may use colours or specific inks for their drawings. A plotter is usually composed of a hard metallic strip on which a pen or pencil holder mechanical hand moves towards left and right. The metallic strip moves forward and backward on the paper surface beneath the strip and draws the specific drawing by moving the mechanical a fashion very similar to human hand. - Speakers:
Just like microphone input audio data to the computer system, speakers are those output devices that generates output in the form of sound. The output of a speaker may be speech of a person as well as some other sound. A wide variety of speakers are available that are different in their capacities of sounds however the purpose of using a speaker to get voice output from the computer system.
Buses and Ports
After discussing the different fundamental components of a computer system, we will now discuss how these components are connected together to make a complete computer system working condition. In order to connect the different components of a computer system within or out of the CPU box we use buses and ports respectively. These are discussed one by one below:
- Buses:
A bus is a set of multiple lines is used to communicate signals in the form of bits from one component to another one within the CPU components like processor, memory and ports for I/O peripherals. It is an important characteristic of a bus that it is a shared transmission medium i.e. multiple devices are connected with the bus and signals transmitted from any of these devices are available to be received by all other devices attached with the bus. A bus is composed of multiple communication lines so that each line is capable to transmit a signal that represents a binary 0 or 1 e.g. 8-bits data can be transmitted over a bus containing eight lines. A bus that connects major computer components (i.e. processor, memory and I/O) is called a system bus.
A system bus is said to be unidirectional if the signals transmission takes place in a single way whereas it will be called a bi-directional bus if there is a two-way transmission of signals in it. Usually system buses may consist of 50 to 100 separate lines so that each line may have particular meaning or function. One any system bus, the lines can classified into three functional groups named as data, address and control lines. These are described below: - Data Line:
Data lines are collectively called data bus. They provide paths for moving data between system modules such as processor, memory and I/O ports. Data buses are typically composed of 8, 16 or 32 separate lines so that the number of lines are referred to as the width of data bus. As each line can carry only one bit at a time, the number of lines determines how many bits can be transferred at a time. Width of data bus is thus a key factor in determining the overall performance of the system as the data transfer rate is dependent up[on it e.g. in case when data bus is 8 lines wide, the processor must access the memory module twice to have a 16 bits long instruction whereas for 16 lines wide data bus, the processor will access the memory module only once. Data buses are bi-directional in nature. - Address Lines:
Like data lines, address lines are also bi-directional in nature and are collectively called address buses. These lines are used to designate the source or destination of data that is too be carried by the data bus. For example if the processor wants to read an instruction or data of 8 or 16 or 32 bits from the memory, it puts the address of the desired memory location on the address lines contained in an address buss. The width of the address bus determines the maximum possible capacity of system's memory. Similarly address buses may also be used to address I/O ports. - Control Lines:
Unlike data and address lines, control lines are unidirectional in nature and are collected together in the form of control bus. These lines are used to control access to data as well as the use of data and address lines. Control signals transmit information about both commands and timing between system modules (i.e. processor, memory and I/O ports). Such control information ensures the smooth and efficient working of the overall computer system. Timing signals indicate the validity of data and address information whereas command signals specify operations to be performed.
In addition to these three lines, there may be power distribution lines that supply power to the attached modules like processor, memory and I/O peripherals. - Ports:
Ports are connections or gateways between CPU and other devices that are connected to the computer system. Each port has a specific processor is capable of addressing upto 65536 ports through their individual addresses. Ports are usually present on the backside of CPU box and are used to connect to input/output peripherals (like keyboard, mouse, speaker, printer and so on) through their cables. Internally, ports are connected to memory module and processor with the help of system buses. Ports exist in two types that are described as: - Serial Ports:
Communication through serial ports may occur in bit-by-bit fashion that is also called serial communication. An example of this may be a speaker control by turning the sound on and off with the help of sound blaster on the main board. Another example of a serial port is serial mouse port. A serial port contains just a single pin or switch for connection. - Parallel Ports:
Unlike serial ports, parallel ports contain multiple pins or switches in order to transmit data as a collection of bits. Transmission through parallel ports is much faster than serial ports. Common example of parallel ports are keyboard and printer ports through which characters are input or output as collections or blocks of bits. - Processor's Registers:
The computer processor is composed of two main components i.e Arithmetic Logic Unit (ALU) where the arithmetic and logical operations are carried out and the Control Unit (CU) that coordinates the activities taking place within the CPU box (i.e. processor, memory and I/O ports) by sending control signals to the various devices.
