Signup - ACTIVIST'S CORNER

The world has desired to go far more than the layman can imagine in the recent times. Today, we feel like the dreams of ours are in vane, but the word of wisdom say: the voices of our father’s will come to play in the generations to come. “THE ACTIVIST CORNER” is a platform to express individual feeling concerning the numerous vices confronting our great society.

To have a conducive environment was the dreams of our forebear and as such should be upheld by any right thinking human being. The new revolution is the mine which speak in our society, most future of the empty figures around are community is bent on destroying and damping the image of our lovely nation.

It is in light of this that the “THE ACTIVIST CORNER” has emerged to look for a possible way to put a stop to all these vices we are experiencing today.

Do you say what to share, has any body done something very unusual in our society, do you have the feelings of airing out what so ever that is disturbing you, do have any wrongs to correct, then this the opportunity you have been waiting for, fire all your comment, suggestions, request, etc, to the “THE ACTIVIST CORNER” (via: E:mail: talk2jerryanytim@yahoo.com, Website: activistcorner.webs.com, Tel: +2348066062435) now and have what ever is your opinion voiced out.

Remember, to have a conducive and a better environment good for living is from our collective effort, so join hands to give our society a fresh air!

SECURE SITES

googlea903b70c9c2e11ce.html

GENERATIONS OF COMPUTER

GENERATIONS OF COMPUTER

The history of computer development is often referred to in reference to the different generations of computing devices. A generation refers to the state of improvement in the product development process. This term is also used in the different advancements of new computer technology. With each new generation, the circuitry has gotten smaller and more advanced than the previous generation before it. As a result of the miniaturization, speed, power, and computer memory has proportionally increased. New discoveries are constantly being developed that affect the way we live, work and play.
Each generation of computers is characterized by major technological development that fundamentally changed the way computers operate, resulting in increasingly smaller, cheaper, more powerful and more efficient and reliable devices. Read about each generation and the developments that led to the current devices that we use today.

First Generation - 1940-1956: Vacuum Tubes

The first computers used vacuum tubes for circuitry and magnetic drums for memory, and were often enormous, taking up entire rooms. A magnetic drum,also referred to as drum, is a metal cylinder coated with magnetic iron-oxide material on which data and programs can be stored. Magnetic drums were once use das a primary storage device but have since been implemented as auxiliary storage devices.
The tracks on a magnetic drum are assigned to channels located around the circumference of the drum, forming adjacent circular bands that wind around the drum. A single drum can have up to 200 tracks. As the drum rotates at a speed of up to 3,000 rpm, the device's read/write heads deposit magnetized spots on the drum during the write operation and sense these spots during a read operation. This action is similar to that of a magnetic tape or disk drive.
They were very expensive to operate and in addition to using a great deal of electricity, generated a lot of heat, which was often the cause of malfunctions. First generation computers relied on machine language to perform operations, and they could only solve one problem at a time. Machine languages are the only languages understood by computers. While easily understood by computers, machine languages are almost impossible for humans to use because they consist entirely of numbers. Computer Programmers, therefore, use either high level programming languages or an assembly language programming. An assembly language contains the same instructions as a machine language, but the instructions and variables have names instead of being just numbers.
Programs written in high level programming languages retranslated into assembly language or machine language by a compiler. Assembly language program retranslated into machine language by a program called an assembler (assembly language compiler).
Every CPU has its own unique machine language. Programs must be rewritten or recompiled, therefore, to run on different types of computers. Input was based onpunch card and paper tapes, and output was displayed on printouts.
The UNIVAC and ENIAC computers are examples of first-generation computing devices. The UNIVAC was the first commercial computer delivered to a business client, the U.S. Census Bureau in 1951.
Acronym for Electronic Numerical Integrator and Computer, the world's first operational electronic digital computer, developed by Army Ordnance to compute World War II ballistic firing tables. The ENIAC, weighing 30 tons, using 200 kilowatts of electric power and consisting of 18,000 vacuum tubes, 1,500 relays, and hundreds of thousands of resistors, capacitors, and inductors, was completed in 1945. In addition to ballistics, the ENIAC's field of application included weather prediction, atomic-energy calculations, cosmic-ray studies, thermal ignition, random-number studies, wind-tunnel design, and other scientific uses. The ENIAC soon became obsolete as the need arose for faster computing speeds.

Second Generation - 1956-1963: Transistors

Transistors replaced vacuum tubes and ushered in the second generation computer. Transistor is a device composed of semiconductor material that amplifies a signal or opens or closes a circuit. Invented in 1947 at Bell Labs, transistors have become the key ingredient of all digital circuits, including computers. Today's latest microprocessor contains tens of millions of microscopic transistors.
Prior to the invention of transistors, digital circuits were composed of vacuum tubes, which had many disadvantages. They were much larger, required more energy, dissipated more heat, and were more prone to failures. It's safe to say that without the invention of transistors, computing as we know it today would not be possible.
The transistor was invented in 1947 but did not see widespread use in computers until the late 50s. The transistor was far superior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficient and more reliable than their first-generation predecessors. Though the transistor still generated a great deal of heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Second-generation computers still relied on punched cards for input and printouts for output.
Second-generation computers moved from cryptic binary machine language to symbolic, or assembly, languages, which allowed programmers to specify instructions in words. High-level programming languages were also being developed at this time, such as early versions of COBOL and FORTRAN. These were also the first computers that stored their instructions in their memory, which moved from a magnetic drum to magnetic core technology.
The first computers of this generation were developed for the atomic energy industry.

Third Generation - 1964-1971: Integrated Circuits

The development of the integrated circuit was the hallmark of the third generation of computers. Transistors were miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speed and efficiency of computers.
A nonmetallic chemical element in the carbon family of elements. Silicon - atomic symbol "Si" - is the second most abundant element in the earth's crust, surpassed only by oxygen. Silicon does not occur uncombined in nature. Sand and almost all rocks contain silicon combined with oxygen, forming silica. When silicon combines with other elements, such as iron, aluminum or potassium, a silicate is formed. Compounds of silicon also occur in the atmosphere, natural waters, many plants and in the bodies of some animals.
Silicon is the basic material used to make computer chips, transistors, silicon diodes and other electronic circuits and switching devices because its atomic structure makes the element an ideal semiconductor. Silicon is commonly doped, or mixed, with other elements, such as boron, phosphorous and arsenic, to alter its conductive properties.
A chip is a small piece of semi conducting material (usually silicon) on which an integrated circuit is embedded. A typical chip is less than ¼-square inches and can contain millions of electronic components (transistors). Computers consist of many chips placed on electronic boards called printed circuit boards. There are different types of chips. For example, CPU chips (also called microprocessors) contain an entire processing unit, whereas memory chips contain blank memory.
Semiconductor is a material that is neither a good conductor of electricity (like copper) nor a good insulator (like rubber). The most common semiconductor materials are silicon and germanium. These materials are then doped to create an excess or lack of electrons.
Computer chips, both for CPU and memory, are composed of semiconductor materials. Semiconductors make it possible to miniaturize electronic components, such as transistors. Not only does miniaturization mean that the components take up less space, it also means that they are faster and require less energy.
Instead of punched cards and printouts, users interacted with third generation computers through keyboards and monitors and interfaced with an operating system, which allowed the device to run many different applications at one time with a central program that monitored the memory. Computers for the first time became accessible to a mass audience because they were smaller and cheaper than their predecessors.

Fourth Generation - 1971-Present: Microprocessors

The microprocessor brought the fourth generation of computers, as thousands of integrated circuits we rebuilt onto a single silicon chip. A silicon chip that contains a CPU. In the world of personal computers, the terms microprocessor and CPU are used interchangeably. At the heart of all personal computers and most workstations sits a microprocessor. Microprocessors also control the logic of almost all digital devices, from clock radios to fuel-injection systems for automobiles.
Three basic characteristics differentiate microprocessors:
• Instruction Set: The set of instructions that the microprocessor can execute.

• Bandwidth: The number of bits processed in a single instruction.

• Clock Speed: Given in megahertz (MHz), the clock speed determines how many instructions per second the processor can execute.
In both cases, the higher the value, the more powerful the CPU. For example, a 32-bit microprocessor that runs at 50MHz is more powerful than a 16-bitmicroprocessor that runs at 25MHz.
What in the first generation filled an entire room could now fit in the palm of the hand. The Intel 4004chip, developed in 1971, located all the components of the computer - from the central processing unit and memory to input/output controls - on a single chip.
Abbreviation of central processing unit, and pronounced as separate letters. The CPU is the brains of the computer. Sometimes referred to simply as the processor or central processor, the CPU is where most calculations take place. In terms of computing power, the CPU is the most important element of a computer system.
On large machines, CPUs require one or more printed circuit boards. On personal computers and small workstations, the CPU is housed in a single chip called a microprocessor.
Two typical components of a CPU are:
• The arithmetic logic unit (ALU), which performs arithmetic and logical operations.

• The control unit, which extracts instructions from memory and decodes and executes them, calling on the ALU when necessary.
In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the Macintosh. Microprocessors also moved out of the realm of desktop computers and into many areas of life as more and more everyday products began to use microprocessors.
As these small computers became more powerful, they could be linked together to form networks, which eventually led to the development of the Internet. Fourth generation computers also saw the development of GUI's, the mouse and handheld devices

Fifth Generation - Present and Beyond: Artificial Intelligence

Fifth generation computing devices, based on artificial intelligence, are still in development,though there are some applications, such as voice recognition, that are being used today.
Artificial Intelligence is the branch of computer science concerned with making computers behave like humans. The term was coined in 1956 by John McCarthy at the Massachusetts Institute of Technology. Artificial intelligence includes:
• Games Playing: programming computers to play games such as chess and checkers

• Expert Systems: programming computers to make decisions in real-life situations (for example, some expert systems help doctors diagnose diseases based on symptoms)

• Natural Language: programming computers to understand natural human languages

• Neural Networks: Systems that simulate intelligence by attempting to reproduce the types of physical connections that occur in animal brains

• Robotics: programming computers to see and hear and react to other sensory stimuli
Currently, no computers exhibit full artificial intelligence (that is, are able to simulate human behavior). The greatest advances have occurred in the field of games playing. The best computer chess programs are now capable of beating humans. In May,1997, an IBM super-computer called Deep Blue defeated world chess champion Gary Kasparov in a chess match.
In the area of robotics, computers are now widely used in assembly plants, but they are capable only of very limited tasks. Robots have great difficulty identifying objects based on appearance or feel, and they still move and handle objects clumsily.
Natural-language processing offers the greatest potential rewards because it would allow people to interact with computers without needing any specialized knowledge. You could simply walk up to a computer and talk to it. Unfortunately, programming computers to understand natural languages has proved to be more difficult than originally thought. Some rudimentary translation systems that translate from one human language to another are in existence, but they are not nearly as good as human translators.
There are also voice recognition systems that can convert spoken sounds into written words, but they do not understand what they are writing; they simply take dictation. Even these systems are quite limited -- you must speak slowly and distinctly.
In the early 1980s, expert systems were believed to represent the future of artificial intelligence and of computers in general. To date, however, they have not lived up to expectations. Many expert systems help human experts in such fields as medicine and engineering, but they are very expensive to produce and are helpful only in special situations.
Today, the hottest area of artificial intelligence is neural networks, which are proving successful in an umber of disciplines such as voice recognition and natural-language processing.
There are several programming languages that are known as AI languages because they are used almost exclusively for AI applications. The two most common are LISP and Prolog.

