1-1 Computer Structure

Transcription

1 1-1 Computer Structure Computer: A computer is a programmable, multiuse machine that accepts data-raw, fact and figures; processes, or manipulates it into information we can use, such as summaries, totals, or reports. Its purpose is to speed up problem solving and increase productivity. Or, Computer: an electronic device that accepts input, stores large quantities of data, execute complex instructions which direct it to perform mathematical and logical operations and outputs the answers in a human readable form. The Computer System consists of the following units: 1- Central Processing Unit (CPU) 2- Input Unit 3- Output Unit 4- Memory Unit 5- Bus Unit It can be useful as a starting point to analyze the processes involved in a manual calculation using pencil and paper. The primary purpose of the paper is information storage. The information stores on paper may include a list of instructions- i.e., an algorithm or program- to be followed in carrying out the calculation, as well as the data (numbers) to be used. During the calculation, intermediate result and, ultimately, the final results, are recorded on the paper. The computational processes needed take place in the brain, which can be called the processor. (1)

2 Central Processing Unit (CPU) Microprocessor Inputs I/Ps Main Memory Outputs O/Ps Secondary Memory Unit Figure (1) Computer System Structure Two major functions performed by the brain can be distinguished: a control function, which interprets the instructions and ensures that they are performed in the correct sequence; and an execution function, which carries out specific calculations such as addition, subtraction, multiplication, and division. The major components of a computing machine are similar. The memory unit corresponds to the paper used by the human calculator; its purpose is to store both instructions and data. The control unit interprets and sequences instructions. The arithmetic-logic unit executes instructions. The control and arithmetic-logic units together form the central processing unit (CPU), which corresponding roughly to the brain in human computations. A significant difference between the human and the machine lies in the way in which they represent information, i.e., instructions and data. Humans employ natural languages with a wide range of symbols and usually represent numbers in decimal (base 10) form. In modern computing machines, however, information is stored and processed in binary form, i.e., using two symbols denoted by 0 and 1, and called bits (binary (2)

3 digits). To provide for communication between the machine and its human users, the box labeled input-output (I/O) unit is used. 1-2 History of Computers From the earliest times the need to carry out calculations has been developing. The first steps involved the development of counting and calculation aids such as the counting board and the abacus. Pascal was the son of a tax collector and a mathematical genius. He designed the first mechanical calculator (Pascaline) based on gears. It performed addition and subtraction. Leibnitz was a German mathematician and built the first calculator to do multiplication and division. It was not reliable due to accuracy of contemporary parts. Babbage was a British inventor who designed an analytical engine incorporating the ideas of a memory and input/output card for data and instructions. Again the current technology did not permit the complete construction of the machine. Hollerith developed an electromechanical punched-card tabulator to tabulate the data for 1890 U.S. census. Data was entered on punched cards and could be sorted according to the census requirements. The machine was powered by electricity. He formed the Tabulating Machine Company, which became International Business Machines (IBM). IBM is still one of the largest computer companies in the world. Aiken a Harvard professor with the backing of IBM built the Harvard Mark I computer (51 ft long) in It was based on relays (operate in milliseconds) as opposed to the use of gears. It required 3 seconds for a multiplication. Eckert and Mauchly designed and built the ENIAC in 1946 for military computations. It used vacuum tubes (valves) which were completely electronic (operated in microseconds) as opposed to the relay, which was electromechanical. It weighed 30 tons, used valves, and required 140 k watts of power. It was 1000 times faster than the Mark I multiplying in 3 milliseconds. ENIAC was a decimal machine and could not be programmed without altering its setup manually. (3)