In addition to the main components, the processor contains circuitry for controlling the interpretation and execution of instructions. Special storage locations that are called registration are included in this circuitry to hold information while the information is being decoded, manipulated or processed. Some of these registers are special purpose where the others are called general-purpose registers. Thus registers can be defined as "fast speed storage location within the processor that are used to temporarily hold the data or information during their processing".
These registers are so fast in speed that no other part of computer is faster or equal to them. This is due to the fact that processor has the highest speed than any other component of the computer system and since these registers are contained within the processor, they are also fast in the storage of data. In the sequence of speed, next to the processor registers is the cache memory that is a limited small memory faster than primary memory but slower than processor registers. After cache memory, primary memory has the next fast speed, then the secondary memory and then the Input / Output devices. Thus Input / Output devices have the slowest speed due to their mechanical movements. One should have now got the answer that why primary memory is online to the processor? The reason is that no other component except cache memory is second to the processor registers in speed than main memory and since cache memory is also considered as a part of processor or somewhere cache memory is not supported by the hardware, so in that case primary memory is the best candidate to be online to the processor.
In above picture, we have the external bus through which certain processor registers are connected to memory. Actually there are two buses i.e. internal and external buses so that:
* Internal buses are those buses that connect the various registers and other internal components of the processor together and transmit signals between them.
* External buses are those buses that connect the processor to memory and other I/O ports within the CPU box.
Various registers, which are mentioned in above picture are described below: - Program Counter (PC):
The Program Counter is a special purpose register that holds address of the next instruction to be executed. Due to this reason, a Program Counter is also called a Sequence Control Register (SCR) or simply Sequence Register (SR).
Whenever a sequence of instruction is being executed, the Program Counter (PC) is automatically incremented to the point where the next instruction begins. That is why it holds the address of the next instruction to be executed. Depending upon the length of the current instruction that is under execution,1,2,3,4............ has to be added to the current contents of Program Counter. In case when the currently executing instruction is a branch or jump instruction, then the address to branch or jump instruction, then the address to branch or jump to is copied to the Program Counter. - Accumulators:
Accumulators are general-purpose registers that are used for providing work area to accomplish arithmetic functions. In some processors, there are upto 16 general-purpose registers. An example to add the contents of memory location say 1000 and 1001 and then store the result into memory location 1002, the following instructions are used: - Load contents of 1000 into the accumulator
- Add contents of 1001 to the accumulator
- Store contents of accumulator in 1002
- Current Instruction Registration (CIR):
This register is also called Instruction Register (IR) and contains information about the operator as well as the operands of current instruction. A current instruction means the one that is already under execution of the processor. This is a special purpose register. An example of current instruction might be LDA 1000 i.e. to load the contents of memory location 1000 into the accumulator. The information that CIR holds about this instruction that LDA is the operator and 1000 is the operand. - Memory Address Register (MAR):
This a special purpose register that holds address of the memory location from which information will be read or to which data will be written. It must be kept in mind that both instruction and data are held in memory so that sometimes the MAR will hold the address of an instruction to be fetched and sometime it will hold the address of data to be used in an instruction. In case when an instruction is to be fetched, the contents of Program Counter (PC) are copied to this register. The processor will know in advance where in the memory to get the next instruction from. Memory address register uses address bus to get specific addresses. - Memory Buffer Register (MBR):
This special purpose register is also called Memory Data Register (MDR) and is used to temporarily store information read from or written to a memory buffer. When the instruction LDA 1000 is decoded, the operand 1000 is placed in MAR whereas the contents of memory location 100 are copied to MBR or MDR. all transfers from memory to processor and from processor to memory occur through this register. Both MAR and MBR serve as buffer register to compensate for the difference in speed between the processor and memory. - Status Register (SR):
This special purpose register contains bits that are set to 1 or cleared to 0 on the basis of the results of an instruction. For example one particular bit will be set to 1 if overflow occurs and another bit will be set to 1 if the result of last instruction becomes negative. One the basis of information provided by this register, the processor can make a decision about what to do in particular circumstances.
These registers are also called Program Status Words or PSW as they also contain information about interrupts. An interrupt is a condition in which the processor is taken away from one process and switched to another process in case of a multiprocessing operation environment. An interrupt can occur due to the finish of time slice for one process and start of the time slice for another one. Time slices are those portions of processor's time that are given to each process for execution by processor in a multitasking or multiprocessing operating system. - Stack Pointer Register:
This is a special register used in most computers that point to the top of a set of memory locations called stack. A stack is an important data structure that is orgainsied into computer memory and is accessible at one end i.e top. Such a stack is known as a system stack and is maintained into a special area of memory called run time stack. When the execution of a program is interrupted due to any reason in a multi-programming environment, the status of this interrupted program and the current contents of all registers associated with it are saved on the run time stack and the Stack Pointer register is updated to show where the information held.
