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In computer science, computer architecture is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit (CPU) performs internally and accesses addresses in memory. It may also be defined as the science and art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals. Computer architecture comprises at least three main subcategories:[1]
Once both ISA and microarchitecture have been specified, the actual device needs to be designed into hardware. This design process is called implementation. Implementation is usually not considered architectural definition, but rather hardware design engineering. Implementation can be further broken down into three (not fully distinct) pieces:
For CPUs, the entire implementation process is often called CPU design. More specific usages of the term include more general wider-scale hardware architectures, such as cluster computing and Non-Uniform Memory Access (NUMA) architectures.
[edit] HistoryThe term “architecture” in computer literature can be traced to the work of Lyle R. Johnson and Frederick P. Brooks, Jr., members in 1959 of the Machine Organization department in IBM’s main research center. Johnson had the opportunity to write a proprietary research communication about Stretch, an IBM-developed supercomputer for Los Alamos Scientific Laboratory. In attempting to characterize his chosen level of detail for discussing the luxuriously embellished computer, he noted that his description of formats, instruction types, hardware parameters, and speed enhancements was at the level of “system architecture” – a term that seemed more useful than “machine organization.” Subsequently, Brooks, one of the Stretch designers, started Chapter 2 of a book (Planning a Computer System: Project Stretch, ed. W. Buchholz, 1962) by writing, “Computer architecture, like other architecture, is the art of determining the needs of the user of a structure and then designing to meet those needs as effectively as possible within economic and technological constraints.” Brooks went on to play a major role in the development of the IBM System/360 line of computers, where “architecture” gained currency as a noun with the definition “what the user needs to know.” Later the computer world would employ the term in many less-explicit ways. The first mention of the term architecture in the referred computer literature is in a 1964 article describing the IBM System/360.[3] The article defines architecture as the set of “attributes of a system as seen by the programmer, i.e., the conceptual structure and functional behavior, as distinct from the organization of the data flow and controls, the logical design, and the physical implementation.” In the definition, the programmer perspective of the computer’s functional behavior is key. The conceptual structure part of an architecture description makes the functional behavior comprehensible, and extrapolatable to a range of Use cases. Only later on did ‘internals’ such as “the way by which the CPU performs internally and accesses addresses in memory,” mentioned above, slip into the definition of computer architecture. [edit] Computer architecturesThere are four types of computer architectures, [edit] Computer architecture topics[edit] Sub-definitionsSome practitioners of computer architecture at companies such as Intel and AMD use more fine distinctions:
[edit] Design goalsThe exact form of a computer system depends on the constraints and goals for which it was optimized. Computer architectures usually trade off standards, cost, memory capacity, latency and throughput. Sometimes other considerations, such as features, size, weight, reliability, expandability and power consumption are factors as well. The most common scheme carefully chooses the bottleneck that most reduces the computer's speed. Ideally, the cost is allocated proportionally to assure that the data rate is nearly the same for all parts of the computer, with the most costly part being the slowest. This is how skillful commercial integrators optimize personal computers. [edit] PerformanceComputer performance is often described in terms of clock speed (usually in MHz or GHz). This refers to the cycles per second of the main clock of the CPU. However, this metric is somewhat misleading, as a machine with a higher clock rate may not necessarily have higher performance. As a result manufacturers have moved away from clock speed as a measure of performance. Computer performance can also be measured with the amount of cache a processor has. If the speed, MHz or GHz, were to be a car then the cache is like the gas tank. No matter how fast the car goes, it will still need to get gas. The higher the speed, and the greater the cache, the faster a processor runs. Modern CPUs can execute multiple instructions per clock cycle, which dramatically speeds up a program. Other factors influence speed, such as the mix of functional units, bus speeds, available memory, and the type and order of instructions in the programs being run. There are two main types of speed, latency and throughput. Latency is the time between the start of a process and its completion. Throughput is the amount of work done per unit time. Interrupt latency is the guaranteed maximum response time of the system to an electronic event (e.g. when the disk drive finishes moving some data). Performance is affected by a very wide range of design choices — for example, pipelining a processor usually makes latency worse (slower) but makes throughput better. Computers that control machinery usually need low interrupt latencies. These computers operate in a real-time environment and fail if an operation is not completed in a specified amount of time. For example, computer-controlled anti-lock brakes must begin braking almost immediately after they have been instructed to brake. The performance of a computer can be measured using other metrics, depending upon its application domain. A system may be CPU bound (as in numerical calculation), I/O bound (as in a webserving application) or memory bound (as in video editing). Power consumption has become important in servers and portable devices like laptops. Benchmarking tries to take all these factors into account by measuring the time a computer takes to run through a series of test programs. Although benchmarking shows strengths, it may not help one to choose a computer. Often the measured machines split on different measures. For example, one system might handle scientific applications quickly, while another might play popular video games more smoothly. Furthermore, designers have been known to add special features to their products, whether in hardware or software, which permit a specific benchmark to execute quickly but which do not offer similar advantages to other, more general tasks. [edit] Power consumptionMain article: low-power electronics Power consumption is another design criterion that factors in the design of modern computers. Power efficiency can often be traded for performance or cost benefits. With the increasing power density of modern circuits as the number of transistors per chip scales (Moore's Law), power efficiency has increased in importance. Recent processor designs such as the Intel Core 2 put more emphasis on increasing power efficiency. Also, in the world of