Adequate care has to be taken of floppy disks or they might get corrupted. Maintenance of floppy disks involves two major functions:
Identification of Data and Software
In contrast to the hard disk, where you store a large quantity of data in well-organized directories, a floppy typically stores specific software or data. The number of floppies you might have to maintain often runs into hundreds, or even thousands. Labeling floppy disks appropriately is the easiest method of identifying data and software.
Paper Labels
Paper labels are used with floppy disk for external identification. As soon as you save any data on a diskette, you should use the label to indicate the contents of the diskette.
Software Labels
Apart from the external labels, software labels, also known as volume labels can be assigned to diskettes. You can assign a character label to the disk to indicate its contents.
While you are formatting a diskette, you can assign a volume label. The DOS command LABEL may also be used for this purpose. It accepts a name from the keyboard and stores it as the volume label. For example, to label the diskette in drive A use the command:
LABEL A:
When a diskette is formatted, the volume label gets erased and you can assign a new one.
Security - Write-protecting Floppy Disks
Protection of data and software is vital, since unauthorized users tend to destroy valuable data and software. Since data and software is going to be stored on floppy disks or hard disks, protecting the floppy disks or hard disks ensures data security to a great extent.
Write-protecting Floppy Disks
You would often create files which have to be used extensively but do not require modification or deletion. These may be data files you are using only for reporting, and not for processing. Or, they may be programs, which you would like to save from being overwritten or accidentally erased.
One way is to make the whole floppy write-protected. In other words, no writing operation can take place on the disk. Data can only be read. In a microdisk, the write-protect tab is a small sticker that has to be stuck onto the floppy disk. In a microdisk, the write-protect tab on the diskette has to be pushed down to enable protection.
Write Protection on a 3½ Disk
Handling Floppy Disks
Floppy disks need special care and handling, as they can get exposed to dirt, heat, and other environmental hazards. Such exposures tend to corrupt the floppy disks. The integrity of the data and software stored is lost. That is why each floppy disk is enclosed in its own protective cover. It also comes with a paper jacket. Some of the things you should observe while handling diskettes are:
- Store diskettes in their boxes.
- Hold a diskette always by the corner and write on labels with a felt-tipped pen (a pencil or ball-point pen can damage the disk if you press too hard).
- Do not place heavy objects on top of diskettes.
- Do not touch the exposed parts of the disk or bend it.
Microdisks have rigid plastic covers with metal shields that protect the disk from dirt and fingerprints. When you insert the disk into the disk drive, the computer automatically moves this shield aside to read the disk.
Whichever type of diskette you are using, it is important to store it in a safe place, away from dust, moisture, magnetism, and extreme temperatures.
File Access
On a PC, if you have files containing confidential information, you can prevent others from accessing them by denying access to the relevant files or to the machine itself.
You can prevent access to files in one of the following ways:
- Giving passwords and permissions to the computer system
- Hiding the files
Some software packages enable you to give passwords for the files you create. The file can be accessed only on supplying the password.
Windows NT provides a multi-layered security architecture. The Security Subsystem of Windows NT is responsible for maintaining security and access-restrictions in the system. The security model provides for discretionary access control so that the owner of a resource (file or folder) can specify which users or groups can access the resources and what type of access they are allowed (such as read, write, and delete). Windows NT also allows the users of the system to specify file access permissions for individual files. The Reference Monitor provides services for validating access to objects, checking user privileges, and generating audit messages.
You can hide files using the DOS ATTRIB command. When you hide a file, the filename will not be displayed in the directory listing. No person can display the file unless the name of the file is known. The file cannot be deleted or copied. The command for hiding one or more files is:
ATTRIB +H <filename>
This hides the specified file(s) from users. The -H option unhides the specified file(s). Figure shows the DIR command before and after hiding the file using the ATTRIB command.
Hiding Files Using the ATTRIB Command
If you want people to read your files but are concerned about them being modified, then you can make individual files read-only. A file is said to be read-only if you are able to examine the contents, but not change them. The command to use is:
ATTRIB +R <filename>
The <filename> can contain wildcard characters to make multiple files read-only. Once a file has been made read-only, it cannot be modified or deleted. An attempt to delete it returns an error message 'Access denied'. Once you have made the required files read-only, your system is secure.
However, if you wish to modify it, you will need to remove its read-only status. This is also done with the ATTRIB utility, with the -R option.
Machine Access
There are two ways of locking a machine. One is to install a mechanical lock that has to be opened by a key. Most of the PCs come with this lock.