CHARACTERISTICS OF COMPUTER

CHARACTERISTICS OF COMPUTER
Speed
The computer was invented as a high-speed calculator. This has led to many scientific projects which were previously impossible. The control of the moon landing would not have been feasible without computers, and neither would today's more scientific approach to weather prediction. If we want tomorrow's forecast today (and not in six months time) meteorologists can use the computer to perform quickly the necessary calculations and analyses. When making flight reservations we want to know well in advance of take-off that a seat will be available - if it is not, then we have time to make other arrangements. The ability to get answers fast enough so that one has time to take action on them (or to make alternative plans, as in the case of airline reservations) makes real-time computing possible.
Electrical pulses travel at incredible speeds and, because the computer is electronic, its internal speed is virtually instantaneous. We do not talk in terms of seconds or even milliseconds. Our units of speed are the microsecond (millionths), the nanosecond (thousand0millionths) and latterly even the picosecond (million-millionths). A powerful computer is capable of adding together two 18-digit number in 300 to 400 nanoseconds.
Consider two examples from non-numerical environments. The manual indexing of the complete works of Thomas Aquinas (approximately 13 million words) would have taken 50 scholars about 40 years to accomplish. With the aid of a computer a few scholars did it in less than one year. Fingerprint identification, in time to catch a criminal before he flees the country, would be impossible without computers. The first example enables us to enjoy knowledge that would otherwise be unobtainable within our own lifetime. In the second example, the police gain time in which to act.
Stage
The speed with which computers can process large quantities of information has led to the generation of new information on a vast scale, in other words, the computer has compounded the information 'explosion'. How can people cope with it? We can't, but computers can. But where do they keep it all?
As a human acquires new knowledge, the brain subconsciously selects what it feels to be important and worth retaining in its memory, and relegates unimportant details to the back of the mind or just forgets them. In computers, the internal memory of the CPU is only large enough to retain a certain amount of information. It is therefore, impossible to store inside the computer the records, for example, of every Premium Bond and the names and address of their owners. All of this data is stored outside of the memory of the CPU, on auxiliary or secondary storage devices. Small sections of the total data can be accessed very quickly by the CPU and brought into the main, internal memory, as and when required for processing.
The internal memory (in CPU) is built up in 1 K or K modules, where K equals 1024 storage locations. Babbage's Analytical Engine would have been capable of holding 1000 numbers, each of 50 digits. Computers come in many sizes. Many small micro-computers have an 8 K or 16 K store whilst 'super computers', such as the CDS CYBER 205 may have up to 1024 K stores (i.e. 1024 * 1024 locations).
Accuracy
In spite of misleading newspaper headlines, the computer's accuracy is consistently high. Errors in the machinery can occur but, due to increased efficiency in error-detecting techniques, these seldom lead to false results. Almost without exception, the errors in computing are due to human rather than to technological weaknesses, i.e. to imprecise thinking by the programr, or to inaccurate data, or to poorly designed systems.
Versatility
Computers seem capable of performing almost any task, provided that the task can be reduced to series of logical steps. For example, a task such as preparing a payroll or controlling the flow of traffic can be broken down into a logical sequence of operations, whereas comparing the tones of a turner with a Vermeer cannot. Yet the computer itself has only limited ability and, in the final analysis, actually performs only four basic operations:

It exchanges information with the outside world via I/O devices,
It transfers data internally within the CUP,
It performs the basic arithmetical operations,
It performs operations of comparison.
In one sense, then, the computer is not versatile because it is limited to four basic functions. Yet, because so many daily activities can be reduced to an interplay between these functions, it appears that computers are highly ingenious. Programming is the craft or reducing a given problem into an interplay between these few operations.
Automation
A computer is much more than an adding machine, calculator or check-out till, all of which require human operators to press the necessary keys for the operations to be performed. Once a program is in the computer's memory, the individual instructions are then transferred, one after the other, to the control unit for execution. The CPU follows these instructions until it meets a last instruction which says 'stop program execution'. When Babbage claimed that his Analytical Engine would be automatic, he meant that once the process had begun, it would continue without the need for human intervention until completion.
Diligence
Being a machine, a computer does not suffer from the human traits of tiredness and lack of concentration. If 3 million calculations have to be performed, it will perform the 3 millionth with exactly the same accuracy and speed as the first. This factory may cause those whose jobs are highly repetitive to regard the computer as a threat. But to those who rely on a continuous standard of output, e.g., quality control in the refining of oil and other chemical processes, the computer will be seen as a considerable help.
The Basic Anatomy of the Computer
Remembering Babbage's Analytical Engine, let us see what happens in a computer. It receives information (input); it processes this information in some way according to set of precise instructions (in the CPU); and it then presents the results in a useful form (output).
On closer inspection we find that the CPU (the computer itself, remember) has to store the information in a memory before it can carry out processing operations. Two kinds of information have to be input, the program and the data. The program is the set of instructions which the computer is to carry out, and the data is the information on which these instructions are to operate. For example, if the task is to sort a list of telephone subscribers into alphabetical orders, the sequence of instructions or procedure which guides the computer through this operation is the program, whilst the list of names to be sorted is the data.
In the Analytical Engine calculations were to be handled by an arithmetic unit which Babbage called the Mill. The computer also has an arithmetic unit. Arithmetic, because all computer operations involve the manipulation of numbers. All information, program and data, are represented in numeric form. The manipulations also include making comparisons and logic type operations as well as arithmetic operations ( + - * / ), and for this reason the unit is referred to in full as the arithmetic and Logic Unit (ALU).
The Electronic Discrete Variable Automatic Computer (EDVAC) in 1952, was to be one such computer. The memory unit stored both the instructions and data to be used for the calculations.
In 1946, Ekert and Mauchly formed their own company, which in 1949 was incorporated as the UNIVAC division of the Remington Rand Company Ltd., In 1951 the UNIVAC I, i.e. the computer developed by Ekert and Mauchly became operational at the Census Bureau. This computer was self checking and used magnetic tape for data input and output. The UNIVAC I was run 24 hours a day until 1963. Yet, another UNIVAC I was put to business by the General Electric Corporation in 1954.

The first generation of computers was marked by the use of vacuum tubes as the electronic components and by the use of either electrostatic tubes or mercury delay lines for storage. Power tapes and punched cards were also used. Electronic time per operation ranged from 0.1 millisecond, while memory access time was 1 millisecond.
SECOND GENERATION COMPUTERS
The WHIRLWIND I was designed and developed at the Massachusetts Institute of Technology in 1952. This computer used a magnetic core memory instead of the vacuum tube, which made it more compact, reliable and fast.
The transistor was invented in 1948, but it was several years before it significantly affected the computer industry. In the early 50's and 60's the switches used in computers were valves. These were expensive. But in 1947 William Sharkley invented the transistor for which he got a Nobel Prize. He used germanium to make the transistor. It was an extremely simple device. The transistor brought an end to the valve industry.
Sharkley then set up a team of 8 brilliant scientists to help him make a transistor of silicon. The drawback of germanium transistors was that hey stopped working when they got warm. The US army needed transistors that would withstand battle conditions, and they contributed a huge sum of develop it. But his 7 scientists left him and with Robert Noyce they set up a company by the name of Fairchild Corporation, here they developed the technique of making several transistors on a single silicon chip. In this way, by 1963, they managed to reduce the cost of transistors 10 times.
The second generation of computers was marked by magnetic core storage and later by the use of transistors in place of the vacuum tubes. The electronic time per operation was 1 to 10 microseconds. Memory access time from the magnetic drum or core was 1-10 microseconds. Magnetic tapes, drums and punched cards were used as secondary storage.
THIRD GENERATION COMPUTERS
The technology of putting more transistors on a single chip developed fast. By 1963, 8 transistors were being put on a chip. This new technology was called the Integrated Circuit (IC). Today the figure is a quarter of a million transistors. Military and Space needs fuelled the research in transistor technology.
At first only a few components could be integrated. This was known as small scale integration (SSI). Later it became possible to integrate up to a hundred components and was known as medium scale integration (MSI).
The third generation was marked by the use of Integrated Solid State circuits, improved secondary storage devices and new input output devices. All this made possible, multi processing, multiprogramming, whereby a number of data processing jobs from different sources could be run virtually at the same time on a single centrally located computer. The electronic time per operation of integrated circuits was 0.1 to 1 microsecond. Central memory access time was 0.1 to 10 microseconds.
FOURTH GENERATION COMPUTERS
The dream of the Fairchild Corporation to put a whole computer onto a single chip heralded the entry of the microprocessor. This was made possible by Robert Nocye. The technology of large scale integration (LSI) technology. It is the LSI technology that led to the development of the microcomputer. It is expected that before 1990 more than one million components will be integrated onto a single chip. This will be known as the Very Large Scale Integration.
A microprocessor is a tiny solid state device, about the size of a thumb nail which in itself is a small computer capable of performing arithmetic and logical operations. The revolution brought about by the microprocessor led to the following developments:
Large computers that are much faster, less expensive and of much greater capacity than equivalent sized third generation computers.
Minicomputers that is equally capable but much less expensive.
Microcomputers which are even further miniaturized computers.
Among the advanced input-output devices employed by the fourth generation computers are the optical readers, by which whole documents can be led into the computer; audio response terminals, by which an operator can vocally communicate with the computer; and graphics display terminals by which pictures can be obtained from the computer.
In is predicted that in the future it will be possible to put more than 10 million components on a single chip. In other words, the size of today's mainframe computer will shrink to the size of a pocket calculator and still be many times more powerful!
FIFTH GENERATION COMPUTERS
These are the so called "thinking computers". They are expected to have capabilities of reasoning, making judgments and the ability to learn. The key to artificial intelligence lies in the newer methods of programming. One can imagine such computers communicating with man via audio visual means. They may be capable of providing invaluable assistance to man in the field of medicine, due to their massive data base. The real significance is that we are on the threshold of new discoveries and new worlds previously beyond man's comprehension.
CLASSIFICATION OF COMPUTER SYSTEMS
Computer Systems
As we have read in the previous chapter, a computer system is a machine which helps us to process data in a fast, efficient and reliable manner. To have a clearer and better understanding of computer systems let us classify different computer systems as well as classify a computer system into its integral parts.
According to data processing
Computer systems may be classified according to the data they are designed to process or they may be classified according to their size and capabilities.
The data required for processing may be obtained either as a result of counting or through some measuring device. Data obtained through counting is known as discrete data, while that obtained through measuring instruments is known as continuous data. An example of discrete data is the number of marks obtained by a student in an examination, while the constant monitoring of the Electro-Cardiogram of a patient is an example of continuous data.
Analog Computers
The analog computers do not directly interact with numbers, but rather deal with variables measured along a continuous scale, like the temperature of a room. Analog computers may be accurate to within 0.1% of the correct value.
Digital Computers