embedded computing, power efficiency has long been and remains the primary design goal next to performance. [edit] System UnitThe system unit is the unit that contains the internal parts of the computer system. It contains hard disk, ram, rom, cooling fan, bus lines etc. [edit] CPUThe Central Processing Unit is the ‘brain’ of a computer. Every machine has a processing unit. Therefore, processor is not a unique feature of the computer. The unique feature of a computer is the memory. The CPU performs all the arithmetic and logical decisions of the computer. Data is fed to the computer by the input devices. This data is processed inside the CPU according to the instructions given by the user. The CPU comprises of 2 essential units, the ALU (Arithmetic and Logical Unit) and the CU (Control Unit). (1) ALU (Arithmetic and Logical Unit) The ALU performs all the arithmetic and logical operations. Arithmetic operations are the basic operations such as addition, multiplication and division. A logical operation simply means the comparisons of two data values. The values may be two numbers or set of alphabetic characters. When running a program, it is generally loaded into memory with relevant data. The data brought to the ALU from the main memory is manipulated by the ALU according to the commands issued by the Control Unit. (2) Control Unit The Control Unit controls the co-ordinates the functioning of all the other units of a computer. It is responsible for fetching all the instructions and data from the main memory to the ALU. It then interprets and executes these instructions. After processing the data, this unit will issue commands to transfer the processed data to the main memory. It also controls the I/O devices and the secondary storage devices. Properties and its use (of CPU) (1) CPU speed - The number of operations which can perform per second. (2) Word size - The largest number of capacity which can handle in 1 operation. (3) Data Path - The largest capacity which can transport into the CPU in operation. (4) Maximum memory- Amount of memory that CPU can handle. [edit] Mother BoardThe main circuit of a microcomputer is the motherboard. The motherboard contains the connectors for attaching additional boards. Typically, the motherboard contains the CPU, memory, mass storage interfaces, BIOS, serial and parallel ports, expansion slots, and all the controllers required to control standard peripheral devices, such as the display screen, keyboard, and disk drive. Collectively, all these chips that reside on the motherboard are known as the motherboard’s chipset. Power Supply Unit (PSU) The main function of the PSU is to convert main electricity as suitable for computer. (e.g. - In Sri Lanka the 230v-AC into 12v-AC) Peripheral Devices Peripherals is a name given to the both input and output devices. Expect memory and CPU, all the devices that are connected to the computer. They are known as peripheral devices. Some peripherals are used only as input or output but there are some devices work as both input and output. The peripheral devices are connecting to the computer by using ports. [edit] PortsPorts are use to connect devices to the computer. They are several ports in a computer. (1) Serial ports – A type of computer interface that complies with the RS-232 standard. They are 9-pin connectors that relay information, incoming or outgoing, one byte at a time. Each byte is broken up into a series of eight bits, hence the term serial port. Serial ports are one of the oldest types of interface standards. Before internal modems became commonplace, external modems were connected to computers through serial ports, also known as communication or "COM" ports. Computer mouse and even keyboards also used serial ports to connect to the computer. Some serial ports used 25-pin connectors, but the common one is 9-pin variety. These ports are controlled by a special chip called a UART (Universal Asynchronous Receiver Transmitter). (2) Parallel ports- A parallel is a port for connecting an external device such as a printer. The parallel port has a 25- pin connector. So the type of this port is DB- 25.The first man made parallel port is named as interface. It is called a interface after the company that designed the original standard for parallel communication between a computer and printer. The modern parallel interface is based on a design by Epson . A newer type of parallel port, which supports the same connectors as the interface, is the EPP (Enhanced Parallel Port) or ECP (Extended Capabilities Port). Both of these parallel ports support bi-directional communication and transfer rates ten times as fast as the interface port. (3) PS/2 port-Type of port developed by IBM for connecting a mouse or keyboard to a PC. The PS/2 port supports a mini DIN plug containing just 6 pins. Most PCs have a PS/2 port so that the serial port can be used by another device, such as a modem. The PS/2 port is often called the mouse port. The PS/2 name is come from the IBM Personal System/2. As in the picture the purple color port is used to connect the keyboard and the green color port is used to connect a mouse. (4) V.G.A card port-Video Graphics Array was first introduced by the IBM. It has 15 pins. The designer of this port is IBM. It is designed on 1987 and today also we use it. It is use to connect computer monitor. (5) Sound Card Port-Sound card port is used to connect speakers, microphones, head phones, sub boofers etc. [edit] Game PortGame port was designed by IBM. Firstly it was released in 1981 as a separate expansion card. This had 15 pins but today the game port is bereave because the coming of the USB port. [edit] USB PortThis port is created on January 1994. By Intel, Compaq, Microsoft, Digital, IBM and Northern Telecom companies. USB is stands for Universal Serial Bus. This device is designed to connect many peripherals to the computer. Such as mice, keyboards, PDA’s, game pads and joysticks, scanners, digital cameras, printers, personal media players, flash drives, and external hard drives. The first man made USB is USB 1.0 in 1994. The data transfer rate of this USB is 12 Mega bits per second. USB 1.0 was created by a core group of companies that consisted of Intel, Compaq, Microsoft, Digital, IBM, and Northern Telecom. Intel produced the UHCI host controller and open software stack; Microsoft produced a USB software stack for Windows and co-authored the OHCI host controller specification with National Semiconductor and Compaq; Philips produced early USB-Audio; and TI produced the most widely used hub chips. One of the co-inventors of the USB was Ajay Bhatt, Then the USB 2.0 was released, On April 2000. It was made by Hewlett-Packard, Intel, Lucent Technologies (now Alcatel, Microsoft, NEC, and Philips joint to the job. To develop a higher data transfer rate, 480 Mega bits per second, than the 1.0 specification of 12 Mega bits per second. Now USB 3.0 specification was released on November 17, 2008 by the USB 3.0 Promoter Group. It has a transfer rate of up to 10 times faster than the USB 2.0 version and has been known as the Super Speed USB. [edit] Network PortAlso known as LAN port or Ethernet port. This was been developed on the time period of 1973-1975 at Xerox PARC. [edit] Storage DevicesStorage devices are used to store data and information for future use.