Under Windows NT, the Logon Process and the Security Subsystem jointly ensure access restrictions and protection from unauthorized access. The Windows NT Logon Process provides for mandatory logon to identify users. Each user must have an account and must supply a password to access that account. The logon name and password provided by the user is authenticated by the Security subsystem. After successful authentication, whenever the user accesses an object, the Security Reference Monitor ensures that the user has permission to access the object.
Protecting Data and Software from Viruses
On May 14, 1988, most computers attached to the Israel University had their hard disks corrupted. It was reported that nearly 115,000 disks were corrupted on that day. Later on, it was found to be a virus that was programmed to change the system areas of all accessible hard disks. This virus was named the Jerusalem Black Hole virus.
In another incident, people purchasing BASIC programs suddenly found that characters on the screen started bouncing up and down. This was attributed to a virus known as the Italian Ping-Pong virus.
These are just two of the numerous news items related to computer viruses, which you have probably read. One of the buzzwords in the field of computers today is virus. A virus is an actively infectious computer program that places copies of itself into other applications and programs. It also infects data files and documents. The reason why a virus attaches itself to an executable program is that, in order to perform its destructive action, it has to get executed. Thus, it remains dormant until you run the application or program to which it is attached. Viruses, which infect document files, load the virus code into the RAM whenever the document is read.
Typically, on running the program to which a virus is attached, the virus infects all portions of memory and then infects other files on your disks (diskettes and hard disks). So the next person using the same diskette on a different computer spreads the virus to the new system.
The reason why viruses cause so much fear is that they destroy valuable data and programs, which could mean that you loose months of work.
When PCs have been connected together to form a network, the damage can be worse. This is because the virus program spreads from one PC to another, thus infecting all the workstations (the computers that form part of the network).
Effects of Viruses
The effect of a virus can sometimes be very destructive, involving large amounts of data. Different viruses have different effects. Some of these effects are listed below:
- Corrupting files
- Increasing file sizes
- Interference with the display on the VDU
- Formatting the hard disk, thus destroying data
- Marking good sectors as bad
- Destroying contents of some sectors at random
- Slowing down the system
- Scrambling file names by manipulating the directory contents; the data is all there, but the user is unable to access it
- Changing all filenames to a single name.
Viruses are triggered off in various ways. For example, some viruses are triggered off by the date, such as the Jerusalem-B virus which waits for Friday the 13th, when it deletes the program files you execute without your permission. Other viruses are triggered by booting a system, such as the PC Stone virus, that occasionally displays the message 'Your PC is now STONED!' as your computer starts, and causes data loss on, both, the hard disk and floppy disks. The Raindrop virus waits till a .COM file is executed and then starts pushing the characters to the bottom of your screen. Hence, no proper display is possible till the virus is eradicated.
Types of Viruses
There are two main types of viruses. They are:
These two types may be detectable or non-detectable. Non-detectable viruses are called Stealth Viruses.
File Virus
A file virus attacks executable program files (files with extension .COM and .EXE). It may replace a portion of the original file with its own code, thus destroying the file, or it may attach itself to the file and yet allow the file to be executed. Examples of file viruses are Raindrop and Jerusalem-B.
Once an infected file is executed, the virus finds itself in the main or internal memory. It continues to exist in the memory even after the file has completed execution. When another file is executed, the virus attaches itself to this file and writes the modified file onto the disk, thus causing one more file to become infected. Some viruses infect a file only once, while others infect a file each time it is executed. The virus continues to infect files till the machine is switched off or re-booted.
How Boot-sector/Partition-table Virus damages your Computer
When an operating system is installed from a floppy disk, the first sector in the outermost track of the floppy contains a short program called the bootstrap loader. This program does the job of loading the operating system into the computer's memory when the computer is switched on. In a hard disk, the first sector contains a program called the master boot program, which does the same job of locating and loading the operating system into the computer's memory. This activity is called booting.
A boot sector virus substitutes itself for the bootstrap loader, and a partition-table virus substitutes itself for the master boot program. These viruses get loaded every time you switch on the system. Once the virus is loaded into RAM, it goes about infecting all files on the disk. Also, to simulate normal processing, the virus program transfers control to the original bootstrap loader so that booting can take place.