A digital computer operates on discrete data. It works basically by directly counting numbers that represent numerals, letters or other functional symbols. Digital computers can be further divided into special purpose and general purpose digital computers.
As the name suggests, a special purpose digital computer is one which has been designed to perform one specific task. The set of instructions required for that task is permanently stored in the computer's memory. What this type of computer lacks in variety, it makes up in speed and efficiency.
A general purpose computer is one which can store different programs and is also re-programmable. The only limitation to the versatility of this type of computer is the extent of imagination of the human mind. In fact, these computers can be made to perform a plethora of different and varied functions.
Hybrid Computers
A hybrid computing system is one in which desirable characteristics of both the analog and digital computers are integrated. In an intensive care unit, analog computers may measure the patient's heart rate, temperature, etc. The measurements may then be converted into numbers and supplied to the digital part of the system which will thereafter regulate the flow of certain medications.
Microcomputers or Personal Computers
This classification, nevertheless, is still very very unconditional. If a survey were to be conducted among the top computer personnel and the difference between a micro and a mini asked, a dozen different answers would result. Computer technology changes so fast that a mini today could be known as a micro after one year's time. In fact the distinction between the different types of classified computers is decreasing day to day.
A personal Computer (PC) or a micro is the smallest general purpose computer system which can execute programs to perform a variety of instruction. It has all the functional elements found in a larger system. These computers usually have an 8, 16 or a 32 bit microprocessor.

The cheapest variety is one with an 8 bit microprocessor. All 8 bit micros are built around a few popular microprocessors like the Z80, 8080 etc. By an 8 bit microprocessor it is understood that it can process 8 bits or 1 byte of data at a single given time. These microprocessors have a 16 line address bus and hence they can identify a maximum of 2 16 i.e. 65,536 or 64K address locations.
In the 16 bit micros the data is processed 16 bits at a time. An example of a 16 bit microprocessor is the Intel 8088 used by the IBM PcJr and the Intel 8086. These microprocessors usually have an expanded 20 line address bus. Hence they identify a maximum of 2 20 i.e. 1,048,576 or 1 M address locations. This eliminates one of the limitations of 8 bit computers i.e. of main RAM memory. Whereas the 8 bit computers can only have a maximum of 64 K RAM the 16 bit computers can have up to 1 M of RAM memory. Usually, however, the 16 bit computers come with 256 K RAM with the facility to expand. If necessary, extra bus lines can be adapted to increase the main memory capacity.
The latest introduction is the Motorola 68000 which is a 32 bit chip. This can process 32 bits of data at a given time. It has an expanded 24 line address bus which gives it a tremendous potential of accessing up to 2, 24 or 16 M address locations, thereby increasing the potential main RAM memory to 16 M.
The peripherals possible with the micros are varied. Virtually all micros come with a QWERTY type keyboard as an input device. A light pen, mouse etc, can also be used as input devices. The VDU is the most common output display unit. It can be connected with a variety of printers, plotters and also speakers. These micros can easily be connected with a modem and hooked up to mainframe system, thereby acting as intelligent terminals. Storage can be done on floppy disks, Magnetic tapes, Winchester etc. The peripherals devices of computer systems will be dealt with later on in this book. Some of the most popular commercially available micros are the IBM PcJr, Apple Commodore, Hewlett Packard, BBC Micro etc.
Minicomputers
Through it is almost impossible to define a minicomputer system anymore; an arbitrary definition can be resorted to. This is the way it goes: "A minicomputer system is a small general purpose computer varying in size from a desktop model to a unit the size of a four drawer filing cabinet". It is quite obvious that there is tremendous amount of similarity between the more powerful micros and the lower end minis. The same situation exists on the other end where the lower priced mainframes are almost similar to the higher priced minis. In fact the minicomputer manufacturers are almost being wiped out with the advent of the super micros. Today’s typical mini will surpass the PC in storage capacity, speed of arithmetic operations and ability to support a variety of peripherals. Minis are usually multi-user computers in contract to the micros. They were previously available with the 16 bit microprocessor but nowadays are available only with the 32 bit one. One of the most important uses of minis is in Distributed data processing networks.
Mainframe Computer
A whole series of mainframe models, ranging in size from small to very large, are typically lumped together under a family designation by mainframe manufacturers. The primary and on-line storage capabilities increase. Several microprocessors are used in place of the single microprocessor used in micro and minicomputer systems. The result is that these systems can process data much faster. These systems have the flexibility to operate automatically from 2 to 8 bytes in the same unit of time. The mainframe vendors also have much large libraries of application programs. One of the most versatile and popular mainframes available is the IBM 370.
Supercomputers
Supercomputers are the most powerful and expensive computers made. Only a few of these computer mostres are made each year because only a few people need it. These computers are a national resources These computers are designed to perform scientific application and hence the computational speed is very important. To maximize the speed each address location holds 64 bits of information. The time required to execute a single operation may be as slow as 4 nanoseconds. The prices of these computers range from 4 million to 215 million dollars. Cray research and control Data Corporation are the primary builders of super computers in U.S.A. Not all super computers are designed for scientific work. Some computers have been used for producing computer generated images in films.
HARDWARE AND SOFTWARE
The first electronic computer was completed in 1946, yet it was not until the mid-50's that the computer industry was firmly established since the time, aided by new technology, the industry has made phenomenal progress for example, notable advances have occurred in the provision of storage and in processor speeds. These in turn have facilitated the development of sophisticated computer systems and complex software. Future developments read like a science fiction story with powerful computers shrunk to the size of a match-box. However, there is one area which, generally speaking. Has not developed to the same extent. This is in the realm of 'man to machine' and 'machine to man' communications, in other words, the input-output devices. The principal reason for this is that speed of communication depends in many cases on mechanical movement and the potential for improvement of such devices is limited.
Input-output units surround the central processor, hence the term peripheral devices. Their purpose is to provide an information link between the outside world and the CPU. In computing parlance, they act as an interface, translating the familiar symbols which we can read into the binary patterns that can be handled electronically within the CPU : they then translate the patterns back again for easily readable output.
Input
Communicating with computers has always been a problem. In the early days, programrs had to communicate directly to the computer in its own language. Main, rather than the machine has to do the translating. The difficulties of having to think in binary led man to develop easier methods of communication. In some cases he was able to do this by making use of existing technology from other fields. For example, the use of punched cards and perforated tape by people unfamiliar with binary permitted the encoding of our familiar symbols into an intermediate stage. This intermediate stage is then translated into binary by the machine.
Since the early days, developments have taken place which permit the computer to 'read' our typed (and even written) symbols directly via mark recognition (mark reading, mark sensing), magnetic ink character recognition (MICRO) and optical character recognition (OCR). There have also developments which have led to typewriter input (teletypewriter) devices and to visual (TV - type) display devices equipped with keyboards for input. More recently, hand-held terminal entry systems and hand-print pads have been introduced, and research into voice input has been successful enough for some commercial companies to use this method of communication in a limited way. The most widely used input devices today are teletypewriter terminals and visual display devices, which also double as output devices.
Mark and Character Recognition

This method involves the recognition of marks or characters, e.g. from work dockets, checks, till roll, and also cards. There are three types of recognition:

 Mark Sense Reading.
 Magnetic Ink Character Recognition (MICR)
 Optical Character Recognition (OCR)
On the whole, to achieve the required standard of accuracy which the computing process demands, the reading devices associated with mark and character recognition operate at slower rates than the punched card reader.
Mark Sense Reading
This is literally what it says. The card or form is divided up into boxes, in which a mark is made by pencil or pen. A character is represented by marking the correct combination of boxes in any one column, as opposed to displaying holes from a punched card. Forms and cards are pre-printed for special purposes so that a mark can be made in a certain position to represent a YES or NO, to answer a market survey question for example, or to signify a number, as on insurance forms, gas and electricity recording cards.
In one form of detection, the conductivity of graphite marks is sensed. The method necessitates the use of a soft pencil, and non-graphite pen or printed marks are not acceptable. Another method uses equipment which reads marks optically. Quite simply a light source senses the presence of a mark. In this case, special pencils are not required to mark the cards or documents. A mark reader may be designed to be insensitive to certain colors. These colors can then be safely used in the pre-printing of the cards or documents without risk of being read when marks made later are sensed.
Magnetic ink character recognition - MICR
Due to the success of mark recognition, investigation turned to the possibility of reading characters. The first successful form of character shapes printed in an ink containing magnetically particles. Early in 1966, two standard MICR fonts (typographical styles) were accepted by the International Standards Organization. One, known as E13B, consists of the numerals 0-9 and four special characters. This is used principally for bank checks. The code number of the bank, the customer's account number, and the check sequence number are all pre-printed in magnetic ink. When a check is submitted to a bank the amount of the transaction is inscribed on it before the check is presented for computer processing.
The magnetized ink induces a current in a reading circuit. The current induced will be proportional to the area of ink being scanned. The patterns of the varying currents can then be compared with and identified as, bit patterns or the selected character. E13B is used in the USA, where it originated and in the UK. Another MICR font, which originated in France and is used in Europe, is CMC7. This includes the digits 0-9, the letters of the alphabet, and five special characters. The symbols are made up of seven magentizable lines with six spaces of varying width between them. A wide space generates a binary one, a narrow space a 0. The speed of reading Micro is around 1200 documents a minute.
MICRO systems employ character styles designed expressly for machine recognition and, therefore, the character has to be accurately formed. They also require magnetic ink. These factors make for expensive printing, but one useful advantage is that characters printed with ink containing magnetically particles can still be read even when over-stamped, as many be the case with bank checks. MICR readers cannot verify, they can only identify. With a check someone still has to verify the amount to be paid, to whom it is to be paid and, most importantly, that the signature authorizing the payment is correct.
Optical character recognition - OCR
It is not only handwriting which varies. Different typewriters and different typesetters produce the letters of the alphabet in a variety of forms, shapes and sizes. Nevertheless, there are certain characteristics which are peculiar to, and common to, each letter, however it is produced.

