Every computer system has a unit whose primary purpose is to process data. This unit is the control centre of the entire computer system. It accepts data from input devices, processes data, and sends results to the printer or other output devices under control of a stored program. This unit is referred to as the microprocessor in a microcomputer and the central processing unit (CPU) in large computer systems. Both units perform basically the same functions.

Function of Microprocessor:

The microprocessor is an electronic device. It is the heart and brain inside every microcomputer. This tiny chip of silicon determines the speed and power of the entire computer by handling most, if not all, of the processing of a microcomputer. Functions of a microprocessor can be summarized as follows:

• The processor first fetches an instruction from the main memory.
• The instruction is then decoded to determine what action is required to be done.
• Based on instruction the processor fetches, if required, data from main memory or I/O module.
• The instruction is then executed which may required performing arithmetic or logical operation on data.
• In addition to execution, CPU also supervises and controls I/O devices. If there is any request from I/O devices, called interrupt, the CPU suspends execution of the current program and transfers control to an interrupt handling program.
• Finally, the results of an execution may require transfer of data to memory or an I/O Module.

Organization of a Microprocessor:

The major components of Microprocessor are an arithmetic/logic unit (ALU) and control unit (CU). The ALU performs the actual computation or processing of data. The control unit controls the movement of data and instructions into and out of the CPU. The internal CPU bus is needed to transfer data between the various registers and the ALU. The ALU operates only on data in the internal CPU memory. The registers in the CPU serve two functions:

• User-Visible Registers: These registers enable the program to minimize main-memory references by optimizing use of registers.
• Control and Status Registers: They are used to control the operations of the CPU and to control the execution of Programs.

User-Visible Registers:

User-visible registers can be characterized in the following categories:

• General purpose registers
• Data registers
• Address registers
• Condintion code registers

General-purpose registers can be assigned to a velocity of functions by the programmer. A general-purpose register can contain the operand for any opcode. This provides true general-purpose register use. However, there are restrictions. There may be dedicated registers for floati0gn-point operation.

General-Purpose registers can be used for addressing functions. In other cases, there is a fractional or clean separation between address registers and data registers, Data registers may be employed only to hold data and cannot be used in the control of an operand address. Address registers may be somewhat general-purpose, or they may be devoted to a particular addressing mode, some examples are:

1. Segment Pointers: A segment register holds the address of the base of a memory segment. There may be multiple registers; for example, one for the operating system and one for the current process.
2. Index Registers: These are used for indexed addressing and may be auto indexed.
3. Stack Registers: Normally the stack is in memory and there is a dedicated register that points to the top of the stack. This allows push, pop, and other stack instructions.

A final category of registers which is minimum partly visible to the user holds condition codes or flags. Condition codes are bits set by the Central Processing Unit (CPU) as the result of operations. For example, an arithmetic or logical function may construct a positive, negative or zero result or an overflow. In addition to the result, it may consequently be checked as element of a conditional branch operation.

Condition code bits are gathered into one or more registers. Generally, they shape part of a control register. Usually, instructions of the machine permit these bits to be read, but they cannot be changed by the programmer.

Control and Status Registers:

A variety of CPU registers control the operation of the CPU. Most of these are not visible to the user. A reasonably complete list of register types, with brief descriptions are presented here. The following four registers are essential for instruction execution:

• Program Counter (PC): Contains the address of an instruction to be fetched.
• Instruction Register (IR): Contains the instruction most recently fetched.
• Memory Address Register (MAR): Contains the address of a location in memory.
• Memory Buffer Register (MBR): Contains a word of data to be written to memory or the world most recently read.

The program counter contains an instruction address. Typically, the program counter is updated by the CUP after each instruction fetch and thus, it always points to the next instruction to be executed. A branch or skip instruction modify the contents of the PC. The fetched instruction is loaded into an instruction register, where the opcode is analyzed. Data are exchanged with memory using the MAR and MBR. In a bus-organized system, the MAR connects directly to the address bus, and the MBR cornets directly to the data bus. User-visible registers, in turn, exchange data with the MBR.