Arithmetic operation also store their intermediate results onto the stack thus using the stack as general-purpose register. Similarly address and parameter information are also stored onto the stack when subroutines or functions are called from the main programs. In case a subroutine is called in a program, the content of Program Counter (PC) comprising the address of the next instruction after the CALL is saved onto the stack. A RETURN instruction fetches this value again from the stack and loads it into the Program Counter (PC). Stack pointer register holds information about current top of the run time stack and this value increases with growth in stack size when something is pushed onto its top and may decrease with the shrink in stack size when existing information is popped away from the top of the run time stack. - The discussion of various components of a computer system is almost over from the computer architecture point of view. The detailed study of registers gives a good understanding about how a processor processes the instructions and where the data and instruction reside during the execution. In this section, we will study how computational operations are carried out i.e simple machine instruction format specified for family of processor to work and the systematic way through which an instruction may pass for its successful execution. These topics are discussed one by one below:
- Simple Machine Instruction Format:
A machine instruction is a binary code that ha special meaning for a computer processor i.e it tells the processor about performing a computational task. The task may be any e.e. to move a number from one location to another one, compare two numbers or to add two numbers. Each machine instruction is precisely defined at the time of manufacturing the processor and it is specific for that family of processor. An instruction set is the entire body of machine instructions available for a single processor and the manufacturer determines it. An instruction to move 5 to the lower half part of accumulator register (AL) in assembly language is.:
MOV AL,5
In 8 bits microprocessor some instructions may just occupy a single byte while the others may occupy two or three bytes also depending upon the number of operands in it. As an instruction comprises an operator that operates on operands, the first byte is always occupied by the op-code that states what is the nature of the operation. The number of operands may be at least 0 and at most 2 according to which the different types of machine instructions for an 8 bit family of processor are: - Zero Address Instructions:
Some instructions such as HLT that are used to stop execution have no operand due to that they will address no or zero operand. These instruction occupy just a single byte as they don't involve any address. Their representation or format is given in terms of address contained in Program Counter (PC) as:
Some other zero address instructions are Shutdown, Restart, DOS command CLS (used to clear the screen and so on).PCOp-Cod - One-Address Instructions:There some instructions that occupy two bytes because they have one operand. In other words they address one operand and are therefore called one address instruction. Their format in terms of Program Counter register is given below asPCOp-Cod
Some common examples of one-address instructions may be ADD X (that adds the contents of X to the accumulator register) and REN Filename.Ext (that is a DOS command used to rename a specific file having a File name and extension).PC+1Operand - Two Address Instruction:
These instructions occupy three bytes either because these instructions involve two operands or due to the fact that address of the operands are too large to fit into one byte. In terms of Program Counter (PC), their format may be.PCOp-CodPC+1Operand 1 or Low Addres
A command example of such an instruction may be ADD A, MENI (that is used to add the contents of memory variable MEN1 to Source Destination (used to copy a file from a source directory to a destination directory) are also two address instructions. The processor determines from the Op-Code how to interpret all the bytes that are making up the instruction.PC+2Operand 2 or High Address - 16-Bit Instruction Format:
Machine that use 15-bits Words also make use of 0-address, 1-address and 2-address formats taking up one, two or three Words respectively. However with 16 bits obviously more information can be stored in each Word and a typical instruction format is shown below:
The first 4 bits are used for the Op-Code whereas 5th and 6th bits are used to indicate the addressing mode that is being used.Function CodeModeOperand Address0123456789101112131415
Microprocessor may have more than one format for one Word instruction depending on the type of instruction being used. Instruction formats not only indicate what operation to be performed but also how many locations are used to hold the actual instruction so that the Program Counter (PC) can be correctly incremented. The number of memory locations used for the address will depend partly on addressing mode that is used. - Processing Machine Instructions (Fetch-Decode-Execute Cycle):
In order to process machine instructions, the Fetch-Decode-Execute cycle is used. It is a cyclic process that describes the working of different processor registers during the execution of instructions. The term Fetch, Decode and Execute are briefly as: - A machine instruction is said to be fetched when the processor gets it.
- A machine instruction is said to be decoded when the processor gets information about its operands and operator from Current Instruction Register (CIR) also called Instruction Register (IR).
- A machine instruction is said to be executed whenever a processor do as it is directed in the given instruction.