OCR readers examine each character as if it were made up of a collection of minute spots. Once the whole character has been scanned, the pattern detected is matched against a set of patterns stored in the computer. Whichever pattern it matches, or nearly matches, is considered to be the character read. Patterns which cannot be identified are rejected. OCR readers can read at a rate of up to 2400 characters per second. They are generally designed to operate at slower speeds, typically 300-800 characters per second, at which they are more accurate and can handle characters which are not quite so perfectly formed. OCR readers are expensive devices of data to process.
A wide range of fonts, using ordinary inks, can now be accepted by OCR readers, including many common typewriter fonts. The standard fonts used are OCR-A (American Standard) and OCR-B (European Standard). Some OCR readers can accept computer print-out and complete pages of type text. It is possible that a computer could be programd to accept some signatures, but it is unlikely that it could ever be programd to accept every type of signature. Even so, devices have been developed which can read neat hand printing (capital letters rather than lower case) in black ink, and with sufficient accuracy for this to become a viable form of input. Refer figure 3.
Terminals

Teletypewriter terminal
With all the forms of input considered so far, the data is first prepared using a separate device. However, it is possible to communicate with the computer directly by using a keyboard machine. A teletypewriter terminal, sometimes called a teleprinter terminal or a keyboard/printer terminal, usually combines a keyboard for manual input of information with a printer for outputting a 'hard copy' (printer record) of the input, system information and program results. The printing device outputs one character at a time at rates between 20 and 50 characters per second on continuous rolls of paper (typically 8 to 20 inch wide) or on fanfold paper, according to the application.
Some terminals display information on a screen as opposed to printing on paper. These are known as visual display units. Some terminals also have a facility for punching and reading paper tape. This enables information to be copied and retained in coded form so that it can be used again later without the information having to be 're-typed'.
One of the features of a terminal is that it can be situated some distance from the computer. It must, therefore, include some means of transmitting information. Some terminals are designed only to send information and some only to receive, but in general both functions are carried out.
Terminals may be connected to a computer in one of two ways. Some are connected locally, by direct cable line. This is known as 'hard-wiring' and does not usually extend more than several hundred feet from the computer itself. The second method is via a remote link, either by telegraph or telephone line, or by microwaves. Whilst it is probable that the 'remote' terminal would be some distance from the computer, it is possible that it may be on the same site as the computer, even in the same room. The terms 'local' and 'remote' refer to the way in which a terminal is linked to the computer, with each terminal making use of the computer in turn. This is known as time-sharing.
Terminals extended the use of the computer to various places of work. They are widely used for such tasks as stock control, entering others, updating accounts-of the computer to various places of work. They are widely used for such tasks as stock control, entering orders, updating accounts and seat reservations. The terminals found in sho9ps and stores might combine several of these tasks while also acting as cash registers. They can be sited at various points on a factory floor to record and receive information on different stages of an industrial process. The development of compact portable terminals as extended their usefulness still further.
Terminals now provide an extremely important and effective communication link with the computer. Their versatility, combined with the fact that they can be situated away from the computer, has led to a rapid growth in their use, and for many applications they are not the standard input device.
With the aid of micro technology many modern terminals are provided with circuitry which enables them, without being linked to a computer to perform some of the simple tasks normally carried out by the computer, example the editing of data or text. Terminals which can perform some computing are often referred to as intelligent terminals. The degree of local intelligence given to terminals is growing rapidly. Refer figure 4.
Bar-code recognition
Lines of bars can be arranged in a code as illustrated in the following figure.
Bar codes are used particularly by the retail trade for labeling goods and by supermarkets for labeling shelves and in stock control. They are also used for numbering books in public libraries so that when a book is borrowed or retuned it can be recorded using a computer.
Bar code reading is performed by a scanner or light pen which is generally linked to a computer via a terminal device. They scanner or light pen is stroked across the pattern of bars, a sequence of bits is generated and information recorded.
Hand-held terminal
Another important innovation is the development of hand-held data entry terminals. A typical hand-held device looks similar to a calculator and may be battery powered. Information is usually entered via the keys. In addition a scanner or light pen may be incorporated in the design to enable to capture of bar-coded data. The hand-held terminal may be provided with up to 32 K or memory for temporary storage of information before transmission to a computer. Typically the transmission is over ordinary telephone lines. Hand-held device enables the collection of data at the place where it is generated and avoids the delay and cost of data preparation at the computer site. The hand-held terminal could be useful to a salesman in the field for direct ordering or for such diverse applications as meter reading, road traffic service, market research and control of medical supplies in hospitals.
Hand-print terminal
A recent development is the introduction of hand-print terminals, variously called data tablets or pads. The device generates a representation of character direct to the computer as the character is hand-printed on paper lying on top of the pad. Different types of pad employ different sensing techniques and a special pen may or may not be required. One method is based on the generation of pulses when to electrically resistive layers beneath the surface of a pad are brought together by the pressure of the pen as the characters are printed. A typical data pad incorporates touch sensitive keyboard for entering non-written characters. It may also be able to digitize, and hence input, drawings as well as characters.
Voice Input
Voice input system accept spoken input. The wage form created by the spoken input is analyzed; patterns are extracted and matched against pre-stored patterns to identify the input. Once identified, the appropriate coding is generated, for handling within the computer. Before a voice input system can be used it must first be provided with a vocabulary. The words and phrases the system is to recognize are spoken with the system operating in a so-called 'training mode'. In this mode the patterns are created and stored for future matching. A system may be trained to recognize both the voice of one or more operators and a given vocabulary for each operator, in which case unwanted inputs or unauthorized speakers are rejected.

The voice linked to a system may be a microphone, telephone or radio communication. Voice input systems are not yet widely used but they are a reality. Vocabularies are comparatively small, typically one hundred to three hundred words. In general as the size of a vocabulary increases recognition reliability decreases. Possible applications or situations in which an operator needs to be free to move about a work are or to have his hands free or where an operator travels and relies on telephone contact with his work base.
Output

We have seen that there are several paths by which man can communicate with the machine. There are, also, several ways in which the machine can communicate with man.

Printed output

Line Printer

The most common method of obtaining output is via a device known as a line printer. Rows of character sets (fonts) are either wrapped around a drum or affixed to a chain. The drum or chain revolves across the path of a series of hammers each of which corresponds to a print position. As the character to be printed is selected, a magnetically controlled hammer presses it onto an ink ribbon and there onto paper, rather like a typewriter works.

A variety of stationery is used, with special designs to suit particular applications, such as pre-printed electricity bills and rate demands. Forms can also be multi-part (where copies are required for different department), with several sheets impregnated with carbon or separated by interleaved carbon paper. In general, though the paper used on drum and chain line printers is known as 'continuous stationery'. The paper can be plain or lined, is usually fan-folded and perforated for separation into convenient lengths (11 inch typically and about 15 inch wide) and comes folded in boxes, usually in ream multiples.

There are normally 132 or 136 print positions per line though some devices are able to print more. Character sets vary in content and size. The 64-set has more special characters than the 48-set and the 96 set prints in lower case letters as well as in upper case. It is even possible to print in Hebrew and in Japanese. The impact method of printing employed by line printers has limitations with respect to quality but it allows for very high speeds, ranging from 300 to 2500 lines per minute, and is associated particularly with large computer output requirements.

Serial Printer

Another category of printer is the serial printer which outputs one character at a time as opposed to one line at a time eg. A teletypewriter terminal. Some devices are print only machines with no keyboard for input purposes. A serial printing device can normally be operated using continuous stationery or separate sheets typically A4 size. A serial or character printer is much slower than a line printer. It is also much cheaper.

A special type is the daisywheel printer, so called because it uses a daisy-shaped disk made of metal or plastic which holds some 96 characters in its petals. Print heads are interchangeable, enabling the use of different character fonts. There are normally 132 or 136 print positions per line and typical printing speeds are 25 to 60 characters per second. Daisywheel printers are noted for their print quality and are often used with word processing systems and other applications where quality of print is desired.

The dot matrix printer is another common type of serial printer. The print head comprises a matrix of tiny needles, typically seven rows with nine needles in each, which hammers out characters in the form of patterns of tiny dots. The shape of each character, i.e. the dot pattern, is obtained from information held electronically in the printer. Matrix printers are faster than daisywheel printers in the range of 45 to 220 cps, but the quality of print inferior.

In addition there are devices which employ non-impact techniques. Thermal printers, for example, use heat to create characters in dot matrix form on special sensitized paper.

Serial printers in general are suitable for applications producing low volume output and are frequently used as output devices for small, special purpose computer systems and microcomputers.

Laser Printer

The most exciting new development is the laser printer. Using a combination of electronics, laser and copier technology, it is possible to design printers capable of converting computer information into print, page by page. Laser printers are very fast, produce very high quality print and can call upon a wide selection of character fonts. However there are few applications which can justify the present high cost of laser printing. Most laser printers operate at speeds between 30 and 250 pages per minute.