The four registers mentioned above are used for the movement of data between the CPU and memory. Within the CPU, data must be presented to the ALU for processing. The ALU may have direct access to the MBR and user-visible registers. Alternatively, there may be additional buffer registers at the boundary to the ALU; these registers serve as input and output registers for the ALU and exchange data with the MBR and user-visible registers.

All CPUs includes a register or set of registers, often known as the program status word (PSW) that contains status information. The PSW typically contains condition codes plus other status information. Common fields or flags include the following:

• Sign: Contains the sign bit of the result of the last arithmetic operation.
• Zero: Set when the result is 0.
• Carry: Set if an operation resulted in carry (addition) into or borrow (subtraction) out of a high-order bit.
• Equal: Set if a logical compare result in equality.
• Overflow: Employed to point toward arithmetic overflow.
• Interrupt Disable or Enable:  Applied to disable or enable interrupts.
• Supervisor: Indicates where the Central Processing Unit is executing in administrator or user mode. Certain privileged instructions can be performed only in supervisor mode, and certain areas of memory can be accessed only in supervisor mode.

In addition to the PSW, there may be a pointer to a block of memory containing additional status information. In machines using vectored interrupts, an interrupt vector register may be provided. If a stack is used to implement certain function (e.g., subroutine cell), then a system stack pointer is needed. A page table pointer is used with a virtual memory system. Finally, registers may be used in the control of I/O operations.

In addition to the above components modern microprocessors may also have the following components:

• Integer unit (iu): To handle the integer operation efficiently.
• Floating point unit (fpu): To perform floating point operations.
• Memory management unit (mmu): This unit handles run-time memory management problems to enhance performance.
• Data cache and instruction cache: These are also called bult-in cache. The built-in cache is a fast primary memory.

It is also noted that currently some microprocessors may have a special component for a specific purpose, for instance multimedia. Some processors have more than one ALU unit to enhance performance. Finally, it is essential to say that the components of a microprocessor may change or new components may be incorporated in the future.

Computer has various peripheral devices such as: modem, vision systems, speech recognition and voice response devices, terminal etc. These parts make a computer very important and attractive instrument in our daily life. Using various peripheral devices of computer we can do many essential tasks easily.

Terminal: 

A terminal is a popular input/output device. Terminals are used for two-way communications with a CPU or with other terminals a few feet or thousands of miles away. With the aid of a terminal, a user can access computers around the world. Terminals, also called workstations, allow a user to interact with a computer. It uses a keyboard to enter data and a cathode ray tube (CRT) screen, or monitor, for displaying data. Because data must be keyed into these devices one character at a time, the possibility of error is high and the data transmission rate is very low; thus limiting the use of terminals to small-volume input and inquiries only. Some of the functions that can be performed using terminals are described below.

Messaging: The communication of information from one terminal to one or more remote terminals.

Data Collection: Data are received by terminals and recorded on secondary storage media for subsequent processing. This eliminates needs to record the information on a source document and then to key the information from the source document into a computer.

Inquiry or transaction processing: Data stored in central data files can be accessed from remote terminals for updating or to determine answers to inquiries about information stored in these files. The system employed by most airlines to maintain and update flight information is an example of such a function.

Remote job processing: Programs can be received from remote terminals directly to a CPU for processing. After execution, the result can be transmitted back to the terminal or to other terminals for output.

Graphic display and design: Data ban be displayed in graphic form, and can also be manipulated and manipulated and modified. Interactive graphic displays, from simple vides games displayed on a television set to sophisticated computerized systems, provide complex designs and three-dimensional displays.

Terminals are available with features to suite the multitude of applications to which they are applied. In general three broad types of terminals are: dumb, smart and intelligent.

Speech Recognition and Voice Response Devices:

Speech recognition devices were introduced in the early 1970s. Typically, these systems contain a database of stored voice patterns. The database of voice patterns in generally stored in a recognition unit or in secondary storage. A microphone, attached to the keyboard or recognition unit, records the spoken word patters. A build-in microprocessor then compares words with the stored patterns and transmits the result of the comparison to a computer for processing. A sentence must be spoken as a serried of Speech recognition devices are generally used in situations where access to a switch is not possible or where a user’s hands are otherwise occupied.

Because voice patterns vary greatly from person to person, most speech recognition services are speaker-dependent and must be fine-tuned to each operation. This is generally accomplished by having the operator speaks each of the words or digits to be stored in the recognition unit dictionary several times. An average of the spoken voice patterns is taken and stored as the standard for future voice communications.