The storage devices are divided into 2 categories:
[edit] Volatile StorageThis data stores temporally if the computer is switched off, this data and information will disappear. Early computers had used Williams’s tubes, delay lines, or rotating magnetic drums as primary storage. But in the 1954 magnetic core memory replaced to it. Then the time came to replace transistors for magnetic core memory. After some time man invented the RAM. Computer needs a RAM because it can store data and retrieve data much faster than the secondary storage. Below diagram shows it.
(1) RAM (Random Access Memory) Random Access Memory is also known as read/write memory. It is a sort of pending tray of data which is currently ‘live’, but not actively being processed. The more RAM we have, the more data that we can have live at a time. Since programs also resides in RAM, a big and complex program will only run if you have enough RAM to hold it RAM is volatile –it only holds information while power is being fed to it usually, this means that it can only retain information while the computer is actually switched on. Most modern computers have at least 32 MB of RAM. The performance of a computer is heavily dependent on the physical size of the RAM. But the power of a CPU cannot be obtained if the RAM is small. (2) Registers A register is a form of memory which works very fast. All devices in a computer have a speed at which they work. Among those devices, the registers work at the highest speed. They are small in physical size. For example, most of the time we find registers of 8-bit length. As I mentioned earlier, many registers works with CPU in relation to arithmetic calculations. Registers also keep information temporally as ROM does. (3) Cache Cache is another form of memory used for keeping commonly used data on temporally basis. The concept of caches allows improve performance of computers. Caches can be found both inside and outside the CPU. The caches, inside the CPU are called internal caches while others are called external caches. The size of the caches also just couple of KB’s only. For example, a computer with 512 KB external cache has enough processing speed to work as a powerful server. [edit] Permanent StorageThe main memory is used to store only those instructions and data items which are to be used immediately. However, a computer has to store a large amount of information permanently. This includes hard disks, floppy disks, CD’s, DVD’s, magnetic tapes, jazz disks, zip disks, pen drives etc. Typically, volatile storage is faster than permanent storage. The below are some examples for permanent storage. [edit] Magnetic TapesThe magnetic tapes were a popular medium for storage of large amount of information. These magnetic tapes are similar to the ones used in audio-cassette recorders. Apart from the use of these tapes as a storage media, these tapes also serve as high speed input and output media. A magnetic tape is made of a plastic material whose one side is coated with a magnetic substance. This substance gets magnetized easily and can retain the magnetism permanently. The disadvantage of these magnetic disks is it takes lot of time to read to read data, because the data access is sequential. More about Magnetic Tapes…… A magnetic tape consists of a number of sections of data, which consists of 2 parts. One part holds and is known as a block and the other is a section of blank tape which is known as Inter Block Gap abbreviated to ‘IBG’. Each block of data is transferred to and from the computer at one time. If the number of characters is large, then each block will correspond to a record. But, if the number of characters is small, then several records will be recorded in one block. An IBG is very useful and serves the following 3 purposes. i. It separates the blocks of records for easy readability by the user. ii. It allows space and time for the tape unit to reach its operating speed. Obviously, the length of an IBG depends upon the operating speed of the tape. iii. It allows the tape to dilacerate after a record is written on the tape or is read from the tape. When the end of a block is reached, the tape cannot be stopped immediately. So this space in the form of an IBG is used to stop the tape and start it again at the beginning of the next block. The inter block gaps which are empty portions of a tape contain no data. No doubt, they result in wastage of a part of the tape and increase the cost of file generation. But, IBG’s are necessary because of the above stated reasons. Magnetic tape is a sequential storage medium. Records are read from it one after another from the first record. This type of access is called sequential access. Magnetic tapes are the second most widely used secondary storage medium. It is used to store data which is not frequently used and processed on a sequential basis; such data is retained on tape because it is economical to do so. Magnetic disks are preferred over tapes because of their improved operational capabilities. A tape is a sequential storage medium while a disk is random access storage medium. When records are stored on tape it is not possible to jump out of order from one record to another as the tapes hold the records in sequential manner. A disk offers a faster method to access data due to random access techniques. Random access means that, records stored anywhere on the disk can be accessed independently without reading through all the records. [edit] Hard disksMagnetic disks are smooth metal plates coated on both sides with thin film of magnetic material. A set of such magnetic plates are fixed to a spindle one below the other to make up a disk pack. The disks pack is mounted on a disk drive. The disk drive consists of a rotate the disk pack about it’s axis at a speed of about 3600 revolutions per minute. But today we can find with hard disks with 5400, 7200, 9200 revolutions per minute. The drive also has a set of magnetic heads mounted on arms. The arm assembly is capable of moving in and out in radial directions. Information is recorded on the surface of a disk as it rotates about its axis. Thus, it is on circular tracks on each disk surface. A set of concentric tracks are recorded on each surface. A set of corresponding tracks in all surfaces of a disk pack is called a cylinder. In a disk pack with 6 plates, there are 12 surfaces. Out of the 12 surfaces, the top-most surface and the bottom-most surface are not used for recording. Thus, there are 10 surfaces on which information is recorded. A cylinder thus consists of 10 tracks in such a disk pack. A rack is divided into sectors (also called blocks). Read and write operations on a disk