Graphical Output

Information can be output in graphical form using graph plotters. These are usually slow but the accuracy (up to within one thousandth of an inch) is more important than speed. Since there is a considerable mismatch between the speeds at which the CPU operates and the speed of the plotting device, output is often transferred to magnetic tape or paper tape first, and then plotted from the information on the tape. Computer systems dedicated to design work may send output direct to a plotter.

The most common type of plotter is flat bed device. As the name suggests, it plots on paper (or some other material) which rest on a flat bed. The pen moves in perpendicular direction across the bed. One directional movement is supplied by a gantry which straddles the bed and runs on rails at either side. The rails allow movement up and down the length of the bed. The other direction is supplied by a pen turret running to and for across the gantry itself. The turret may contain different colored pens (felt tip, ball point or ink) for multi-contain different colored plotting. The plot size is restricted by the area of the bed. Some may be as small as A2 size whilst some very large beds used in aircraft design, for such things as wing profiles, can be up to 20 ft by 50 ft. Plotters are used to trace out conventional graphs and to assist with design, e.g. in textiles. Some plotters can etch plastic or metal plates.

Visual Display Units - VDU

A visual display device uses a cathode ray tube (CRT) to display information. It looks like a television screen and is similar in other respects. VDUs are used particularly in situations where information is required quickly and where perhaps there is little advantage in having a permanent record of the information.

The VDU is really a type of terminal, with a keyboard for manual input of characters to the computer and with a screen for character display of the input or output. The screen displays information as it is keyed in enabling a visual check before the input is transferred to the computer. Information is displayed very much more quickly than by the convenient keyboard/printer terminal (teletypewriter) and it is almost silent in its operation. One disadvantage is that the device provides no hardcopy of the output, but it is normally possible to add a printing device which can be switched on to provide a 'hard' copy of the display when it is needed. However, in the type of applications for which VDUs are particularly suited, where the display provides information on which action is taken immediately, there is often no requirement for printed output.

The most common display method is to generate characters from a 'dot matrix'. A selected pattern of dots is illuminated to form a character. Screens vary in size (12 in. and 15 in. are common) and in the number of characters which they can display. A maximum display is typically 24 lines of 80 characters, with sometimes an optional display of 132 characters per line.

Initially used in situations where information is required quickly, for example in airline seat reservations where speed is the essence in handling customer enquirer, VDUs are now widely used for general data entry and retrieval of stored information. The VDU is fast becoming as common a piece of office equipment as the typewriter. Many of today's models are intelligent terminals, incorporating microprocessors, and able to carry out some computing functions.

There are not some VDUs equipped with touch sensitive screens which allow data to be input by touching the screen with the fingertip. The screen surface consists of a number of 'touch points' as defined by the program in use. When touched the terminal sends the co-ordinates of the point to the computer.

Graphics VDU

This type of VDU is able to display graphics and diagrams as well as alphanumeric characters. It is a specialized piece of equipment normally very much more expensive than the conventional VDU, and is used particularly as an aid to design. Via suitable software it can enable a design to be viewed from different angles and can display intricate detail at varying levels of magnification. The design can then be modified as necessary. Designs may be enhanced by different shades of grey and some device display in color. Graphics devices are used as an aid in car and boat design, in constructional and civil engineering applications and by architects and interior designers. Graphics often make considerable programming demands on the system and these sophisticated types of display are usually linked to large, powerful computer systems, or to smaller machines which are used solely for this one purpose.

A copying device can be used in conjunction with a graphics VDU to provide hard copy of any display. A link up with a Computer Output Microfilm Device is particularly significant. It enables the recording of a whole series of graphs or designs which can then be viewed later at leisure to assist in the selection of an optimum design.

Computer Output Microfilm - COM

A computer output microfilm device translates information normally held on magnetic tape into miniature images on microfilm. The device displays the information as characters on a CRT screen and then using photographic methods records the display onto film, usually 16 or 35 mm. Drawings or pictures can usually be displayed as well as narrative text. A full display (perhaps equivalent to a page of line printer output) is recorded as a single frame.

A special reader or reader/printer can be used subsequently to view the processed film. The reader operates on a 'back projection' principle displaying a frame at a time on a translucent screen, typically about A4 size. The printer can then produce a hard copy of what is presented on the screen, probably using an electrostatic method.

Microfilm, in roll form or microfiche, is small and easily stored and the speed of recording is some 25 - 30 times faster than the average line printer. The equivalent of thousands of pages of computer output can be stored in a small drawer and the cost of the microfilm for a page of output is less than a sheet of line printer paper. Once the film has been processed it can easily be duplicated and full size hard copy prints made quickly and inexpensively.

A COM system is ideal for use in applications where there is a large amount of information to be retained which is required only for manuals, industrial catalogues and archives. Companies may need to retain records of such things as bills and invoices for a number of years before destroying them. COM provides an easy way of retaining the information, of retrieving it in a matter of seconds using a compact desk top viewer and is ideal when multiple copies of reports or information are required. The information may be distributed in microfiche form.

Complete COM systems are relatively expensive to install and are associated more with big computer users. Small and medium-sized computer users who need microfilm are more likely to take their files for conversation into microfilm to a bureau offering a COM processing service. Desk top microfiche readers, ideal for use on workshop benches as well as in offices, are comparatively inexpensive. There are also portable readers small enough to fit into briefcases which run off mains or batteries.

Attention has now turned to providing a mechanism to input directly from microfilm. Some KIM (Computer Input Microfilm) equipment is already on the market but it is not yet widely used.

Audio Response Unit

The computer can be used effectively to trigger verbal communication via an audio response unit and this may be an appropriate method to use if standard replies to requests for information are all that are required.

Messages are composed and transmitted in coded form, may be over telephone lines, using perhaps a keyboard for input or even a voice-input system. The unit analysis the input assembles the response from pre-recorded words and phrases and delivers the verbal reply. For the sake of clarity the response is delivered more slowly than words are normally spoken. The digitized format in which the spoken word is retained by the system makes heavy demands on storage and typical systems can only store around 400 spoken words or phrases. However, within the context of a single application, the limitation of a small vocabulary may not be unduly restrictive.

Speech synthesis is getting cheaper and voice output is starting to make sense in a variety of applications. Transient information, that is information which only needs to be conveyed once, may be a good candidate for voice output. A good test of whether information is transient is whether it can usefully be conveyed over the telephone. Potential application areas include remote enquiry of stock quotations and product availability, and instructional sequences for various procedures. An audio response unit, sometimes called a voice output terminal may be attached to a large computer system or be supported by a minicomputer dedicated to the one purpose. Very small devices in the form of micro chips or boards can be added to some micros systems to provide limited voice output, for example with spelling instruction and learning games.
Output

We have seen that there are several paths by which man can communicate with the machine. There are, also, several ways in which the machine can communicate with man.

Printed output

Line Printer

The most common method of obtaining output is via a device known as a line printer. Rows of character sets (fonts) are either wrapped around a drum or affixed to a chain. The drum or chain revolves across the path of a series of hammers each of which corresponds to a print position. As the character to be printed is selected, a magnetically controlled hammer presses it onto an ink ribbon and there onto paper, rather like a typewriter works.

A variety of stationery is used, with special designs to suit particular applications, such as pre-printed electricity bills and rate demands. Forms can also be multi-part (where copies are required for different department), with several sheets impregnated with carbon or separated by interleaved carbon paper. In general, though the paper used on drum and chain line printers is known as 'continuous stationery'. The paper can be plain or lined, is usually fan-folded and perforated for separation into convenient lengths (11 inch typically and about 15 inch wide) and comes folded in boxes, usually in ream multiples.

There are normally 132 or 136 print positions per line though some devices are able to print more. Character sets vary in content and size. The 64-set has more special characters than the 48-set and the 96 set prints in lower case letters as well as in upper case. It is even possible to print in Hebrew and in Japanese. The impact method of printing employed by line printers has limitations with respect to quality but it allows for very high speeds, ranging from 300 to 2500 lines per minute, and is associated particularly with large computer output requirements.

Serial Printer

Another category of printer is the serial printer which outputs one character at a time as opposed to one line at a time eg. A teletypewriter terminal. Some devices are print only machines with no keyboard for input purposes. A serial printing device can normally be operated using continuous stationery or separate sheets typically A4 size. A serial or character printer is much slower than a line printer. It is also much cheaper.

A special type is the daisywheel printer, so called because it uses a daisy-shaped disk made of metal or plastic which holds some 96 characters in its petals. Print heads are interchangeable, enabling the use of different character fonts. There are normally 132 or 136 print positions per line and typical printing speeds are 25 to 60 characters per second. Daisywheel printers are noted for their print quality and are often used with word processing systems and other applications where quality of print is desired.

The dot matrix printer is another common type of serial printer. The print head comprises a matrix of tiny needles, typically seven rows with nine needles in each, which hammers out characters in the form of patterns of tiny dots. The shape of each character, i.e. the dot pattern, is obtained from information held electronically in the printer. Matrix printers are faster than daisywheel printers in the range of 45 to 220 cps, but the quality of print inferior.

In addition there are devices which employ non-impact techniques. Thermal printers, for example, use heat to create characters in dot matrix form on special sensitized paper.

Serial printers in general are suitable for applications producing low volume output and are frequently used as output devices for small, special purpose computer systems and microcomputers.

Laser Printer

The most exciting new development is the laser printer. Using a combination of electronics, laser and copier technology, it is possible to design printers capable of converting computer information into print, page by page. Laser printers are very fast, produce very high quality print and can call upon a wide selection of character fonts. However there are few applications which can justify the present high cost of laser printing. Most laser printers operate at speeds between 30 and 250 pages per minute.

Graphical Output

Information can be output in graphical form using graph plotters. These are usually slow but the accuracy (up to within one thousandth of an inch) is more important than speed. Since there is a considerable mismatch between the speeds at which the CPU operates and the speed of the plotting device, output is often transferred to magnetic tape or paper tape first, and then plotted from the information on the tape. Computer systems dedicated to design work may send output direct to a plotter.

The most common type of plotter is flat bed device. As the name suggests, it plots on paper (or some other material) which rest on a flat bed. The pen moves in perpendicular direction across the bed. One directional movement is supplied by a gantry which straddles the bed and runs on rails at either side. The rails allow movement up and down the length of the bed. The other direction is supplied by a pen turret running to and for across the gantry itself. The turret may contain different colored pens (felt tip, ball point or ink) for multi-contain different colored plotting. The plot size is restricted by the area of the bed. Some may be as small as A2 size whilst some very large beds used in aircraft design, for such things as wing profiles, can be up to 20 ft by 50 ft. Plotters are used to trace out conventional graphs and to assist with design, e.g. in textiles. Some plotters can etch plastic or metal plates.