Speaker-independent systems are less common and have a very restricted vocabulary: generally then digits and “yes” or “no” response. Despite their restricted vocabulary, speaker-independent systems are widely usable since they do not have to be fine-turned but can be understood by anyone. Clearly, speaker-independent systems are more desirable than speaker-dependent systems. Expense, large database requirements and the limitations or current technology have made the development of voice recognition systems slow. Manufactures are beginning to offer sophisticated speech recognition devices for the popular microcomputer.

One of the strongest impacts made on the use of voice response has come from the manufacturers of microcomputers. The pricing and availability of voice response units are economically feasible for even the smallest concern. Voice response is no longer an isolated discipline but another among the multitude of computer output techniques.

Vision Systems:

This system utilizes a digitizer, camera, computer, and a technique termed as image processing. Image processing is concerned with digitizing and storing of computer-processed images and with pattern recognition.

Familiar examples of computer-processed images are: computer generated digitized portraits at amusement parks, computer-produced special effects in movies, digitized images of Jupiter and Saturn beamed from image processors of spacecraft to earth etc. All these examples have one thing is common, that is to digitize an image. In a visual system, all images that must be recognized or interpreted are digitized and stored in a database. Only after the database has been established, the visual system can be applied to pattern recognition. Pattern recognition, the process of interpreting images, begins when the system digitizes the image of the object to be interpreted. The digitized image is then compared to those in the database to determine a probable match. As it is unlikely that a perfect match will be achieved, there is always a small possibility or error.

Modem:

A modem is the bridge between digital and analog signals. It converts digital data into analog signals by varying or modulation an electrical wave, a process known as modulation. Modulation is carried out for sending digital data thorough analog telephone system. On the receiving end of a phone connection, a modem does just the opposite. It demodulates the analog signals back into digital code. The two terms MOdulate and DEModulate give the modem its name. Modem communications involve three elements of personal computing: serial ports, modem commands, and communications software.

Classification based on purposes:

There are either special-purpose or general-purpose computers. A special-purpose computer is designed for a specific application. It is also known as dedicated computer. Many such computers have instructions permanently programmed into them that are designed to perform only one major function. Special-purpose computers are used to control traffic lights, to control the collection of tolls on highways, and in automobiles, weapons, appliances and games etc.

General-purpose computers are used to handle a variety of tasks. This is possible by the stored-program concept. By this concept, a program containing a series of instructions is prepared for an application and temporally stored in memory. Once stored in the computer’s memory, the program can be executed to perform the specific function. After the completion of the execution of one program, another program can be used for some other task. That is, the same hardware can be used to execute many different programs.

General-purpose computers are more versatile than special-purpose computer. But when applied to the same task it is seen that general-purpose computers are less efficient and also slower than special-purpose computers 

Classification based on signals:

Two types of computers based on internet signaling are analog computers and digital computers. An analog computer actually represents quantities by the physical analogies. These kinds of computer represent the physical quantities, such as distance, acceleration, velocity, temperature, or angular position, pressure, force etc. by electrical or mechanical parameters.

The example of an analog-computing device is automobile speedometer. It changes the rotating rate of the drive shaft of a car into the numerical value of the speed of the automobile. The example of an analog device is a thermometer. It converts the movement of a column of mercury into a temperature reading.

Analog computers are ideal in situations where data can be accepted straightly from measuring instruments. The capability to gather data at high speeds and to process data at uniformly high speeds, makes analog computers exclusively suited to controlling processes of oil refineries, weapon systems steel mills, and similar other procedures. An analog computer does not need any storage space ability. The out from an analog computer is usually in the form of reading on a dial (as in the speedometer of a car) or a graph plotted on paper.

Analog computers were in use before the invention of the digital computers. There are far more digital computers in use today than analog computers. 