The virus infects other diskettes in the following manner. Whenever a diskette is accessed, the virus first checks whether the diskette is infected. If it is already infected, the requested disk access is performed and control is returned to the user. If the diskette is not infected, the virus moves the original boot record of the diskette to some other safe sector and copies its own code onto the boot sector, thus infecting this diskette. It then proceeds with the requested disk access. Some examples of boot-sector/partition-table viruses are C Brain, PC Stone and Birthday Joshi.
Stealth Virus
Stealth viruses are those written in a way so as to avoid detection by standard anti-virus software. A particular virus may be a file virus and a stealth virus, or a boot-sector virus and a stealth virus. The techniques used to avoid detection are self-encryption (where the virus converts its code into some sort of program code), and alteration of disk directory data. One example of a stealth virus is the Whale virus. This is, both, a file infector and a stealth virus. This virus infects .EXE files and adds 9216 bytes to each file size. The virus then subtracts 9216 bytes from this file's entry in the directory to give the impression that no change has taken place.
Virus Detection and Cleaning
Your machine first contracts a virus from an infected diskette. It then passes the infection to other diskettes. When those diskettes are used on other machines, they also get infected. In a networking environment, your workstation can get infected when you interact with other machines on the network.
Viruses are normally identified by the standard messages they use, such as 'Your PC is now Stoned' or 'Welcome to the Dungeon'. If they do not use messages, they can be detected by their signature, i.e. a series of bytes unique to a virus. The detection of viruses is normally carried out by standard software.
Virus Attack Symptoms
You look out for the following symptoms, which may indicate a possible virus attack, viz.
- Programs take a long time to load.
- Accessing the disk takes a long time.
- Strange or irrelevant error messages or screen displays appear.
- Files disappear; strange files may appear.
- Executable files change in size.
- Drive lights come on without reason.
Anti-virus Programs
Cleaning a disk of file viruses involves deleting the virus code from the infected file, while cleaning a disk of boot-sector/partition-table viruses involves deleting the virus code from the respective sectors and copying the boot programs back to their original space.
Although all this seems easy, the actual process is very complicated, and sometimes drastic measures have to be adopted to delete viruses, such as deleting infected files or formatting disks.
Anti-virus programs are mainly of two types:
- Scanners
- Antidotes
Scanners scan the hard disk or floppy disk and detect many viruses; for example, the program VirusScan by McAfee Associates. These programs normally check for a known number of viruses. Figure 2.5 shows the starting screen of McAfee virus scanner for Windows NT.
The scanner checks all the files on the disk that you specify for viruses and reports if it finds any. Then you can either delete the infected file or let McAfee clean the file. The scanning process is shown in Figure.
The Starting Screen of McAfee Virus Scanner
Antidotes destroy viruses which they are programmed to detect. Examples are CPAV from Central Point and Norton Anti-Virus from Symantec Corp and McAfee.
McAfee Virus Scanner in Operation
Some virus scanners reside in the computer's memory and check for the entry of a virus after every I/O operation. On detection of a virus, a warning is displayed and all further operations are suspended.
Examples are Smartdog and UTRes.
However, these days most anti-virus software perform scanning as well as eradication of viruses. A typical example of this category is Norton Anti-virus.
Virus Preventior
Viruses spread when infected floppy disks are transported from one computer to another. It is always better to prevent virus attacks than cure infected disks. Here are a few precautions you should observe to prevent virus attacks.
- Make sure that the system is always booted from the hard disk or a virus-free, write-protected system diskette.
- Take frequent backups of all your files; in case of a virus attack, you can always replace your data.
- Buy software from authorized selling agents; unauthorized software, besides being illegal, has a higher chance of having a virus.
- Use write-protect tabs on floppy disks that do not need data to be written onto them.
- Avoid updating the date of your system to match a date on which viruses tend to attack, like Friday the 13th.
- Do not lend your program disks, as they may come back infected; if you have to give them, make a copy and check for viruses or format the disk when it is returned.
- Do not let anyone use your system; if this is not possible, do not allow others to use their own diskettes.
- Never run unknown programs before making sure that they have no virus.
- Do not copy game-programs from other users who work on different computer systems. Since games are the most frequently distributed type of software, they are frequently infected with viruses.
Worms and Trojan Horses
Whenever viruses are discussed, both Trojan Horses and Worms are discussed in the same context. A Worm is a program that duplicates itself without attaching its code to other programs. Worms normally exist on networks rather than single machines. A Worm creates a number of copies of itself, all running simultaneously, and thus slowing down the system. An example of a Worm is the US Internet Worm, which was injected into the Internet network. This program multiplied uncontrollably, eating up the available memory space of all the infected computers on the network until they could no longer function.