Visual Display Units - VDU

A visual display device uses a cathode ray tube (CRT) to display information. It looks like a television screen and is similar in other respects. VDUs are used particularly in situations where information is required quickly and where perhaps there is little advantage in having a permanent record of the information.

The VDU is really a type of terminal, with a keyboard for manual input of characters to the computer and with a screen for character display of the input or output. The screen displays information as it is keyed in enabling a visual check before the input is transferred to the computer. Information is displayed very much more quickly than by the convenient keyboard/printer terminal (teletypewriter) and it is almost silent in its operation. One disadvantage is that the device provides no hardcopy of the output, but it is normally possible to add a printing device which can be switched on to provide a 'hard' copy of the display when it is needed. However, in the type of applications for which VDUs are particularly suited, where the display provides information on which action is taken immediately, there is often no requirement for printed output.

The most common display method is to generate characters from a 'dot matrix'. A selected pattern of dots is illuminated to form a character. Screens vary in size (12 in. and 15 in. are common) and in the number of characters which they can display. A maximum display is typically 24 lines of 80 characters, with sometimes an optional display of 132 characters per line.

Initially used in situations where information is required quickly, for example in airline seat reservations where speed is the essence in handling customer enquirer, VDUs are now widely used for general data entry and retrieval of stored information. The VDU is fast becoming as common a piece of office equipment as the typewriter. Many of today's models are intelligent terminals, incorporating microprocessors, and able to carry out some computing functions.

There are not some VDUs equipped with touch sensitive screens which allow data to be input by touching the screen with the fingertip. The screen surface consists of a number of 'touch points' as defined by the program in use. When touched the terminal sends the co-ordinates of the point to the computer.

Graphics VDU

This type of VDU is able to display graphics and diagrams as well as alphanumeric characters. It is a specialized piece of equipment normally very much more expensive than the conventional VDU, and is used particularly as an aid to design. Via suitable software it can enable a design to be viewed from different angles and can display intricate detail at varying levels of magnification. The design can then be modified as necessary. Designs may be enhanced by different shades of grey and some device display in color. Graphics devices are used as an aid in car and boat design, in constructional and civil engineering applications and by architects and interior designers. Graphics often make considerable programming demands on the system and these sophisticated types of display are usually linked to large, powerful computer systems, or to smaller machines which are used solely for this one purpose.

A copying device can be used in conjunction with a graphics VDU to provide hard copy of any display. A link up with a Computer Output Microfilm Device is particularly significant. It enables the recording of a whole series of graphs or designs which can then be viewed later at leisure to assist in the selection of an optimum design.

Computer Output Microfilm - COM
A computer output microfilm device translates information normally held on magnetic tape into miniature images on microfilm. The device displays the information as characters on a CRT screen and then using photographic methods records the display onto film, usually 16 or 35 mm. Drawings or pictures can usually be displayed as well as narrative text. A full display (perhaps equivalent to a page of line printer output) is recorded as a single frame.
A special reader or reader/printer can be used subsequently to view the processed film. The reader operates on a 'back projection' principle displaying a frame at a time on a translucent screen, typically about A4 size. The printer can then produce a hard copy of what is presented on the screen, probably using an electrostatic method.
Microfilm, in roll form or microfiche, is small and easily stored and the speed of recording is some 25 - 30 times faster than the average line printer. The equivalent of thousands of pages of computer output can be stored in a small drawer and the cost of the microfilm for a page of output is less than a sheet of line printer paper. Once the film has been processed it can easily be duplicated and full size hard copy prints made quickly and inexpensively.
A COM system is ideal for use in applications where there is a large amount of information to be retained which is required only for manuals, industrial catalogues and archives. Companies may need to retain records of such things as bills and invoices for a number of years before destroying them. COM provides an easy way of retaining the information, of retrieving it in a matter of seconds using a compact desk top viewer and is ideal when multiple copies of reports or information are required. The information may be distributed in microfiche form.
Complete COM systems are relatively expensive to install and are associated more with big computer users. Small and medium-sized computer users who need microfilm are more likely to take their files for conversation into microfilm to a bureau offering a COM processing service. Desk top microfiche readers, ideal for use on workshop benches as well as in offices, are comparatively inexpensive. There are also portable readers small enough to fit into briefcases which run off mains or batteries.

Attention has now turned to providing a mechanism to input directly from microfilm. Some KIM (Computer Input Microfilm) equipment is already on the market but it is not yet widely used.
Audio Response Unit
The computer can be used effectively to trigger verbal communication via an audio response unit and this may be an appropriate method to use if standard replies to requests for information are all that are required.
Messages are composed and transmitted in coded form, may be over telephone lines, using perhaps a keyboard for input or even a voice-input system. The unit analysis the input assembles the response from pre-recorded words and phrases and delivers the verbal reply. For the sake of clarity the response is delivered more slowly than words are normally spoken. The digitized format in which the spoken word is retained by the system makes heavy demands on storage and typical systems can only store around 400 spoken words or phrases. However, within the context of a single application, the limitation of a small vocabulary may not be unduly restrictive.
Speech synthesis is getting cheaper and voice output is starting to make sense in a variety of applications. Transient information, that is information which only needs to be conveyed once, may be a good candidate for voice output. A good test of whether information is transient is whether it can usefully be conveyed over the telephone. Potential application areas include remote enquiry of stock quotations and product availability, and instructional sequences for various procedures. An audio response unit, sometimes called a voice output terminal may be attached to a large computer system or be supported by a minicomputer dedicated to the one purpose. Very small devices in the form of micro chips or boards can be added to some micros systems to provide limited voice output, for example with spelling instruction and learning games.
BACKING STORES
We indicated earlier that a computer's memory is limited in size, is needed as a working space for the current program, and only retains information on a temporary basis. However computers may often work on vast amounts of data and backing stores are used to retain the data on a permanent basis. Information stored on these devices can be retrieved and transferred speedily to the CPU when it is required.
Several different devices can provide this additional storage space, but the one selected will depend mainly on how the information needs to be accessed. There are two methods of access, serial and direct. Information on a serial device can only be considered in the same sequence in which it is stored. This would be suitable, for example, for dealing with a mailing list where each address needs to be accessed in turn. However, should an address be required out of order, it can only be retrieved by searching through all those addresses which are stored before it. Frequently we need to access information in a more direct manner than serial devices allow. For example, at any given moment in a bank, some customer will be requesting details about his account. Backing storage devices exist which permit access to individual information in this more direct or immediate manner. These direct devices are also called random access devices because the information is literally available at random, i.e. it is available in any order.
Magnetic Tape
Magnetic tape provides only serial access. It can be referenced many times without the need for replacement. In addition, information can be erased by recording new information in its place. The tape has a ferromagnetic coating on a plastic base, is usually 1/2 in. wide and comes in reels of 50 to 2400 feet. It is similar to the tape used on a tape recorder except that it is of higher quality and more durable.
Information is retained on the tape in the form of magnetized and non-magnetized spots (representing 1's) which are arranged in tracks, typically seven or nine, running the length of the tape. To represent a character in tracks, special codes are need, just as they are for paper tape. Information recorded on magnetic tape is stored in varying densities, typically 556 characters to the inch, or 800 or 1600 or even as dense as 6250 characters to the inch, with the higher densities applicable to the more modern systems using nine-track tapes.
Turning to the tape drive itself, it can be seen that the tape runs from a supply reel to a pick-up reel via two vacuum channels and between a set of read/write heads. The two vacuum channels are designed to take up slack tape, acting as buffers to prevent the tapes from snapping or stretching when starting from a stationery position or slowing down from full speed. The read/write heads are present either to access information on the tape. They are a single unit, made up of one read/write head per track.
Even though information can only be accessed serially, magnetic tape is very widely used. Frequently it is necessary to copy information and to retain it in the same order for use on another occasion. Tape is ideal for this purpose as it is cheap and the transfer rate to and from the computer's main memory are relatively fast. A reel of tape is also a convenient way of carrying information from one place to another, i.e. transferring information between computer systems which are not linked together. In addition, tape is widely used to back-up information on magnetic disk and there is increasing use of cartridge tape systems for this purpose.
Besides acting as an area of secondary storage, magnetic tape is also an input/output medium in its own right. Information is input to the computer from the tape for processing and information is output to tape where it resides until it is needed again or until it becomes redundant and is erased.
Magnetic Disk
This device has direct access. In shape, a disk resembles an LP record. A disk pack consists of a number of these disks, six or more, monted about half-an-inch apart on a central hub which rotates, spinning the disks at speeds of 50 or more revolutions a second. Information is recorded on both sides of each disk as a series of magnetized or non-magnetized spots, i.e. similar to magnetic tapes.
Information is stored on tracks arranged in concentric circles, with each character represented by a pattern of bits in sequence on one track. Although varying in length, each track contains the same number of characters, which means that tracks on the outer reaches of the disk are less densely packed with characters than those towards the centre. Each track and sector address (76;5, that is 76th track and 5th sector).
The diameter of a standard sized disk is 14 in. and there may be several hundred tracks per surface, each with a storage capacity of thousands of characters. Disk packs are potentially very high capacity storage device typically in the range 20 t0 1000 megabytes (M bytes).
The disk pack on some disk storage devices is permanently fixed in position, whilst on other the pack can be removed and replaced by another in a matter of seconds. Not all disks are stacked in packs. A single removable disk is generally known as a cartridge disk. The facility to change packs or cartridges means that storage space can be increased without the heavy expense of buying another complete device.
The are two types of read/write head units for magnetic disk devices, a moving-head unit and a fixed-head unit. In the moving-head unit, the head moves horizontally across the surface of the disk so that it is able to access each track individually. There is a head for each surface and all the heads move in unison. Information stored on the tracks which constitute a cylindrical shape through the disk pack are therefore accessed simultaneously, a significant factor in storage arrangements. Exchangeable disk packs are only associated with moving-head units. In the case of the fixed-head unit, there is one read/write head for each track, as a result of which not head movement is needed and information is therefore traced more quickly. The heads do not have direct contact with the surface but 'rest on a cushion of air. The movement caused by the revolving disk forces the head to 'fly' 1/400th of an inch from the surface.
The time taken to access information on these direct, or random, devices varies considerably, but the fixed-head device is quicker than the moving-head device. As with magnetic tape, information on magnetic disk can be accessed again When fresh data is recorded it simply replaces the existing information.
There may be as many as 256 tracks and 32 or more sectors.
Floppy Disk
The Floppy disk is a comparatively new storage device developed in the early 1970s as a cheap and faster alternative to storage on magnetic tape. It is a small, random access disk which, like all secondary storage devices, can be used both for input and output operations. The disk is made of flexible plastic and coated in magnetic oxide. For protection it is normally contained within a plastic cardboard sleeve, often referred to as a cartridge. The cartridge is readily loaded into, and unloaded from a drive unit. Unlike the moving-head read/write mechanism on conventional disk drives, the heads on a floppy disk unit make contact with the disk surface, when reading or writing, and disk therefore get worn with constant use.
There are two standard sizes, 8 in, and 51/4 in., frequently referred to as diskette and mini-floppy respectively. Storage capacity is small compared with other conventional disk devices but quite impressive for size. The capacity of an 8 in. diskette is typically between 250 K bytes and 1.5 M bytes and the capacity of a 51/4 in. mini-floppy is between 125 K bytes and 500 K bytes, depending on density. The floppy disk is a low-cost device particularly suited to supporting personal computer systems and for use with small business systems and word processing systems.
Winchester Disk
The most significant development in disk technology in recent years is the introduction of hermetically sealed units, generally knows as 'Winchester disk drives, in which the read/write heads are designed to take off and land on the disk surface. The disk is coated with a special lubricant which reduces the friction when the heads and the sealed chamber prevent contamination from dust and other airborne particles. The technology enables greater precision of alignment, an increase in the number of tracks on the disk surface and a higher storage density per track. Winchester disks are fast and highly reliable, yet low priced compared with conventional hard disk devices. Because the units are sealed preventive maintenance is not required.
Standard sizes are 51/4 in., 8 in and 14 in., with 8 in. the most common. Storage capacities of 10, 20 and 40 megabytes are typical, with double capacity on dual disk drives. A magnetic tape cartridge transportable magnetic tape form. Winchester disks are used to support minicomputers and are competing with 'floppies' for a share of the expanding word processing and business systems market.
Disks are getting better all the time and research aimed at increasing capacity and reliability continues. The cost of disk storage is decreasing significantly and 8 in. Winchester devices of up to 180 megabytes capacity are eagerly awaited.