start at sector boundaries. If the number of bytes to be stored in a sector is less than the capacity of a sector, then the rest of the sector is padded up with the last byte recorded. Typically 512 bytes are stored per sector. There are 50 sectors per track, 400 tracks per surface and 10 surfaces. The total capacity of the disk pack is thus 512 X 50 X 400 X10= 102400000 bytes A set of disk drives are connected to a disk controller, which is an electronic circuits. It accepts commands from the computer and positions the read-write heads of the specified disk for reading or writing. In order to read or write on a disk pack, the computer must specify the drive number, cylinder number, surface number and the sector number. Drive number should be specified, because a controller normally controls more than one drive. Normally, one or more tracks are reserved on a disk with permanent recording to aid the controller in positioning the heads at the specified address. When a read write-command is received by the disk controller, the controller first positions the arm assembly so that the read-write head reaches the specified cylinder. The time taken to reach the specified cylinder is known as the seek time. The seek time varies depending upon the position of the arm assembly when the read-write command is received by the controller. The maximum seek time is the time taken by the head assembly to reach the innermost cylinder from the outermost cylinder or vice versa. The minimum seek time is 0 if the head assembly happens to be positioned on the selected cylinder. The average seek time (usually 30ms) is specified by the manufacture. Once ht head assembly is positioned on the specified cylinder, the head corresponding to the specified surface is switched. This switch is electronic and is almost instantaneous. After the head is selected there is a further delay because the specified sector has to reach the read-write head. This rotation delay is variable. The average rotational delay equals half the time taken by the disk to rotate once. This time is known as the latency time. For a disk rotating at 3600 rpm(revolutions per minute) is0.5/3600 minutes=8.3 milliseconds. The sum of average latency and seek time is known as the average access time. When a sector is reached, the data in it is read at a speed determined by the speed of rotation of the disk. One full revolution of the disk takes 1/3600 minutes. In one revolution, one track, which has 50 sectors, is read. The number of bytes read per revolution is thus 50 X 512=25600 bytes. The rate at which information is read from the disk known as the transfer rate. The transfer rate is thus 25600 X 3600/60 bytes per second 1536000 milliseconds. Bytes/sec or 1.536 Mega bytes/sec. The time to read one sector is 512/1.536 X10=1000000 =0.333 milliseconds. Thus, the average time to access a sector is 30+8.3 milliseconds and the time to read the information in it is 0.333 milliseconds. Besides moving the head to the correct sector for read-write, and initiating read-write, the disk controller also checks whether the data sent from the main memory of the computer is correctly written on the disk. It is done by reading back through a read-head, which is physically located next to the write-head, the Information written in the sector. Each byte so read is compared with the contents of a buffer register in the controller which contains the byte sent from the main memory for writing. The major delay in reading information from a disk is the seek time which is caused by the need for the head to move from track to track. This delay may be eliminated by providing one read-write head for each track of the disk. Such a hard disk is known as a head per-track disk. As the physical dimension of a head is large relative to the width of a track, the number of tracks in such a device is limited. [edit] CD-ROMThe term CD-ROM is stands for Compact Disk Read Only Memory. Data can be written to a CD is only once. In the recent times, there are various forms of CD- ROM are in the market. For example, we can write to some CD-ROMs more than once. They are called Re-recordable CDs. In contrast to floppy or hard disks, a CD-ROM has one long spiral track on which data is recorded. Firstly the CD-ROM is developed by the Sony and Philips in 1985. It is well known that magnetic media can be destroyed by factors such as heat, humidity, high impact etc. therefore, data on both hard disks and floppy disks can be destroyed if such efforts are given. However, optical storage is independent of most of such factors except heat. In fact excess heat can damage the plastic media of a CD-ROM, but not the storing pattern of data. Nevertheless, one can damage a CD-ROM by scratching on it or putting dust particles. In the past, CD-ROM had a much bigger capacity than that of hard disks. However, nowadays, hard disks offer even bigger capacities. [edit] Floppy DisksThe growing market for low-coast micro computers created the need of low-coast high capacity storage. This led to the development of floppy disks. Floppy disks are made of magnetic oxide-coated flexible plastic material. The flexible material is cut into circular pieces of 5.25 inches in a diameter. Mini floppy drives use 3.5 inches diameter floppy disks. The floppy disk with diameter 3.5 is common in use. As the material used is not a hard plate, but a flexible plastic, it is called a “floppy disk”. The flexible floppy disk is packed in a plastic envelope inside which floppy disk can rotate The disk driver holds the envelope and flexible disk is rotated inside the envelope by the drive mechanism. The inner side of the envelope is smooth and permits free rotation. The floppy disk, along with the envelope, is slipped into the floppy disk driver. When inserting the floppy disk into the driver, the sliding door moves, opens a slit and gives access to the read/write head of the floppy disk. So, the read-write head of the disk driver is held in physical contact with the floppy disk. The head is moved radialy along the slit open by the sliding door. [edit] Jazz DisksJazz disk is a very similar disk to the floppy disk and zip disk. But it can store a larger capacity of data than floppy disks or zip disks. It can store up to 1GB or 2GB. [edit] DVDDVD stands for Digital Versatile disk or Digital Video Disc, DVDR stands for DVD Recordable and DVDRW for DVD Rewriteable. If you're familiar with regular audio/music CDs or regular DVD-Video discs, then you will know what a recordable DVD looks like. A recordable DVD stores up to 2 hours of very good quality DVD-Video, including several audio tracks in formats like stereo, Dolby Digital or DTS and