Bulletin boards on networks are the most common source of viruses. In a network, most users wishing to share software, place such software in a place called the bulletin board along with messages for other users. Sometimes virus programmers place infected programs onto the bulletin board and, as a result, the network gets infected. Network users should only access bulletin boards, but not download software from them to their workstations without checking the software for infection.
A Trojan Horse is a supposedly useful program, which encourages you to run it. But its real purpose is to damage files on your system, or to plant a virus or a worm. An example of a Trojan horse is Egabtr, (pronounced as eggbeater). Egabtr is an acronym for enhanced graphics adaptor beater. It is supposed to enhance the images on your screen, but it actually destroys data and programs on disks.
Another Trojan Horse called Stars, is a program to display visual effects on the screen. It does that but, at the same time, locates files containing the network users' passwords and destroys them.
Note: The anti-virus programs for Win NT are different from those for the Windows95 machines. These programs differ in construction as well as usage.
Troubleshooting
Most PC users suffer from an acute fear of something going wrong with the hardware and not knowing what to do about it. Many of the problems that PCs develop are minor ones that can be fixed by the user. Some faults require the attention of a qualified engineer. Identifying the fault and being able to decide whether to call in the hardware engineer or attempt some troubleshooting yourself will go a long way into making you a complete PC user.
Some elementary troubleshooting techniques are listed here:-
Troubleshooting the Mouse
| Problem | Possible Causes | Remedies |
| Moving the mouse does not produce corresponding movement of the on-screen cursor | Ball is coated with dirt | Extract the ball by opening the cover underneath the mouse and wash in warm water with soap; dry and replace |
| The movement of the onscreen cursor is erratic | The mouse cable is knotted/twisted | Straighten the cable |
| The mouse is not responding at all to any movement | The mouse ball may be stuck | Clean the mouse ball |
| The mouse cable may be broken | Replace the cable |
Troubleshooting the Mouse
Troubleshooting the System Unit
| Problem | Possible Causes | Remedies |
| The computer does not get powered on | Bad power supply | Use another power point and check if it works |
| System problem | Call your customer support engineer | |
| "Non-system disk or disk error" | Non-bootable floppy in the A drive | Remove floppy and boot again |
| Corrupted system files | Boot from a new floppy; reinstall the OS if you know the procedure or call an engineer | |
| Virus Infection | Boot from a clean bootable diskette, scan hard disk and remove virus with anti-virus software |
Troubleshooting the System Unit
Troubleshooting the Keyboard
| Problem | Possible Causes | Remedies |
| Keyboard does not respond at all or prints wrong characters | Bad or loose cable | Re-connect cable |
| Bad circuitry | Call engineer | |
| One or more keys do not work | Bad key or chip | Call engineer and get key or chip replaced |
| Keyboard allows you to enter either upper-case or lower-case characters only | The Shift or Caps Lock key may be stuck | Loosen key |
| Bad Shift or Caps Lock key | Call engineer and get key replaced |
Troubleshooting the Keyboard
Troubleshooting the Printer
| Problem | Possible Causes | Remedies |
| Printer does not power on. | Power supply not connected or switched on | Connect or switch on supply Check connection or change cable and test |
| Bad cable or loose connection | Change power point and get supply checked | |
| Carriage does not move after power on | Loose connections between components within the printer | Check connections if you are familiar with them or call engineer |
| Internal problems | Call engineer | |
| Does not print during self test | Loose connections between components within the printer | Check connections if you are familiar with them or call engineer |
| Internal problems | Call engineer | |
| Prints a blank line after each line | Switch is set to add a line after each line is printed | Change switch settings; refer to the printer manual for exact details |
Troubleshooting the Printer
Troubleshooting the Visual Display Unit (VDU)
| Problem | Possible Causes | Remedies |
| First line of display not on the top line of the screen | Alignment problem | Use the V-shift control to adjust the display |
| Display not covering the whole screen but is compressed in the center of the screen | Alignment problem | Use the H-shift control to adjust the display |
| Display not stable | Power supply fluctuations | Change power supply point |
| Interference from other devices, such as an electric heater, vacuum cleaner, etc. | Shut off these devices or shift the computer to a different location | |
| Display suddenly vanishes and reappears | Loose contact in VDU socket | Check and replace if necessary |
| Bad connections within the system | Call engineer | |
| No display | Bad cable | Change cable |
| Brightness maladjustment | Adjust brightness to maximum | |
| Bad monitor or system problem | Call engineer | |
| Bad or no color | Monitor maladjustment | Adjust monitor controls |
| Color signals are not proper | Call engineer |
Troubleshooting the Visual Display Unit (VDU)
Communications
You have already seen in the module on Local Area Networks how computers can be linked together to form networks, enabling them to communicate, share information and resources. While information requirements within a limited area (like an office building or university campus) can be handled by Local Area Networks (LANs), communication requirements beyond that need Metropolitan Area Networks (MANs) or Wide Area Networks (WANs).