A long as the computer system is on, this cyclic process is continued and instructions are fetched, decoded and executed one after the other. - Types of Software:
After having a good understanding of machine instructions and their execution with the help of processor, we can now discuss about software and its different types. Software is set of instructions to the computer system that are followed according to a proper way in order to carry out a specific computing task. It is the non-physical component of a computer system i.e. one can use it and see its functions but can't touch it. Software has two major types i.e Application and System Software, the different sub-types of these two main types and their functionalities are briefly described below: - General Purpose Application Software:
This category includes common software packages that are found on most of the desktop computers. These include: - Word Processing Software like MS-Word, Word Perfect, WorkStar & so on. These software packages are used for producing and saving well laid out documents such as business letters, technical manuals, books memos and reports.
- Desktop Publishing Software such as PageMaker, Ventura, Frame Maker, Page Plus so on. Among other things these are used to produce magazine, newsletters, advertisements and books.
- Spreed Sheet Packages like MS-Excel, Lotus 123, Super Calc and so on. These packages are mostly used for working with numbers, producing accounts and tabulated numerical information of all types.
- Database Packages like MS-Access, Paradox, Dbase III Plus and FoxPro etc. These are used for the storage and retrieval of information. Databases are used in thousands of different applications from Airline Booking system to Mail Order and Invoicing system.
- Graphics Package like Paint, Paint Brush and Coral Draw are used to produce artwork, 3-Dimensional images and special textual effects.
- Computer Aided Design (CAD) Packages like Auto CAD and Turbo CAD are used for producing accurate engineering and architectural drawings.
- Multimedia Authoring Tools like Authorware and Macro Media Director combines text, graphics, animation, sound and video clips for presentation, games, interactive tutorial and so on.
- Telecommunication Software for example Microsoft's Internet Explorer, Netscape Navigator, Yahoo Messengers, MSN Messenger and different Search Engines etc enable us to send and receive data over a Wide Area Network via a MODEM, access the Internet, send and receive the Email and browse or surf the World Wide Web. These telecommunication software are supported with the help of data communication protocols during their work.
- Expert System Software for example Crystal, ELSIE and so on. These are programmed on the basis of facts and rules about a specific file of knowledge like Geological knowledge of soil around the oil fields or any other mineral. The system can then be used to predict the likelihood of finding oil in a new location when given its Geological profile.
- Special Purpose Application Software:
Whenever an organisation wants to computerised some or all aspects of its business, it is often possible to buy an "off the shelf" package to do more or less exactly what the organisation wants. There are literally thousands of specialist applications that are readily available to perform such task as: - Keeping Business Accounts
- Stock Control Information
- Payroll
- General Practice managements and appointment
- The booking of rails using railway tickets
In such cases, the alternative to buy one of these packages is either to: - Buy a database package and build a customised application that suits the exact requirements.
Or - To write programs using a programming language as a tool such as Pascal, BASIC, C or others which can perform the required tasks.
- Programming Languages:
Computer languages such as C, Pascal, Fortran and so on are English like languages in which programs can be written, debugged and executed. These languages have their translators which are actually translation programs. These translation program and turn that code into their equivalent machine executable form. Common types of such translators are. - Compilers are system programs that convert the source code contained in programs into their equivalent target code in machine executable form so that a file having the program name with extension .exe is created.
- Interpreters are system programs that read and convert a program into machine executable code in instruction-by-instruction manner. However unlike compiler, no file with .exe extension is created during interpretation process.
- Assemblers are system programs that are used to translate the symbolic code of Assembly language into its machine executable from and an executable file with .exe extension is created.
An important difference between compiler and assembler is that a compiler is used for the execution of High Level Language programs whereas an assembler is used for the execution of Assembly Language programs. Also due to the creation of a file with .exe extension, compilers are faster than interpreters.
Those programmers who write programs for compilers, interpreters or assemblers are called system programmers. Different application software such as word processors, spreadsheets and database packages can be developed with the help of these programming languages. - Operating System:
An operating system works in the background when the application programs are running. It is the most significant kind of system software which is responsible for: - Managing the resources of computing system.
- Controlling the functionalities of computing system resources.
- Providing users with the user-friendly interfaces that enable them to work with computer system.
The operational parts of an operating system are loaded from secondary storage into memory when the computer is switched on. This process is called booting. System booting is monitored by the ROM routines called firmware. - Utility Programs:
Utility programs are also called operating system' utilities and are used to perform common tasks such as formatting disks, copying and deleting files or repairing damaged files. Many utility programs are provided within the operating system software and can be called directly from the user interface. Other utility programs can be purchased from a number of software manufactures such as - PC-Tools is a collection of utilities for PC system maintenance, management and protection.
- Dr. Solomon's Anti-Virus Toolkit that is a system used for detection of virus and protection of computer system from it.
- After Dark Screen Savers which are screen displays the appear when the computer is left alone for some specific time.