4 Von Neumann was a scientific genius and was a consultant on the ENIAC project. He formulated plans with Mauchly and Eckert for a new computer (EDVAC) which was to store programs as well as data. This is called the stored program concept and Von Neumann is credited with it. Almost all-modern computers are based on this idea and are referred to as Von Neumann machines. He also concluded that the binary system was more suitable for computers since switches have only two values. He went on to design his own computer at Princeton which was a general-purpose machine. Alan Turing was a British mathematician who also made significant contributions to the early development of computing, especially to the theory of computation. He developed an abstract theoretical model of a computer called a Turing machine, which is used to capture the notion of computable i.e. what problems, can and what problems cannot be computed. Not all problems can be solved on a computer. Note that Turing machine is an abstract model and not a physical computer. Table 1 Computer History Date Inventor: machine Capability 500 B.C Babylonians: abacus Addition, subtraction 1642 Pascal: Pascaline Addition, subtraction 1671 Leibniz Addition, subtraction, multiplication, and division 1827 Babbage: Difference Engine Polynomial evaluation by method of finite differences 1834 Babbage: Analytical Engine (never General-purpose computation completed) 1944 Aiken: Harvard Mark I (The first operational general-purpose computers) General-purpose computation 1-3 Computer Generations From the 1950 s, the computer age took off in full force. The years since then have been divided into periods or generations based on the technology used. (4)

5 1. First Generation Computers ( ): Vacuum Tubes These machines were used in business for accounting and payroll applications. Valves were unreliable components generating a lot of heat. They had very limited memory capacity. Magnetic drums were developed to store information and tapes were also developed for secondary storage. They were initially programmed in machine language (binary). A major breakthrough was the development of assemblers and assembly language. 2. Second Generation ( ): Transistors The development of the transistor revolutionized the development of computers. Invented at Bell Labs in 1948, transistors were much smaller, more rugged, cheaper to make and far more reliable than valves. Core memory was introduced and disk storage was also used. The hardware became smaller and more reliable, a trend that stills continues. Another major feature of the second generation was the use of high-level programming languages such as Fortran and Cobol. These revolutionized the development of software for computers. The computer industry experienced explosive growth. 3 Third Generation ( ): Integrated Circuits (ICs) IC s were again smaller, cheaper, faster and more reliable than transistors. Speeds went from the microsecond to the nanosecond (billionth) to the picoseconds (trillionth) range. ICs were used for main memory despite the disadvantage of being volatile. Minicomputers were developed at this time. Terminals replaced punched cards for data entry and disk packs became popular for secondary storage. IBM introduced the idea of a compatible family of computers, 360 family, easing the problem of upgrading to a more powerful machine. Operating systems were developed to manage and share the computing resources and time-sharing operating systems were developed. These greatly improved the efficiency of computers. Computers had by now pervaded most areas of business and administration. The number of transistors that be fabricated on a chip is referred to as the scale of integration (SI). Early chips had SSI (small SI) of tens to a few hundreds. Later chips were MSI (Medium SI): hundreds to a few thousands,. Then came LSI chips (Large SI) in the thousands range. (5)

6 4. Fourth Generation (1971-Present): VLSI (Very Large SI) The term fourth generation is occasionally applied to VLSI-based computer architecture. VLSI has made it possible to fabricate an entire CPU, main memory, or similar devices with a single IC. This has resulted in new classes of machines such as inexpensive personal computers, and high-performance parallel processors that contain thousands of CPUs. VLSI allowed the equivalent of tens of thousands of transistors to be incorporated on a single chip. This led to the development of the microprocessor a processor on a chip. Intel produced the 4004 which was followed by the 8008, 8080, 8088 and 8086 etc. Other companies developing microprocessors included Motorola (6800, 68000), and Zilog. Personal computers were developed and IBM launched the IBM PC based on the 8088 and 8086 microprocessors. Mainframe computers have grown in power. Memory chips are in the megabyte range. VLSI chips had enough transistors to build 20 ENIACs. Secondary storage has also evolved at fantastic rates with storage devices holding gigabytes (1024MB = 1 GB) of data. On the software side, more powerful operating systems are available such as UNIX. Applications software has become cheaper and easier to use. Software development techniques have vastly improved. Fourth generation languages 4GLs make the development process much easier and faster. [Languages are also classified according to generations from machine language (1GL), assembly language (2GL), and high level languages (3GL) to 4Gls]. Software is often developed as application packages. We can summarize the major characteristics of the four generations of electronic computers in the figure as follows: (6)