also advanced menu systems, subtitles and still pictures that can be played by many standalone DVD Players and most computer DVD-ROMs. If you choose to lower the video quality it is possible to store several hours video on a recordable DVD using low bit rates and low resolution with video quality more like VHS, SVHS, SVCD, CVD or VCD. It is also possible to have up to 4.37GB ordinary data or mix DVD-Video and data on a recordable DVD that can be played by most computer DVD-ROMs. DVD-R was the first DVD recording format released that was compatible with standalone DVD Players. DVD-R is a non-rewriteable format and it is compatible with about 93% of all DVD Players and most DVD-ROMs. DVD-RW is a rewriteable format and it is compatible with about 80% of all DVD Players and most DVD-ROMs. DVD-R and DVD-RW supports single side 4.37 computer GB DVDs (called DVD-5) and double sided 8.75 computer GB* DVDs(called DVD-10). These formats are supported by DVD Forum. DVD+R and DVD+RW DVD+R is a non-rewritable format and it is compatible with about 89% of all DVD Players and most DVD-ROMs. DVD+RW is a rewritable format and is compatible with about 79% of all DVD Players and most DVD-ROMs. DVD+R and DVD+RW supports single side 4.37 GB DVDs (called DVD-5) and double side 8.75 GB DVDs (called DVD-10). These formats are supported by the DVD+RW Alliance. DVD+R DL DVD+R DL or called DVD+R9 is a Dual Layer writeable DVD+R. The dual layered discs can hold 7.95GB (called DVD-9) and dual layered double sides 15.9GB (called dvd-18). DVD-R DL DVD-R DL or called DVD-R9 is a Dual Layer writeable DVD-R. The dual layered discs can hold 7.95 computer GB* (called DVD-9) and dual layered double sides 15.9GB (called dvd-18). DVD-RAM DVD-RAM has the best recording features but it is not compatible with most DVD-ROM drives and DVD-Video players. Think more of it as a removable hard disk. DVD-RAM is usually used in some DVD Recorders. This format is supported by DVD Forum. [edit] Input DevicesAn input device is any peripheral used to provide data and control signals to an information processing system like computers. Without a input device the computer cannot get data and instruction. Then it will like be a television, only displaying. There are a variety of input devices which are used by computers. The most commonly used units of this type are punched card readers, magnetic floppy disk readers, video terminals, mouse and keyboards, scanners, joystick, tracker ball, light pen, touch screen etc. Special purpose units magnetic ink characters, optical mark readers, optical character readers and bar code readers. [edit] Punched cardsThe punch cards are no longer used but, it is still worth discussing its basic features. A punch card is usually a rectangular thin card of size 18.8cm X 8.3cm. It is divided into 80 columns and 12 rows. Rows are numbered from 9 to 0 from button to top. Rows 11 and 12 are above row 0. Punched holes in these rows are called zone punches. Data to be processed by computers are punched on a machine called a card punching machine. The punching machine has a keyboard similar to typewriter keyboard. When a key is pressed, it punches a set of holes along a column on the card and also prints the character on top of the column. On a card, it is possible to punch 80 characters, i.e. one character per column. Thus, if we want to record a student’s examination results on card, we may use columns1 to 6 for index number, columns 7 to 32 for name, 33to 50 for the marks in six different subjects(3 columns per subject). If we have 100 students in a class, we will have 100 cards. This deck of 100 cards is called Card file. The punch cards are read as a card moves passing the reading station of the reader. The reading station senses the holes in a card mechanically by means of wire-brushes passing through the holes and making contact with a conducting spot. Electrical pulses corresponding to the holes in a card are transmitted and stored in the memory of the computer. [edit] KeyboardThe keyboard can be considered as the most commonly used in input device. The computer keyboard has a similar appearance to a keyboard of a typewriter. However, computer keyboard has various keys such as typewriter keys, numbers, special function keys and control keys. Despite there are various input devices emerged, keyboard is still considered as the main device for entering data to computer. Note that as time goes on there are also various types of keyboards supporting some special activities. It should also be noted that keys on a computer keyboard can be programmed to do various things. For example, you can press the key character Q (or another) to go out of a particular system. However, as you know keys on a typewriter cannot be programmed. A few key technological developments created the transition of the typewriter into the computer keyboard. The teletype machine, introduced in the 1930s, combined the technology of the typewriter (used as an input and a printing device) with the telegraph. Elsewhere, punched card systems were combined with typewriters to create what was called keypunches. Keypunches were the basis of early adding machines and IBM was selling over one million dollars worth of adding machines in 1931. Early computer keyboards were first adapted from the punch card and teletype technologies. In 1946, the ENIAC computer used a punched card reader as its input and output device. In 1948, the BINAC computer used an electromechanically controlled typewriter to both input data directly onto magnetic tape (for feeding the computer data) and to print results. The emerging electric typewriter further improved the technological marriage between the typewriter and the computer. Video Display Terminals By 1964, MIT, Bell Laboratories and General Electric had collaborated to create a computer system called Multics; a time sharing, multi-user system. Multics encouraged the development of a new user interface, the video display terminal. The video display terminals (VDT) combined the technology of the cathode ray tube used in televisions and electric typewriters. Computer users could now see what text they were typing on their display screens making text easier to create, edit and delete, and computers easier to program and use. Computer Keyboards Send Direct Electronic Impulses Earlier computer keyboards had been based either on teletype machines or keypunches. There were many electromechanical steps in transmitting data between the keyboard and the computer that slowed things down. With VDT technology and electric keyboards, the keyboard's keys could now send electronic impulses directly to the computer and save time. By