In this session, you will see how the services available on Wide Area Networks help in expanding the usage of a PC. The rapid increase of such networks has been responsible for transforming the PC from an information-processing machine into a communications center.
Wide Area Networks may be commercial networks which anyone can subscribe or they may be run privately by companies for their internal use only. There are international networks with thousands of users as well as smaller ones run by local computer clubs. Computer equipment manufacturers have networks to give their customers the latest information on their products. Customers can also report problems or place orders through them.
Networks like CompuServe offer their users (subscribers) many useful facilities like:
- Communication with other users through E-Mail (Electronic Mail) or FAX (Facsimile).
- Transfer of files from one computer to another. You may want to send a file to a particular user, or you may want to share a game or utility that you have developed with other users. In such a case, you can upload (send) it to the network, from where other users can download it for their use.
- Access to centralized databases. These are like libraries on computers containing business, technical or academic information. They have complete text of publications or abstracts that you can browse through, as in a library. Some well known centralized databases are:
- Dow Jones & Co's Dow Jones News/Retrieval Service.
- Lockheed's Dialog Information Service. It has 518 separate databases covering science, engineering, business and economics.
- Mead Data Central's NEXIS/LEXIS. It offers full text from 15 newspapers like New York Times and 31 magazines like Byte, Newsweek and Business Week, etc.
- West Publishing Co's Westlaw. It allows lawyers to browse through court cases and federal regulations.
- Special Interest Groups (SIGs) or forums where you can ask questions, get responses and exchange information about your specific areas of interest, that could be medicine, cricket or computers.
- On-line services offered by various banks, airlines and shopping centers allow you to transact with them through the network.
- Support forums maintained by various hardware and software companies provide technical expertise and solutions to problems to their customers.
- Computing Time, i.e. allowing you to link to mainframes and utilize their processing capacity.
Wide Area Networks that offer such services are called Bulletin Board Services (BBSs).
Electronic Mail (E-Mail)
Sending Electronic Mail, or E-Mail, using computers is similar to sending letters through ordinary mail. When you write a letter, address it and post it, it is collected, transported and delivered at that address.
In a network, each user is given a unique identification, called E-Mail address. If you type the letter and the E-Mail address of the receiver, the E-Mail facility will take care of sending it over the network to the right person. You can send a message to someone in another office, another city or another continent with equal ease, using your computer.
While most WANs like CompuServe also offer E-Mail as one of their services, there are networks set up specifically for this purpose. Examples of such networks are MCl-Mail and Telecom Gold.
Receiving, storing, forwarding and monitoring of mail are handled by dedicated software called E-Mail software. Some well-known E-Mail software are cc:Mail and Microsoft-Mail.
The advantage of E-Mail is that the receiver's machine need not be working when the mail arrives. It is put in the receiver's mailbox, that is essentially a subdirectory that stores all the mail received. When the machine is accessed, the user is informed of the arrival of mail. Since E-Mail is very fast and reliable, many offices and universities have started using it for sending letters, memos and applications, instead of normal mail.
Facsimile or FAX
Facsimile or FAX is another area where computers are taking over. FAX machines have been used for a long time to transfer documents by scanning and sending over telephone lines.
You can plug in an add-on card, called the FAX card, into your PC. This will enable you to send the file on the disk to a FAX machine at the other end, and vice versa.
On a Local Area Network, if the FAX card is installed on one PC, other workstations of the LAN can utilize its services to send FAXs.
FAX Machine
Requirements to Connect your PC to a Network
Unlike a Local Area Network, where the workstations are normally connected through cables, connection to a Wide Area Network is done through telephone lines, and so a telephone connection is a pre-requisite.
To connect to a WAN, you need a device called a modem, and a communication software for dialing and establishing a connection with another PC. ProComm, ProComm Plus, pcANYWHERE, Telix and Crosstalk are some well-known communication packages.