Classification based on capacity:

Capacity of a computer refers to the volume of data that a computer system can process. Formerly a computer’s size was a sign of its capacity. With the current state of smallness, dimension of capacity is based on throughput of the computer. Throughput is the quantity of processing that can be performed in a given amount of time. Based on throughput computer systems can be divided into four major categories:

• Microcomputers
• Minicomputers
• Mainframe computes
• Supercomputers

Microcomputers:

Microcomputers are generally known as personal computers-PCs and are microprocessor based small notebook or laptop or desktop systems with changeable capacity. Personal digital assistants (PDAs) are very small portable computers. PDAs are also recognized as the palmtop computers. The brain of a microcomputer is the microprocessor; it is a silicon chip containing essential circuits to execute logic or arithmetic operations and to manage the input/output operations. A microprocessor is an integrated circuit which usually contains millions of transistors squeezed onto a small silicon chip. A microcomputer system is formed by adding input and output facility along with memory to the microprocessor.

At the initial time the microcomputers had very limited processing power and limited choice of input/output devices. But at modern days they have wider processing capabilities and maintain a wide range of input/output devices. Today microcomputers are available with a collection of input/output devices varying from a tape recorder to a voice synthesizer. In addition to general-purpose computations, microcomputers are also used for exceptional purpose applications in automobiles, airplanes, toys, clocks, appliances etc.

Workstations: The High-end microcomputers are also recognized as workstations. They symbolize the bridge between the microcomputers and minicomputers. It is a microcomputer with many of the facilities and abilities of bigger minicomputers but price much less. At first it was designed for use by designers and engineers who need extremely powerful processing and output capabilities.

Servers: Servers are not designed to be used directly. They make programs and data available for users having access to a computer network. A computer network is a collection of computers connected together.

Clients: To use servers, users run desktop programs called clients, which know how to contact the server and obtain information from the server. Use of desktop clients and centralized servers is called client/server computing.

Terminals: Although terminals look like the personal computers, they have some limitations when compared with personal computers. Terminals have only a screen and a keyboard and the electronics that allow them to communicate with the computer to which they are connected. Because they lack the ability to process data on their own, they are called dumb terminals. There is a variety of dumb terminals that can perform limited processing. These are called smart terminals. A personal computer is an example of a smart terminal.

Minicomputers:

A minicomputer system performs the basic arithmetic and logic functions and supports some of the programming language used with large computer systems. They are physically smaller, less expensive, and have small storage capacity compared to mainframes. Minicomputers are ideally suited for processing tasks that do not require access to huge volumes of stored data. As a result of low cost, ease of operation, and versatility, minicomputers have gained repaid acceptance since their introduction in the mid-sixties. Some of the larger and expensive minicomputers are capable of supporting a number of terminals in a time-shared mode. Uses of minicomputers are gradually being diminished with the rapid development of microcomputers.

Mainframe Computers:

A larger computer normally consists of modules accumulated on a chassis and is terms as a mainframe computer. They differ in size, from those a little larger than a minicomputer to supercomputers. These computer systems present extensive benefits over minicomputers or microcomputers. Some of these are: greater storage facility, greater processing speed, a larger assortment of input/output devices, and support for a number of high-speed storage devices, multiprogramming, and time sharing.

Owing to wonderful expense, a mainframe computer system must be operated powerfully. Operating mainframes at the necessary level of effectiveness requires a very large and highly trained staff. These are normally used by government agencies, large business, military and the universities. These systems are often coupled with other computer systems in a large network to give massive computing power. This is referred to as a distributed data processing system. 

Supercomputer:

A very powerful and large mainframe computer is known as a supercomputer. The astronomical cost of super-computers has limited their development to only a few hundred worldwide. The example of a supercomputer is the Cray X-MP. Such supercomputers are applied to the solution of very difficult and complicated scientific and technical problems. Supercomputers are also used for the various national security purposes of some advanced nations.

From the fastest supercomputer and largest mainframe, right down to the smallest embedded microprocessor in a microwave oven or watch, every computer uses an operating system. Over the years, a variety of operating systems have been developed and many remain in use. When the IBM PC set the standard for microcomputers, DOS (Disk Operating System) become the most widely used microcomputer operating. Today, DOS has been replaced by more powerful operating systems such as Windows, UNIX, Windows NT, Linux and OS/2 etc. These operating systems make it possible to run tasks on less expensive microcomputers that were once performed on expensive mainframes and minicomputers.