the late ‘70s and early ‘80s, all computers used electronic keyboards and VDTs. Nevertheless, the layout of the computer keyboard still owes its origin to the inventor of the first typewriter, Christopher Latham Sholes who also invented the QWERTY layout. However, the computer keyboard does have a few extra function keys. [edit] MouseMouse is a unique input device used with computers. Mouse is connected to the computer through a suitable port. This device has set of buttons on the body to send signals to the computer. Commonest mouse has three buttons on it and is called three-button mouse. There are also two-button mouse, whose usage is little difficult. Depending on the application program, the mouse buttons can be programmed. In general the left most buttons is used to activate something. Clicking twice consecutively on the left button you can open an application. Further for the purpose of dragging and dropping icon. The left mouse button can be used. The middle button generally ends tasks. The right mouse button opens and allows choosing from secondary menus. [edit] Video TerminalsA video terminal consists of a television screen and a keyboard similar to a type writer keyboard with additional control keys. This is not the same as a collection of a monitor and a keyboard. In video terminals, when a key is pressed, the corresponding character is displayed on the screen. Simultaneously a cursor moves to the position where the next character will be displayed. A cursor is a small arrow, underline or a small square which can be moved horizontally or vertically to indicate the position of a character. Note that some video terminals are provided with a special pen which can touch the screen thereby giving inputs. Our ordinary monitor is generally considered as an output device rather than an input device. However, with the emerging technology, the monitor, one day will be a peripheral with both input and output capabilities. Magnetic ink character reader (MICR) MICR devices were developed in the late 1950s, mainly to assist the banking industry. These devices assist the banking industry in the automation of the processing of input cards and paper documents. These cards and paper documents are written with a magnetic ink. This magnetic ink contains iron oxide particles in it. The characters have a standard configuration. This makes them recognizable to humans and at the same time provides signals, produced by the read head. These signals are then sent to an electronic circuitry. Then, the signals are analyzed to identify the characters used and then these are transmitted to the memory. The MICR device can be considered as a special type of canner. It should be noted that input from MICR like device is much better and more accurate than using manual entry using keyboard. This avoids clerical errors. Further, giving inputs using MICR like devices is much faster than manual entry through a keyboard. However, MICR can recognize only a less number of characters. This makes the use of MICR is limited to specific purpose of data processing. [edit] Optical Mark ReaderIn this method, special preprinted forms are designed with boxes which can be marked with a dark pencil or Ink. Each box is annotated distinctly so that the user clearly understands what response he is marking. Such a document is read by a document reader which transcribes the marks into electrical pulses. These pulses are then transmitted to the computer. This type of documents is applicable in areas where responses are one out of a small number of alternatives. Thus, they are used for; i. Multiple choice type answer papers in examinations in which large number of candidates appear. ii. Market surveys, population surveys etc. where responses can be restricted to one or more out of a few possibilities. iii. Order forms containing a small choice of items. The advantage of this method is that Information is entered at its source and no further transcription is required. This minimizes the unreliability of data. The main disadvantage is the need for accurate alignment of printing on forms and the need for good quality expensive paper. The form cannot be redesigned frequently because any change will require reprinting of the form, which is expensive. [edit] Optical Character Reader (OCR)This is an input device, which is designed to read numeric and alphabetical characters from printed documents. However, the recent OCRs can read hand written characters with a rare rejection rate. This is a photoelectric device as the shape of the character is recognized with the help of light source which converts characters into electrical signals and then these are analyzed and identified. Optical readers read the source documents directly, so it saves time and money in transferring the data from the source document to the other input media. Bar Code Reader In this method, small bars of varying thickness and spacing are printed on packages, badges, tags etc. which are read by optical readers and converted to electrical pulses. The patterns of bars are unique and standardized in some countries. For example, in USA each grocery product has been given a unique 10 digit code and this is represented in bar code form on every container of the. The input method of bar code reading is generally used in the commercial sector as a price tag of particular items. Further, this is used in book cataloging in libraries. [edit] Output DevicesThe output devices are the devices which output the information and show us the information. There are lots of devices to output information from a computer. When we refer to the term output, we generally mean the output which can be read by an ordinary person. That is output in the form of machine code is not considered for this discussion. Generally, output can be 2 types, known as soft and hard. The hard copy or the hard output is generally considered as the printed form of an output into a paper. So, a printout is a hard copy. In contrast, the output written to a floppy or the hard disk is known as soft copy or the soft output. [edit] PrintersNowadays printers can be seen as the most popular type of output device. A printer produces a character by putting large number of dots on a paper. The quality of a printer is decided by number of dots printed for producing a character. This is expressed by a measure called dots per inch (dpi). Higher the dpi is better the quality. You can print out information that is in the computer onto paper. By printing you create what is known as a 'hard copy'. There are different kinds of printers which vary in their speed and print quality. Mainly the printers are divided into 2 groups.