Lastly, you must subscribe and get an account to become a user of a WAN or BBS that you want to use.
The general features provided by a communication package are:
- Operation in host and remote modes: If the communication software is running in the host mode on your computer, other computers can connect themselves to it and utilize its services. Your machine is called the host as it provides its resources to the others. When you connect to another host and utilize its services, you are said to be a remote user. In host mode, the communication software can handle the task of providing services while the host computer can be left unmanned.
- Sending and receiving of files: This could be used to send a file to one or multiple destinations. Files are normally compressed and then transferred to save time on the network.
- Automatic logging of incoming data to printer or disk: You may want to use this when you want messages/files that are received to be saved on disk or printed without prompting and disturbing your other activities on the PC.
- Recording and playback of sessions and calls: This helps you keep track of all the calls made, duration and amount of data transfer for each one of them. You can also re-run a call. This is useful if the transfer of the same data has to be repeated.
- Providing a Process Control Language: This helps you to program repetitive functions such as unattended file transfer to multiple destinations after business hours.
The System Unit and Its Components
The system unit is a box-like unit filled with a number of useful components, each performing a discrete function. These components work together to accomplish the main function of the computer, viz. accept and process input and deliver output. This section will elaborate on these components one by one.
Internal Power Supply
The system unit draws power from the AC mains through a power protection device. This power is not directly supplied to the internal components. Instead, one of the components, called the internal power supply, converts the AC input into DC output of 5 and 12 volts. Normally, the internal power supply is referred to as Switched Mode Power Supply (SMPS). The SMPS provides cable connectors to supply the required voltage to the other internal components like the floppy drives, the hard disk drive, the motherboard and external device such as the keyboard. The ON/OFF switch of the system unit is actually a part of the SMPS. Thus, when you switch the system unit on or off, you actually switch the SMPS on or off. Figure shows the various components of the system unit.
Components of System Unit
Exhaust Fan
The SMPS has a small fan, called the exhaust fan, attached to it. This fan rotates as long as the computer is switched on. Its function is to cool the SMPS unit.
Speaker
The system unit also has a small audio speaker attached to it. This speaker is connected to the motherboard and produces a sound whenever instructed by software programs. For example, when the machine starts a self-test program is executed that uses beep to indicate that everything is working satisfactorily. It is also used by entertainment programs to produce sound effects.
Motherboard
When you open the system unit, a large board containing a number of tiny electronic circuits and other components are visible. This is called the motherboard. All peripheral devices are connected to the motherboard. In an IBM PC, the motherboard is called the system board. The components of the motherboard are:
Representation of Characters
Before proceeding with the understanding of a microprocessor chip, it is necessary to understand as to how characters are represented in the computer. The term byte refers to a single character of storage. A byte is essentially a collection of 1s and Os representing a character.
One is familiar with the ten distinct decimal digits 0 to 9 commonly used in the decimal arithmetic system. The computers being electronic machines work better with just two signals (on/off) thus giving them only two distinct digits 1 (for on) and 0 (for off) to work with.
This inherent ease of working with two digits makes the computer amenable to binary arithmetic (as opposed to the decimal arithmetic that one is familiar with).
Hence, in the binary system used by computers, each number or character is represented in terms of the binary digits 1 and 0. Each of these binary digits is called a bit. A collection of 8 bits or a byte is used to represent a character in the computer.
The number of bits used to represent a character will determine the number of unique characters that can be represented.
If only 3 bits were used, there would be 8 (23) possible combinations of 1s and 0s. These are:
000,
001,
010,
011,
100,
101,
110,
111.
Each could represent a unique character in the computer. This would mean that 8 different characters can be represented using 3 bits. In a similar way, by using 8 bits, the PC can represent a larger set of characters (28).
As to which combination of 1s and 0s will represent which number (0-9) or character (A-Z, *, /... etc.) is decided by a representation scheme used by the computer. The most widely-used representation scheme is called ASCII.
A sample of the ASCII representation for some of the characters is shown in Table.
Character | ASCII Representation |
| 6 | 0011 0110 |
| 7 | 0011 0111 |
| A | 0100 0001 |
| B | 0100 0010 |
ASCII Representation
Microprocessor
The entire design of the PC is based on the microprocessor chip. The microprocessor can take in data, perform arithmetic operations like addition, subtraction, division or multiplication on the data and send out the result. It also performs comparison operations on data to check if they are equal, or if one is greater than the other. It also has the ability to perform logical operations on data. The microprocessor controls the activities of the various components of the computer and also responds to requests from the peripheral devices; for example, printer signals indicating that it has run out of paper.