Microsoft Disk Operating System (MS DOS):

MS-DOS, developed by Microsoft Corporation, became the defector standard for machines designed with Intel microprocessors. MS-DOS was selected by IBM for its PCs. This established the popularity of MS-DOS. Since IBM hardware and its compatibles hold a large share of the microcomputer market, application software became abundant for MS-DOS. The IBM version of MS-DOS is known as PC-DOS.

DOS is helpful to organize disks and use them efficiently. It helps to create and manage files on disks, make copies of them or remove unwanted files from the disks.

MS-DOS 5.0 comes equipped with a graphical user interface (GUI) called the DOS Shell. This interface helps the user with everyday tasks such as starting a program or formatting a floppy disk. With the GUI the desired task can be easily selected and started with the mouse, by just pointing to the item on the screen and clicking the mouse button.

The DOS shell also provides an overview of the contents of the hard disk and floppy disk any time. Several programs can be started at once but only one program runs at a time, the others are suspended.

Because DOS was developed for early 16-bit processors, it cannot task full advantage for advanced capabilities of Intel 32-bit or higher microprocessors. However, use of DOS is declining rapidly.

Mac Operating System (Mac OS):

Mac Os introduced the graphical user interface bringing the UNIX idea to microcomputer. The original Mac OS released in 1984, consisted of System and Finder. By late 1980s, the Mac’s operating system was the most technologically advanced n personal computing, but Apple computer was unable to capitalize on its lead, and Mac OS fell behind Microsoft Window. Mac OS is widely considered to be easiest operation system for computer users. Mac Os released an improved version in 1988, which is up to the standard of Microsoft Windows 98.

Windows 3.XX:

Microsoft Windows 3.XX is an extension to the DOS operating system and cannot run without DOS. It is a graphical user interface whose features extend far beyond those of the DOS Shell. Although it was designed for 32-bit, full advantage of it is limited as it has to run in cooperation of DOS.

Microsoft released Windows in 1984. The goal of this graphical user interface is to make the computer friendlier to the users. With Windows, applications files are presented as symbols (icons) on the screen. Simply pressing a key or clicking the mouse activates a command. Owing to the success of Windows, Windows-compatible application software becomes very popular. Also Windows accessory programs are available to accomplish many tasks within Windows itself. Windows permits multitasking whereby several programs can be run at once. However, it uses cooperative multitasking, which means that the failure of any one application in likely to crash the whole system

Microsoft Windows 95, 98 and 2000:

Microsoft Windows 95 is an operating system which fully supports 32-bit processing capabilities. It also supports preemptive multitasking, which enables the operating system to regain control if an application crashes.

Windows 95, or Win 95, is an operating system, poised between Windows 3.XX (a 16-bit operating system) and Windows NT (32-bit with advanced features). Win 95 is designed to run many existing 16-bit applications designed for windows 3.XX as well as new 32-bit applications.

Microsoft Windows 98, or Win 98 (released in 1988), is an improved version of Win 98 that offers

• Better stability
• Improved Internet connectivity
• Drivers for new peripherals

A new version of Windows called Windows 2000 is available with a lot of additional facilities which include networking applications.

Microsoft Windows 95, 98 and 2000

Microsoft windows CE:

Microsoft windows CE is simplified version of Windows, designed for palmtop computer. Windows CE includes an interactive scheduling calendar, an address book for contacts, electronic mail, and web browsing. By means of an automatic active synchronization program, users can quickly synchronize the corresponding utilities on their computers; Windows CE includes handwriting recognition and support for voice recording as well.

Microsoft Windows NT:

Microsoft Windows NT (New Technology) is a 32-bit operating system, specifically designed for client/server system, formerly a strong hold of UNIX. It has two components to support namely, client and server.