Examples for impact printers are Dot matrix printer The most popular serial printer is called a dot matrix printer. In such a printer the print head generally consists of 9 pins. However, modern dot-matrix printers have high quality printing heads with 24 pins. Characters to be printed are sent one character at a time from the memory to the printer. The character code is identified by the printer electronics and the appropriate pins in the print head will be activated. These pins are moved forward to from a character. They hit the carbon ribbon In front of the paper thereby printing that character. Many dot matrix printers are bi-directional, that is, they print from left to right as well as from right to left on return. This speeds up printing. An advantage of dot matrix printers is the possibility of converting them to print alphabets other than English. It is possible to adopt them to print Sinhalese script, Tamil script etc. Letter quality printer (Daisy wheel printer) As a character produced by a dot matrix printer is made up of a finite number of dots, the appearance of the printer output is not very good. For better looking outputs where characters are represented by sharp continuous lines, a character printer known as the Daisy wheel printer is used. The daisy wheel is mounted on a carriage which can move from left to right (and back) and can also rotate. Characters to be printed are sent serially to the printer. The movable carriage positions itself at the print position and rotates, so that the character to be printed appears in front of the printer ribbon. The carriage now tilts and embossed character strikes the ribbon and an impression is made on the paper mounted behind the ribbon. The carriage now moves on character position to the right and rotates the daisy wheel so that the next character to be printed is positioned correctly and the character is printed. (2) Non impact printers Non-impact printers are much quieter than impact printers as their printing heads do not strike the paper. Most non-impact printers produce dot-matrix patterns. Several different technologies have been used to provide a variety of printers. The main types of non-impact printer are: o Thermal Printer o Laser Printer o Ink Jet Printer Laser printers The basic limitation of line and serial printers is the need for a head to move and impinge on a ribbon to print characters. This mechanical movement is relatively slow due to the high inertia of mechanical elements. Intensive research and development with the goal to eliminate mechanical motion in printers has been conducted by computer by computer manufactures. One such effort by international business machine (IBM) has led to the development of laser printers. In these printers, an electronically controlled laser beam traces out the desired characters to be printed on to a photo-conductive drum. The drum attracts an ink toner onto the exposed areas. This image is transferred to the paper which comes in contact with the drum. At present, these printers are very expensive. They give excellent quality outputs. These printers do not have a type head striking on a ribbon. Inkjet printers Inkjet printers are cheaper than laser printers. However, these printers can provide a substantial quality documents. As a result, Inkjet printers are used inkjet most of the places today inkjet printers follow a simple mechanism for printing. That is the printer is supplied with a cartridge of ink and it is designed to be activated to send jet of ink to produce characters inkjet a paper. The characters on the paper are produced as a collection dots as inkjet the case of laser printer. Note that since the Inkjet printers send drops of ink to the paper, it is necessary for leaving the drops to get dry up before touching the paper. Otherwise, ink can spread on the paper. Due to this reason, inkjet printers should be used very carefully when the document has many pages. It should be noted that Inkjet printers can also produce color printouts. As compared with color printouts using laser printers, the Inkjet color printouts are much cheaper. Bubble jet printers Bubble jet printers are an improved version of inkjet printers. The bubble jet printers send ink bubble which is heated with extra energy. Thus the printing quality is better than using a spray of raw ink. Inkjet fact you do not need to wait longer for drying up the printed material as in the case of inkjet. Inkjet the market, bubble jet printers are slightly expensive than inkjet printers. However, they are still cheaper than laser printers. Nowadays, many people refer to buy bubble jet printers for office and home use. Plotters Besides printer plotters, Special plotters to produce good quality drawings and graphs have been designed and are available inkjet the market. There are 2 types of plotters. i. Drum plotter In a drum plotter, the paper on which the graph is to be drawn is mounted on a rotating drum. A pen which can move linearly, i.e. perpendicular to the direction of drum rotation, is mounted on a carriage. The drum rotate inkjet either clockwise or anti-clockwise direction under the control of the plotting instructions sent by the computer. The pen can move left to right to left. The pen can also move up or down. The movements of the pen and drum are controlled by the graph plotting program. The program can thus draw various graphs and also annotate them by using the pen to draw characters. ii. Flat bed plotter A flat bed plotter has a stationary horizontal plotting surface on which paper is fixed. The pen is mounted on a carriage which can move inkjet either X or Y direction. The pen can also be moved up or down. A graph plotting computer program is used to move the pen to draw the desired graph. [edit] MonitorOften referred to as a monitor when packaged in a separate case, the display is the most-used output device on a computer. The display provides instant feedback by showing the text and graphic images as the work or play. Most desktop displays use a cathode ray tube (CRT), while portable computing devices such as laptops incorporate liquid crystal display (LCD), light-emitting diode (LED), gas plasma or other image projection technology. Because of their slimmer design and smaller energy consumption, monitors using LCD technologies are beginning to replace the venerable CRT on many desktops. Displays have come long way since the blinking green monitors in text-based computer systems of the 1970s. Just look at the advances made by IBM over the course of a decade: In 1981, IBM introduced the Color Graphics Adapter (CGA), which was capable of rendering four colors, and had a maximum resolution of 320 pixels horizontally by 200 pixels vertically. IBM introduced the Enhanced Graphics Adapter (EGA) display in 1984. EGA allowed up to 16 different colors and increased the resolution to 640x350 pixels, improving the appearance of the display and making it easier to read text. In 1987, IBM introduced the Video Graphics Array (VGA) display system. Most computers today support the VGA standard and many VGA monitors are still in use. IBM introduced the Extended Graphics Array (XGA) display in 1990, offering 800x600 pixel resolutions in true color (16.8 million colors) and 1,024x768 resolutions in 65,536 colors. Most displays sold today support the Ultra Extended Graphics Array (UXGA) standard. UXGA can support palette of up to 16.8 million colors and resolutions of up to 1600x1200 pixels, depending on the video memory of the graphics card in your computer. The maximum resolution normally depends on the number of colors displayed. For example, your card might require that you choose between 16.8 million colors at 800x600, or 65,536 colors at 1600x1200. The combination of the display modes supported by your graphics adapter and the color capability of your monitor determine how many colors can be displayed. For example, a display that can operate in Super VGA (SVGA) mode can display up to 16,777,216 (usually rounded to 16.8 million) colors because it can process a 24-bit-long description of a pixel. The number of bits used to describe a