The microprocessor consists of the Arithmetic-logic unit (ALU), the Control Unit and some special purpose storage areas called Registers. The ALU section is where the actual arithmetic and logic instructions are executed. During its ALU operations, the microprocessor holds its intermediate results in special purpose registers. These registers are storage areas which are physically a part of the microprocessor chip. These are not accessible to the programmer.
The control section makes available to the ALU, the requisite data that it needs to work upon. It does this by keeping track of the next instruction to be executed and the address of the data referred to by this instruction. The control also uses some of the special purpose registers of the processor to hold the next instruction and the data addresses.
There are many types of microprocessors available in the market that are manufactured by different companies like Intel, Advanced Micro Devices, Motorola, etc. The most widely-used are the range of Intel microprocessors. The microprocessors in the family of Intel processors are the Intel 8004, 8085, 8086, 8088, 80286, 80386, 80486 and the Pentium. The Pentium is the most powerful microprocessor from Intel.
The capacity of a microprocessor is measured in terms of the number of bits it can send or receive and the number of bits it can process internally. The 8088 is an 8/16 bit processor, indicating that it can send or receive 8 bits of data and internally process 16 bits of data at a time. The 80286 is a 16/16 bit processor. Thus, it is a true 16 bit processor and is faster than the Intel 8088 chip. Also, the 80286 can work in two different modes: the real and the protected. When working in the real mode, the 80286 works just like the 8088. In the protected mode, the 80286 makes available many facilities that are not available on the 8088. For example, it provides features that allow more than one program or task to be executed simultaneously. The protection feature prevents one program from tampering with any part of the memory that does not belong to it.
The 80386 chip has a 32-bit processing capability with a 32-bit data path and, as a result, is much faster than either the 8088 or the 80286 chip. Like the 80286, it can also work in the protected mode.
The 80486 chip was introduced by Intel in late 1989 and for the next four years, enjoyed the distinction of being the best performing microprocessor in the Intel family. The Intel 80486, like the 80386, is a 32-bit processor. It has architectural enhancements, which makes it perform better than the 80386 chip.
The Pentium is a 32-bit processor with a 32-bit data path, and was introduced in 1993. It is three times faster than the 80486, and is considered the best in the Intel family.
Note: DOS is a 16-bit operating system and most applications that run under it are 16-bit applications, including Windows 3.1. Windows NT however, is a 32-bit operating system that runs 32-bit applications. To maintain a degree of compatibility, older 16-bit applications are also allowed to run on the 32-bit platform. But what is required today is not just better hardware that runs applications faster, but also better software that matches the changing hardware.
Interrupts
Although the processor executes the instructions in a specified sequence, it should be able to respond to adhoc requests for its attention, such as pressing keys on the keyboard, or the printer signaling that it has run out of paper, and take the appropriate action required.
This is done through interrupts. An interrupt signal makes the microprocessor respond to such adhoc requests, even though it is busy working on something else. However, the microprocessor stores the status of the current work being processed before it diverts itself to handle the interrupt request. The microprocessor stores this status, i.e. the next instruction that was to be executed and all the intermediate results generated etc. into an area of the memory called the stack and retrieves this status to continue processing after it has completed the handling of the interrupt request.
Clock Chip
The components in a computer are designed to operate in perfect synchronization. To do this, they need a time keeper. The clock chip provides the timing signal in the form of electronic pulses that are used by the computer components to set up a working pace. The chip generates a regular beat (like the ticking of a clock) and the operations of a computer are timed to this beat. It is like the baton held by the conductor in an orchestra, the rhythmic movements of which set the pace for the individual instruments of the orchestra to follow. The faster the rhythmic movements, the faster the speed at which the orchestra plays. Thus, a computer, that operates at higher clock speeds is faster. For its timing, the clock chip uses a quartz crystal like the one used in quartz watches.
Clock Chip
Speed of Operation
The speed of a clock is measured in terms of frequency of pulses generated. The unit used for measuring this frequency is MHz (Mega Hertz). If the clock speed is 1 MHz, it means that the clock produces 1 million pulses per second. The clock speed of PCs ranges from a low of 16 MHz to a high of 200 MHz (used by the Pentium microprocessor). The clock speed is one way of measuring the speed of a computer.