7 Table 1 Computer Generations Generation Technologies Hardware features Software features First ( ) Vacuum tubes; CRT memories Fixed-point arithmetic Machine language; assembly language Second ( ) Discrete transistors; ferrite cores; magnetic disks Floating-point arithmetic; index registers; IO processors High-level languages; subroutine libraries Third ( ) Integrated circuits (SSI and MSI) Microprogramming; pipelining; cache memory Multiprogramming; multiprocessing; operating systems; virtual memory Fourth (1971- Present) LSI/VLSI circuits; semiconductor memories Microprocessors; microcomputers Parallel and problem-oriented languages 1-4 Three Directions of Computer Development: Since the days of ENIAC, computers have developed in three directions and are continuing to do so. 1) Miniaturization (smaller size): Everything has become smaller. ENIAC's vacuum tubes gave way to the smaller, faster, more reliable transistor. A transistor is a small device used as a gateway to transfer electrical signals along predetermined paths (circuits). The next step was the development of tiny integrated circuits. Integrated circuits are entire collections of electrical circuits or pathways that are now etched on tiny squares of silicon half the size of your thumbnail. Silicon is a natural element found in sand. In pure form, it is the base material for computer processing devices. The miniaturized processor, or microprocessor, in a personal desktop computer today can perform calculations that once required a computer filling an entire room. 2) Speed: Thanks to miniaturization, computer makers can cram more hardware components into their machines, providing faster processing speeds and more data storage capacity. (7)

8 3) Affordability (Less expense): Processor costs today are only a fraction of what they were 15 years ago. A state-of-the-art processor costing less than $1000 provides the same processing power as a huge 1980s computer costing more than $1 million. 1-5 The Future (fifth generation) The race is now on building the next or fifth generation of computers, machine that exhibit artificial intelligence (AI). Thus new generations of computers will involve robotics and computer networks. Developments are still continuing. Computers are becoming faster, smaller and cheaper. Storage units are increasing in capacity. Distributed computing is becoming popular and parallel computers with large numbers of CPUs have been built. 1-6 The Categories of Computers: Computers now come in a variety of shapes and sizes, which could be roughly classified according to their processing power into five sizes: super large, large, medium, small, and tiny. 1) Supercomputers-super large computers: Supercomputers are high capacity machines with hundreds of thousands of processors that can perform more than 1 trillion calculations per second. These are the most expensive but fastest computers available. "Supers," as they are called, have been used for tasks requiring the processing of enormous volumes of data, such as doing the U.S. census count, forecasting weather, designing aircraft, modeling molecules, breaking codes, and simulating explosion of nuclear bombs. The fastest computer in the world that looks like 48 refrigerator-size blue boxes is Blue Mountain, located at Los Alamos Laboratory in New Mexico. 2) Mainframe computers-large computers: The only types of computer available until the late 1960s, mainframes are water- or air-cooled computers that vary in size from small, to medium, to large, depending on their use. Small mainframes are often called midsize computers; they used to be called minicomputers. Mainframes are used by large organizations such as banks, airlines, insurance companies, and colleges-for processing millions of (8)