pixel is known as its bit depth. With a 24-bit bit depth, 8 bits are dedicated to each of the three additive primary colors -- red, green and blue. This bit depth is also called true color because it can produce the 10,000,000 colors discernible to the human eye, while a 16-bit display is only capable of producing 65,536 colors. Displays jumped from 16-bit color to 24-bit color because working in 8-bit increments makes things a whole lot easier for developers and programmers. Briefly, the measure of how much space there is between a display's pixels. When considering dot pitch, remember that smaller is better. Packing the pixels closer together is fundamental to achieving higher resolutions. A display normally can support resolutions that match the physical dot (pixel) size as well as several lesser resolutions. For example, a display with a physical grid of 1280 rows by 1024 columns can obviously support a maximum resolution of 1280x1024 pixels. It usually also supports lower resolutions such as 1024x768, 800x600, and 640x480. In monitors based on CRT technology, the refresh rate is the number of times that the image on the display is drawn each second. If your CRT monitor has a refresh rate of 72 Hertz (Hz), then it cycles through all the pixels from top to bottom 72 times a second. Refresh rates are very important because they control flicker, and you want the refresh rate as high as possible. Too few cycles per second and you will notice a flickering, which can lead to headaches and eye strain. Differences between CRT and LCD monitors The two types of monitor technologies available to consumers are available in CRT monitors and LCD monitors. There are big differences between LCD and CRT, and while LCD technology has advanced to the point where it’s viewing quality is comparable to CRTs, many people today still choose to purchase a CRT monitor. CRTs are bigger and bulkier than an LCD, they consume more power and are prone to screen flicker. LCD monitors, however, are more expensive when compared to CRTs, they introduce the problem of viewing angles, and generally have less accurate color replication. Each type of monitor, as you can see, has its advantages and disadvantage. In this article we will provide a comparison of CRT and LCD monitors, along with defining some of the many specifications and terminology you should be aware as you decide between an LCD and CRT monitor. Sort for cathode-ray tubes, CRT monitors were the only choice consumers had for monitor technology for many years. Cathode ray tube (CRT) technology has been in use for more than 100 years, and is found in most televisions and computer monitors. A CRT works by moving an electron beam back and forth across the back of the screen. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. By drawing many such lines from the top to the bottom of the screen, it creates an entire screen of images. [edit] LCD/Flat panel monitorsShort for liquid crystal display, LCD technology can be found in digital watches and computer monitors. LCD displays use two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light. Color LCD displays use two basic techniques for producing color: Passive matrix is the less expensive of the two technologies. The other technology, called thin film transistor (TFT) or active-matrix, produces color images that are as sharp as traditional CRT displays, but the technology is expensive. Resolution & Viewing Quality Resolution on a CRT is flexible and a newer model will provide you with viewing resolutions of up to 1600 by 1200 and higher, whereas on an LCD the resolution is fixed within each monitor. The resolution on an LCD can be changed, but if you're running it at a resolution other than its native resolution you will notice a drop in performance or quality. Both types of monitors (newer models) provide bright and vibrant color display. However, LCDs cannot display the maximum color range that a CRT can. In terms of image sharpness, when an LCD is running at its native resolution the picture quality is perfectly sharp. On a CRT the sharpness of the picture can be blemished by soft edges or a flawed focus. A CRT monitor can be viewed from almost any angle, but with an LCD this is often a problem. When you use an LCD, your view changes as you move different angles and distances away from the monitor. At some odd angles, you may notice the picture fade, and possibly look as if it will disappear from view. Refresh Rate Some users of a CRT may notice a bit of an annoying flicker, which is an inherent trait, based on a CRTs physical components. Today's graphics cards, however, can provide a high refresh rate signal to the CRT to get rid of this otherwise annoying problem. LCDs are flicker-free and as such the refresh rate isn't an important issue with LCDs. Dot Pitch Dot pitch refers to the space between the pixels that make up the images on your screen, and is measured in millimeters. The less space between pixels, the better the image quality. On either type of monitor, smaller dot pitch is better and you're going to want to look at something in the 0.26 mm dot pitch or smaller range. Screen (viewable) Size Most people today tend to look at a 17-inch CRT or bigger monitor. When you purchase a 17-inch CRT monitor, you usually get 16.1 inches or a bit more of actual viewing area, depending on the brand and manufacturer of a specific CRT. The difference between the "monitor size" and the "view area" is due to the large bulky frame of a CRT. If you purchase a 17" LCD monitor, you actually get a full 17" viewable area, or very close to a 17". Physical Size There is no denying that an LCD wins in terms of its physical size and the space it needs. CRT monitors are big, bulky and heavy. They are not a good choice if you're working with limited desk space, or need to move the monitor around (for some odd reason) between computers. An LCD on the other hand is small, compact and lightweight. LCDs are thin, take up far less space and are easy to move around. An average 17-inch CRT monitor could be upwards of 40 pounds, while a 17&-inch LCD would weigh in at around 15 pounds. Price As an individual one-time purchase an LCD monitor is going to be more expensive. Throughout a lifetime, however, LCDs are cheaper as they are known to have a longer lifespan and also lower power consumption. The cost of both technologies has come down over the past few years, and LCDs are reaching a point where smaller monitors are within many consumers' price range. You will pay more for a 17" LCD compared to a 17" CRT, but since the CRT's actual viewing size is smaller, it does bring the question of price back into proportion.
[edit] Computer SpeakersEven though most of the interaction between computer and user is visual, computer speakers are still an important part of a home computer system. There are many programs that send audio cues for different information, not to mention the usefulness of speakers for watching videos on DVD or the internet. And computer speakers are absolutely essential for web conferencing and internet phone service. Computer speakers allow users to have a greater experience with computers, and they have become standard equipment when buying a computer. Most computers contain a low-quality speaker in the computer itself that can only emit beeps and tones. But manufacturers include a set of stand alone speakers that can handle a wider range of sounds, including voice and music. These additional speakers are not intended for high quality sound; but manufacturers also make a wide variety of high-quality speaker systems that can even rival home entertainment systems for sound quality. These systems are rarely offered with the computer, but instead are bought and installed later. Some of the more extensive computer speakers systems include things like surround sound and subwoofers, although the average consumer might not need that much audio equipment. The stand alone speakers that come with computers are usually enough for most gaming, movie watching, and computing needs. [edit] See also
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