Another unit for measuring the speed of a computer is MIPS (Million Instructions Per Second). It gives the number of instructions executed per second. The standard PC is rated to have a speed of 0.4 MIPS.
Memory
The internal memory of a PC is present on the Motherboard in the form of chips. The computer uses two types of internal memory:
Random Access Memory (RAM)
This is the memory that the computer uses for storing the programs and their data while working on them. So, if you are drafting a letter using a wordprocessor, say MS-Word, you are working with a document loaded into the RAM. To store your letter for later use, it has to be saved on the hard disk. Later, you can retrieve it from the disk and continue working on the document. Thus, RAM functions as a scratch pad for the computer and is sometimes called the scratch pad memory.
RAM has the following characteristics:
- Data within the RAM can be read or modified, i.e. you can either read from the RAM or write onto it. Hence it is called read/write memory.
- The contents of the RAM are lost when the computer is switched off. Hence, the RAM is said to be volatile.
The capacity of RAM can vary from 640 KB to 64 MB and more, depending on the number of memory chips installed, which in turn depends on the capacity the microprocessor can handle. The RAM size is an important parameter in determining the size and complexity of problems that a computer can handle. When people refer to the amount of memory that a computer has, they are talking about the amount of RAM available.
Read Only Memory (ROM)
Another portion of the internal memory found on the motherboard is called ROM. The ROM contains permanently recorded instructions that are vital for starting up a computer. One set of instructions found in ROM is called the ROM-BIOS, that stands for Read-Only Memory Basic Input-Output Services. These programs perform the most basic control and supervisory operations for the computer. For example, they check whether the I/O devices have been connected properly to the system unit. They also handle the basic needs of the hardware involved, which include all input and output devices. Any set of programs residing in ROM is called firmware.
Instructions in ROM can be executed but cannot be changed and hence the name Read-Only Memory. Further, these instructions are not erased when the power goes off. Therefore, ROM is said to be nonvolatile.
Note: In recent years, PCs have become increasingly memory-hungry, and the minimum desirable memory seems to go up every year. Now, 8 MB of memory is no longer considered large; it is the minimum required in most situations. Windows 3.1, for example, can function with 4 MB of RAM. However, it will spend a vast amount of time swapping data to disk unless you have a minimum of 8 MB of RAM.
Bus
As stated earlier, the microprocessor works on the data stored in RAM. Further, to execute the instructions stored in ROM, it has to fetch these instructions into RAM. Communication between the microprocessor and the memory chips as well as other chips found on the motherboard is accomplished through a set of wires running between them. These are called the bus. The bus comprising a set of 8 wires for carrying data is called the data bus. When data is sent from one unit to another, the address (location in memory at which the data is stored) is also sent with the data.
The bus has a set of 20 wires (in an 8088-based system) for carrying these addresses. This type of bus is called the address bus.
Ports
All input and output devices like the keyboard, the printer and the mouse are connected to the system unit through ports. These ports are actually the inlets and outlets of the microprocessor. The microprocessor communicates with the outside world through them. These ports can be either serial or parallel. To transfer a byte through a serial port, eight bits are queued and sent bit by bit. However, in a parallel port, all the eight bits are transferred simultaneously.
If all PC components were made by the same manufacturer, there would be no problems in connecting the input and output devices to the system unit. But this is not always the case, and each manufacturer follows a different standard for transferring data. Thus, you may not be able to connect a brand X monitor to a brand W computer. The solution to this problem lies in standardizing the way data is transferred.
Serial Port Standard
The industry standard for serial transfer of data is the RS-232C port. This standard defines the various parameters for data transfer, one of them being the speed of transfer. A mouse, for example, uses RS-232C port for communicating with the CPU.
Parallel Port Standard
An industry acknowledged parallel port standard is the Centronics printer interface developed by the makers of the Centronics printers. Printers normally use this standard.
Expansion Slots
Your PC comes with a limited number of serial and parallel ports for connecting input and output devices. However, you may find it necessary to attach an extra input or output device to the PC. The motherboard has a set of sockets called expansion slots, that help you attach extra input and output devices, in case you run out of ports. These slots can be used to add extra memory and additional devices.
Attaching a device to a PC is an easy task. You need to buy an appropriate add-on card or an option board. This card contains the electronic components, which interface the input and output device with the motherboard. In addition, it provides a port to which you can connect your device. Next, you open the system unit, plug the card into an empty expansion slot and attach the device to the card's port. Now the device is connected to the motherboard.
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