Windows NT Workstation for desktop computers is similar to but more powerful than Win 98. IT is faster than Win 98 and supports network environment. Windows NT desktops can be linked to servers running Windows NT server. Windows NT server offers

• Strong security to network recourses
• Rmote network administration
• Web server scope to make Web pages available
• Directory services to map files and applications available on the network

UNIX:

It is a multi-user operating system for powerful 16-bit and 32-bit machines. UNIX set the standard for multi-user systems for personal computers. Developed at AT&T’s Bell Laboratories in USA, UNIX held monopoly in the multi-user OS market for quite some time before other operating systems were introduced. It was first written in the assembly language. Later in 1973, it was rewritten in C language. This gives it portability i.e., it could be run easily on different types of machines. This is one important reason for the popularity of UNIX. There are several implementations of this system. The greatest success of UNIX lies in client/server computing, a type of computer usage that is widely found in corporations today. UNIX also provides

• Highest level of security for applications and access control and
• Faster services

Linux:

Linux a new flavor of UNIX operating system is the fastest growing operating system for Intel based personal computers. Two important characteristics make Linux attractive. These are:

• It is powerful
• It is free

Linux brings all the maturity and sophistication of UNIX to the PC, including multitasking, virtual memory, internet support, multiprocessor support and graphical user interface.

Linux is distributed with General Public License. In practice, most people buy a Linux distribution, a CD-ROM containing Linux and a collection of drivers, utilities, GUI interfaces and application programs.

Linux, a derivation of UNIX, is a freeware operating system. However, Linux cannot run most popular Microsoft Office applications. Linux is gaining increasing acceptance for web and other applications.

Xerox PARC:

Xerox PARC (Palo Alto Research Center) was the first GUI (Graphical User Interface) operating system developed in 1970. Xerox PARC originated the idea of desktop icon, on screen fonts, windows and pull-down windows. Although Xerox released a GUI based computer in 1981, the company was never able to capitalize on its innovations.

At the initial time of business computing, information system development was a muddled procedure commonly producing discouraging results. To progress the quality of information systems and services an approach for system development life cycle (SDLC) is developed. The system development life cycle can be considered in five phases. Each phase in intended to address key issues and to produce outcome, which form the input for the next phase. The phases are as follows:

1. Preliminary investigation – determining the problem and scope
2. Analysis – understanding the existing system
3. Design – planning the new system
4. Development – doing the work to bring the new system into working condition
5. Implementation – converting to the new system

System development life cycle and activities:

Phases	Activities
Preliminary investigation	Problem definition Nature of problem Problem scope Objectives of the problem Feasibility test Report of the preliminary investigation
System Analysis	Data gathering Written documents Interviews Questionnaires Observation Sampling Data analysis Charts Tables System requirement Report to the management
System design	Preliminary design Prototyping Detail design Output requirement Input requirement Files and databases System processing Controls and backup
System Development	Scheduling activities Programming Testing
Implementation	Training Equipment conversion File conversion System conversion Audition Evaluation Maintenance

Waterfall Model:

The SDLC described above is a systematic step by step by process. However, it is not always wise to keep going if work in a later stage turns up problems with the work performed in an earlier stage. For example, in the system analysis phase, the analyst may discover that the problem has not been formulated correctly. To overcome this problem, the most popular waterfall model builds improvement pathways into the model that allow a return to a previous phase. It is the widely used way to implement the system development life cycle.

Many companies face shrinking budgets for software development. This means that the scale of investment in software projects are greatly reduced. This results in the need for shorter development times and lower manpower costs. With the increasing swing toward distributed systems, a new set of powerful visual tools is available. These tools bring with them a new approach towards system development. However, the following issues need to be resolved to overcome this crisis:

• Designing systems with open interfaces
• Ensuring reusability and extensibility of modules
• Developing modules that are tolerant to changes
• Improving productivity and decreasing cost
• Improving quality of software.
• Managing time schedules
• Industrializing software development process.

Traditional methodologies cannot meet the above mentioned challenges and the object oriented methodology is gaining ground to meet the challenges.

System and Analyst:

System: A system is a collection of components purposefully organized to accomplish a certain task. There are natural systems, such as the cardiovascular system. However, many systems that have been planned and deliberately put into place by people are known as artificial system. For example, a library has a system for serving a member (borrow, return and search of book)

System Analysis: System analysis is the process of studying an existing system to determine how it works and how it meets user need. System analysis lays the groundwork for improvements to the system. The analysis involves an investigation, which in turn usually involves establishing a relationship with the client for whom the analysis is done and with the users of the system.

Client: The client is the person or organization contracting to have the work done. The users are people who contact with the system (usually employees and consumers). For instance, in a library system, the client is probably the librarian, and the users are the members of the library who have library cards.