Input Direct to Backing Stores
In early computer systems punched cards were the most common form of input but today the emphasis has switched to magnetic and optical media. In office and business organizations, where traditionally data to magnetic type or disk is now standard practice, superseding the use of the traditional punched card. Three methods are distinguishable:
Key-to-tape
Key-to-cassette/cartridge
Key-to-disk/diskette
When the information is eventually transferred from these backing stores, the backing stores themselves become 'input' devices. From keyboard to computer is yet another communication path.
Key-to tape
A key-to-tape device, sometimes referred to as a magnetic tape encoder, permits the recording of information directly on magnetic tape. An operator, copying from documents, keys the data electronically using a typewriter-like keyboard. The data is stored temporarily by the device and typically displayed on a CRT for visual checking before being transferred to magnetic tape.
Key-to-cassette/cartridge
Information can also be keyed direct to small magnetic cassette tapes or cartridges and later transferred to standard magnetic tape for processing. A cassette, typically two and a had inches by four inches in size is capable of storing around 200 000 characters of information. A typical cartridge is smaller in capacity, averaging around 30,000 characters, These key input devices are easy to use and compact, and are therefore most suitable for the collection of data at source, for example at places remote from the computer installation.
Key-to-disk/diskette
As with key-to-tape systems, information is entered via a keyboard and a copy displayed on a CRT to allow a visual check. A key-to-disk/diskette system normally comprises a minicomputer, number of key stations (one or more for a diskette system, typically 8 to 64 for a disk system) and a disk drive. The minicomputer is required to control the input from the various stations, enabling the data to be held temporarily for verification and editing before allocation to the disk store. The process of keying to disk or diskette is more efficient and cost effective than punching data on cards for card input and is now the preferred method.
Backing stores serve two purposes. They supplement the internal memory of the computer when linked to the CPU, and they also store programs and data for future use. It is important to appreciate that information from backing stores has to pass into the internal memory (under the direction of control) before it can be used. This means that the CPU handles information passing to and from conventional input/output devices.
WHAT IS SOFTWARE
Instructing the computer

The circuitry of the computer interprets and understands the data and the instructions which are passed on to it after translation into a set of 1s and 0s. All internal working is really done using only two numbers-one and zero, these being represented inside by two voltage levels. This method of representation and computation is called binary arithmetic. The user however need not know anything about binary arithmetic to use the computer as an effective tool.
We have said that the computer understands only ones and zeros. But we see the computer being instructed in the usual English character set in what may be called pseudo-English. How does this get translated into the binary notation that the computer understands?
This is where software comes into the picture. Software is the means of communicating with the computer. The set of instructions or programs is software.
Software exists at many levels. There is software that can translate instructions into the language the computer understands-the machine language of zeros and ones. There is software that translates the instructions given into something that this first type of software can understand, such that the two types of software in tandem result in a machine understandable version of the given set of instructions. Various types of software are available and can be used to put together a set of instructions to make the hardware do what you want.

THE FUNCTIONS OF A COMPUTER

THE FUNCTIONS OF A COMPUTER
A TYPICAL COMPUTER SYSTEM

A typical digital computer consists of:

a) A central processor unit (CPU)
b) A memory
c) Input/output (1/0) ports

The memory serves as a place to store Instructions, the coded pieces of information that direct the activities of the CPU, and Data, the coded pieces of information that are processed by the CPU. A group of logically related instructions stored in memory is referred to as a Program. The CPU "reads" each instruction from memory in a logically determined sequence, and uses it to initiate processing actions. If the program sequence is coherent and logical, processing the program will produce intelligible and useful results.

The memory is also used to store the data to be manipulated, as well as the instructions that direct that manipulation The program must be organized such that the CPU does not read a non-instruction word when it expects to see an instruction. The CPU can rapidly access any data stored in memory; but often the memory is not large enough to store the entire data bank required for a particular application. The problem can be resolved by providing the computer with one or more Input Ports. The CPU can address these ports and input the data contained there. The addition of input ports enables the computer to receive information from external equipment (such as a paper tape reader or floppy disk) at high rates of speed and in large volumes.

A computer also requires one or more Output Ports that permit the CPU to communicate the result of its processing to the outside world. The output may go to a display, for use by a human operator, to a peripheral device that produces "hardcopy," such as a line-printer, to a peripheral storage device, such as a floppy disk unit, or the output may constitute process control signals that direct the operations of another system, such as an automated assembly line. Like input ports, output ports are addressable. The input and output ports together permit the processor to communicate with the outside world.

The CPU unifies the system. It controls the functions performed by the other components. The CPU must be able to fetch instructions from memory, decode their binary contents and execute them. It must also be able to reference memory and 1/0 ports as necessary in the execution of instructions. In addition, the CPU should be able to recognize and respond to certain external control signals, such as INTERRUPT and WAIT requests. The functional units within a CPU that enable it to perform these functions are described below.

THE ARCHITECTURE OF A CPU

A typical central processor unit (CPU) consists of the following interconnected functional units:
• Registers
• Arithmetic/Logic Unit (ALU)
• Control Circuitry

Registers are temporary storage units within the CPU. Some registers, such as the program counter and instruction register, have dedicated uses. Other registers, such as the accumulator, are for more general purpose use.

Accumulator:

The accumulator usually stores one of the operands to be manipulated by the ALU. A typical instruction might direct the ALU to add the contents of some other register to the contents of the accumulator and store the result in the accumulator itself. In general, the accumulator is both a source (operand) and a destination (result) register.

Often a CPU will include a number of additional general purpose registers that can be used to store operands or intermediate data. The availability of general purpose registers eliminates the need to "shuffle" intermediate results back and forth between memory and the accumulator, thus improving processing speed and efficiency.

Program Counter (Jumps, Subroutines and the Stack):

The instructions that make up a program are stored in the system's memory. The central processor references the contents of memory, in order to determine what action is appropriate. This means that the processor must know which location contains the next instruction.

Each of the locations in memory is numbered, to distinguish it from all other locations in memory. The number which identifies a memory location is called its Address.

The processor maintains a counter which contains the address of the next program instruction. This register is called the Program Counter. The processor updates the program counter by adding "1" to the counter each time it fetches an instruction, so that the program counter is always current (pointing to the next instruction)

The programmer therefore stores his instructions in numerically adjacent addresses, so that the lower addresses contain the first instructions to be executed and the higher addresses contain later instructions. The only time the programmer may violate this sequential rule is when an instruction in one section of memory is a Jump instruction to another section of memory.

A jump instruction contains the address of the instruction which is to follow it. The next instruction may be stored in any memory location, as long as the programmed jump specifies the correct address. During the execution of a jump instruction, the processor replaces the contents of its program counter with the address embodied in the Jump. Thus, the logical continuity of the program is maintained.

A special kind of program jump occurs when the stored program "Calls" a subroutine. In this kind of jump, the processor is required to "remember" the contents of the program counter at the time that the jump occurs. This enables the processor to resume execution of the main program when it is finished with the last instruction of the subroutine.

A Subroutine is a program within a program. Usually it is a general-purpose set of instructions that must be executed repeatedly in the course of a main program. Routines which calculate the square, the sine, or the logarithm of a program variable are good examples of functions often written as subroutines. Other examples might be programs designed for inputting or outputting data to a particular peripheral device.

The processor has a special way of handling subroutines, in order to insure an orderly return to the main program. When the processor receives a Call instruction, it increments the Program Counter and stores the counter's contents in a reserved memory area known as the Stack. The Stack thus saves the address of the instruction to be executed after the subroutine is completed. Then the processor loads the address specified in the Call into its Program Counter. The next instruction fetched will therefore be the first step of the subroutine.