9 transactions. Often users access a mainframe using a terminal, which has a display screen and a keyboard and can input and output data but cannot by itself process data. 3) Workstations-medium computer: Introduced in the early 1980s, workstations are expensive, powerful computers usually used for complex scientific, mathematical, and engineering calculations and for computer-aided design and computer-aided manufacturing. Providing many capabilities comparable to midsize mainframes, workstations are used for such tasks as designing airplane fuselages, prescription drugs, and movie special effects. Workstations have caught the eye of the public mainly for their graphics capabilities, which are used to breathe three-dimensional life into movies such as Jurassic Park and Titanic. The capabilities of low-end workstations overlap those of high-end desktop microcomputers. 4) Microcomputer- small computers: Microcomputers, also called personal computers (PC), can fit next to a desk or on a desktop, or can be carried around. They are either stand-alone machines or are connected to a computer network, such as a local area network. A local area network (LAN) connects, usually by special cable, a group of desktop PCs and other devices, such as printers, in an office or a building. Microcomputers are of several types: desktop PCs, tower PCs, laptops (or notebooks), and personal digital assistants-handheld computers or palmtops. Desktop PCs: are those in which the case or main housing sits on a desk, with keyboard in front and monitor (screen) often on top. Tower PCs: Is that Microcomputer in which the case sits as a "tower," often on the floor beside a desk, thus freeing up desk surface space. Laptop computers (also called notebook computers): are lightweight portable computers with built-in monitor, keyboard, hard-disk drive, battery, and AC adapter that can be plugged into an electrical outlet; they weigh anywhere from 1.8 to 9 pounds. (9)

10 Personal digital assistants (PDAs) (also called handheld computers or palmtops) combine personal organization tools-schedule planners, address books, to-do lists. Some are able to send and faxes. Some PDAs have touch-sensitive screens. Some also connect to desktop computers for sending or receiving information. 5) Microcontrollers-tiny computers: Microcontrollers, also called embedded computers, are the tiny, specialized microprocessors installed in "smart" appliances and automobiles. These microcontrollers enable PDAs microwave ovens, for example, to store data about how long to cook your potatoes and at what temperature. 1-7 How Computers Work? We will need to understand three key concepts: 1) FIRST: The purpose of a computer is to process data into information. Data consists of the raw facts and figures that are processed into information-for example, the votes for different candidates being elected to student government office. Information is data that has been summarized or otherwise manipulated for use in decision making-for example, the total votes for each candidate, which is used to decide who won. 2) SECOND: You should know the difference between hardware and software. Hardware consists of all the machinery and equipment in a computer system. The hardware includes, among other devices, the keyboard, the screen, the printer, and the "box" the computer or processing device itself. Software, or programs, consists of all the instructions that tell the computer how to perform a task. These instructions come from a software developer in a form (such as a CD-ROM disk) that will be accepted by the computer. Examples you may have heard of are Microsoft Windows 98 or Office 2000.etc. 3) THIRD: Regardless of type and size, all computers follow the same four basic operations: (1) input, (2) processing, (3) storage, and (4) output. To this we will add (10)

11 (5) communications. 1. Input operation: Input is whatever is put in ("input") to a computer system. Input can be nearly any kind of data-letters, numbers, symbols, shapes, colors, temperatures, sounds, or whatever raw material needs processing. When you type some words or numbers on a keyboard, those words are considered input data. 2. Processing operation: Processing is the manipulation a computer does to transform data into information. When the computer adds to get 4 that is the act of processing. The processing is done by the Central Processing Unit frequently called just the CPU a device consisting of electronic circuitry that executes instructions to process data. 3. Storage operation: Storage is of two types-temporary storage and permanent storage, or primary storage and secondary storage. Primary storage, or memory, is the computer circuitry that temporarily holds data waiting to be processed. This circuitry is inside the computer. Secondary storage, simply called storage, is the area in the computer where data or information is held permanently. A floppy disk is an example of this kind of storage. (Storage also holds the software-the computer programs.) 4. Output operation: Output is whatever is output from ("put out of") the computer system, the results of processing, usually information. Examples of output are numbers or pictures displayed on a screen, words printed out on paper in a printer, or music piped over some speakers. 5. Communications operation: These days, most (though not all) computers have communications ability, which offers an extension capability-in other words, it extends the power of the computer. With wired or wireless communications connections, data may be input from afar, processed in a remote area, stored in several different locations, and output in yet other places. However, you don't need communications ability to write term papers, do calculations, or perform many other computer tasks. (11)