The last instruction in any subroutine is a Return. Such an instruction need specify no address. When the processor fetches a Return instruction, it simply replaces the current contents of the Program Counter with the address on the top of the stack. This causes the processor to resume execution of the calling program at the point immediately following the original Call Instruction.

Subroutines are often Nested; that is, one subroutine will sometimes call a second subroutine. The second may call a third, and so on. This is perfectly acceptable, as long as the processor has enough capacity to store the necessary

return addresses, and the logical provision for doing so. In other words, the maximum depth of nesting is determined by the depth of the stack itself. If the stack has space for storing three return addresses, then three levels of subroutines may be accommodated.

Processors have different ways of maintaining stacks. Some have facilities for the storage of return addresses built into the processor itself. Other processors use a reserved area of external memory as the stack and simply maintain a Pointer register which contains the address of the most recent stack entry. The external stack allows virtually unlimited subroutine nesting. In addition, if the processor provides instructions that cause the contents of the accumulator and other general purpose registers to be "pushed" onto the stack or "popped" off the stack via the address stored in the stack pointer, multilevel interrupt processing (described later in this chapter) is possible. The status of the processor (i.e., the contents of all the registers) can be saved in the stack when an interrupt is accepted and then restored after the interrupt has been serviced. This ability to save the processor's status at any given time is possible even if an interrupt service routine, itself, is interrupted.

Instruction Register and Decoder:

Every computer has a Word Length that is characteristic of that machine. A computer's word length is usually determined by the size of its internal storage elements and interconnecting paths (referred to as Busses); for example, a computer whose registers and busses can store and transfer 8 bits of information has a characteristic word length of 8 bits and is referred to as an 8bit parallel processor. An eight-bit parallel processor generally finds it most efficient to deal with eight-bit binary fields, and the memory associated with such a processor is therefore organized to store eight bits in each addressable memory location. Data and instructions are stored in memory as eight-bit binary numbers, or as numbers that are integral multiples of eight bits: 16 bits, 24 bits, and so on. This characteristic eight-bit field is often referred to as a Byte.

Each operation that the processor can perform is identified by a unique byte of data known as an Instruction Code or Operation Code. An eight-bit word used as an instruction code can distinguish between 256 alternative actions, more than adequate for most processors.

The processor fetches an instruction in two distinct operations. First, the processor transmits the address in its Program Counter to the memory Then the memory returns the addressed byte to the processor. The CPU stores this instruction byte in a register known as the Instruction Register, and uses it to direct activities during the remainder of the instruction execution.

The mechanism by which the processor translates an instruction code into specific processing actions requires more elaboration than we can here afford. The concept, however, should be intuitively clear to any logic designer.

The eight bits stored in the instruction register can be decoded and used to selectively activate one of a number of output lines, in this case up to 256 lines. Each line represents a set of activities associated with execution of a particular instruction code. The enabled line can be combined with selected timing pulses, to develop electrical signals that can then be used to initiate specific actions. This translation of code into action is performed by the Instruction Decoder and by the associated control circuitry.

An eight-bit instruction code is often sufficient to specify a particular processing action. There are times, however, when execution of the instruction requires more information than eight bits can convey

One example of this is when the instruction references a memory location. The basic instruction code identifies the operation to be performed, but cannot specify the object address as well In a case like this, a two or three-byte instruction must be used. Successive instruction bytes are stored in sequentially adjacent memory locations, and the processor performs two or three fetches in succession to obtain the full instruction. The first byte retrieved from memory is placed in the processor's instruction register, and subsequent bytes are placed in temporary storage; the processor then proceeds with the execution phase. Such an instruction is referred to as Variable Length.

Address Register(s):

A CPU may use a register or register pair to hold the address of a memory location that is to be accessed for data If the address register is Programmable, (i e., if there are instructions that allow the programmer to alter the contents of the register) the program can "build" an address in the address register prior to executing a Memory Reference instruction (i.e., an instruction that reads data from memory, writes data to memory or operates on data stored in memory).

Arithmetic/Logic Unit (ALU):

All processors contain an arithmetic/logic unit, which is often referred to simply as the ALU The ALU, as its name implies, is that portion of the CPU hardware which performs the arithmetic and logical operations on the binary data .

The ALU must contain an Adder which is capable of combining the contents of two registers in accordance with the logic of binary arithmetic. This provision permits the processor to perform arithmetic manipulations on the data it obtains from memory and from its other inputs.

Using only the basic adder a capable programmer can write routines which will subtract, multiply and divide, giving the machine complete arithmetic capabilities. In practice, however, most ALUs provide other built-in functions, including hardware subtraction, Boolean logic operations, and shift capabilities

The ALU contains Flag Bits which specify certain conditions that arise in the course of arithmetic and logical manipulations. Flags typically include Carry, Zero, Sign, and Parity. It is possible to program jumps which are conditionally dependent on the status of one or more flags. Thus, for example, the program may be designed to jump to a special routine if the carry bit is set following an addition instruction

Control Circuitry:

The control circuitry is the primary functional unit within a CPU. Using clock inputs, the control circuitry maintains the proper sequence of events required for any processing task After an instruction is fetched and decoded, the control circuitry issues the appropriate signals (to units both internal and external to the CPU) for initiating the proper processing action. Often the control circuitry will be capable of responding to external signals, such as an interrupt or wait request An Interrupt request will cause the control circuitry to temporarily interrupt main program execution, jump to a special routine to service the interrupting device, then automatically return to the main program. A Wait request is often issued by a memory or 1/0 element that operates slower than the CPU. The control circuitry will idle the CPU until the memory or 1/0 port is ready with the data.

COMPUTER OPERATIONS

There are certain operations that are basic to almost any computer A sound understanding of these basic operations is a necessary prerequisite to examining the specific operations of a particular computer.

Timing:

The activities of the central processor are cyclical. The processor fetches an instruction, performs the operations
required, fetches the next instruction, and so on. This orderly sequence of events requires precise timing, and the CPU therefore requires a free running oscillator clock which furnishes the reference for all processor actions The combined fetch and execution of a single instruction is referred to as an Instruction Cycle. The portion of a cycle identified with a clearly defined activity IS called a State. And the inter vat between pulses of the timing oscillator is referred to as a Clock Period. As a general rule, one or more clock periods are necessary for the completion of a state, and there are several states in a cycle.

Instruction Fetch:

The first state(s) of any instruction cycle will be dedicated to fetching the next instruction. The CPU issues a read signal and the contents of the program counter are sent to memory, which responds by returning the next instruc tion word. The first byte of the instruction is placed in the instruction register. If the instruction consists of more than one byte, additional states are required to fetch each byte of the instruction. When the entire instruction is present in the CPU, the program counter is incremented (in preparation for the next instruction fetch) and the instruction is decoded. The operation specified in the instruction will be executed in the remaining states of the instruction cycle. The instruction may call for a memory read or write, an input or output and/or an internal CPU operation, such as a register to register transfer or an add registers operation.

Memory Read:

An instruction fetch is merely a special memory read operation that brings the instruction to the CPU's instruction register. The instruction fetched may then call for data to be read from memory into the CPU. The CPU again issues a read signal and sends the proper memory address; memory responds by returning the requested word. The data received is placed in the accumulator or one of the other general purpose registers (not the instruction register).

Memory Write:

A memory write operation is similar to a read except for the direction of data flow. The CPU issues a write signal, sends the proper memory address, then sends the data word to be written into the addressed memory location.

Wait (memory synchronization):

As previously stated, the activities of the processor are timed by a master clock oscillator. The clock period determines the timing of all processing activity.

The speed of the processing cycle, however, is limited by the memory's Access Time. Once the processor has sent a read address to memory, it cannot proceed until the memory has had time to respond. Most memories are capable of responding much faster than the processing cycle requires. A few, however, cannot supply the addressed byte within the minimum time established by the processor's clock.

Therefore a processor should contain a synchronization provision, which permits the memory to request a Wait state. When the memory receives a read or write enable signal, it places a request signal on the processor's READY line, causing the CPU to idle temporarily. After the memory has had time to respond, it frees the processor's READY line, and the instruction cycle proceeds

Input/Output:

Input and Output operations are similar to memory read and write operations with the exception that a peripheral 1/0 device is addressed instead of a memory location. The CPU issues the appropriate input or output control signal, sends the proper device address and either receives the data being input or sends the data to be output.
Data can be input/output in either parallel or serial form. All data within a digital computer is represented in binary coded form. A binary data word consists of a group 5
of bits; each bit is either a one or a zero. Parallel 1/0 consists of transferring all bits in the word at the same time, one bit per line. Serial 1/0 consists of transferring one bit at a time on a single line. Naturally serial 1/0 is much slower, but it requires considerably less hardware than does parallel 1/0.
Interrupts:

Interrupt provisions are included on many central processors, as a means of improving the processor's efficiency. Consider the case of a computer that is processing a large volume of data, portions of which are to be output to a printer. The CPU can output a byte of data within a single machine cycle but it may take the printer the equivalent of many machine cycles to actually print the character specified by the data byte. The CPU could then remain idle waiting until the printer can accept the next data byte. If an interrupt capability is implemented on the computer, the CPU can output a data byte then return to data processing. When the printer is ready to accept the next data byte, it can request an interrupt. When the CPU acknowledges the interrupt, it suspends main program execution and automatically branches to a routine that will output the next data byte. After the byte is output, the CPU continues with main program execution. Note that this is, in principle, quite similar to a subroutine call, except that the jump is initiated externally rather than by the program.

More complex interrupt structures are possible, in which several interrupting devices share the same processor but have different priority levels. Interruptive processing is an important feature that enables maximum utilization of a processor's capacity for high system throughput.

Hold:

Another important feature that improves the throughput of a processor is the Hold. The hold provision enables Direct Memory Access (DMA) operations.

In ordinary input and output operations, the processor itself supervises the entire data transfer. Information to be placed in memory is transferred from the input device to the processor, and then from the processor to the designated memory location. In similar fashion, information that goes from memory to output devices goes by way of the processor.

Some peripheral devices, however, are capable of transferring information to and from memory much faster than the processor itself can accomplish the transfer. If any appreciable quantity of data must be transferred to or from such a device, then system throughput will be increased by having the device accomplish the transfer directly. The processor must temporarily suspend its operation during such a transfer, to prevent conflicts that would arise if processor and peripheral device attempted to access memory simultaneously. It is for this reason that a hold provision is include d on some processors.