The Visual Display Unit
The Visual Display Unit (VDU) is a device used for interactive processing, i.e. data that is being keyed in, is displayed on the screen or monitor. Messages and processed information are also displayed on the screen. The combination of keyboard and the VDU is usually referred to as Video Display Terminal (VDT), which is an input/ output (I/O) device.
The formation of images is controlled by the video controller. The video controller along with the memory is termed as the display adaptor.
The number of dots (pixels) on the screen is the measure of resolution of monitor.
Display adaptors are of various types and are normally classified on the basis of resolution, colour and display mode:
MGA: Monochrome Graphics Adaptor (MGA). The term graphics is a misnomer since the device supports text only. Although the option is monochrome or single colour, it offers various attributes e.g. the brightness and intensity of characters which can be changed.
CGA: Colour Graphics Adaptor supports both text and graphics mode.
It functions in colour and monochrome modes in various resolutions. The CGA works with different types of monitors. However, it gives poor display quality in the text mode. The typical resolution is 640 * 200 (i.e. 640 rows * 200 columns) in the graphics mode.
HGA: Hercules Graphics Adaptor is a monochrome adaptor with an additional graphics display mode which provides high resolution monochrome graphics. In text mode it functions like the MGA but in the graphics mode it offers a resolution of 750 * 350 dots, even better than the CGA. In the graphics mode the HGA has no colour although it offers brightness and intensity variations.
EGA: Enhanced Graphics Adaptor was developed in an effort to unify the variety of display adaptors. The EGA combines all the features of other adaptors and provides higher resolutions using higher quality colour monitors. The EGA supports 16 colours at a time.
VGA: Video Graphics Array supports 256 colours at a time with a high resolution.
SVGA: Super Video Graphics Adaptor supports 24-bit true colour upto 1024* 768 dots.
The results of processing could be written by the computer onto a tape or disk, to be used at a later time or to be given to another computer as input.
However, the most common form of computer output is printed output - also called hard copy output. Printers are classified by how they print and how fast they operate.
Character Printers print one character at a time and are used for low-volume printing jobs.
Dot Matrix Printer
The dot matrix printer is a versatile low-cost device capable of printing in various languages, printing letters of various types, in bold, italics or underlined. It can also be used to print graphics.
Ink Jet Printers
Droplets of ink are electrically charged after leaving a nozzle. The droplets are guided to the proper positions on the paper by electrically charged deflection plates. Print quality is good because each character is formed by dozens of ink dots. Text and graphics produced are of better quality and the printer is comparatively fast. They also have the ability to use multiple-nozzle print heads, thus enabling it to print in several colours.
Laser printers are page printers. A page of text or pictures is composed at a time. A laser printer utilises a laser beam that sensitized selected areas on a print page. The laser - exposed areas attract a toner (an ink powder) that attaches itself to the lasergenerated charges on the drum. The toners is then permanently fused on the paper with heat or pressure. The resolution of print image is upto 1200 dots per inch.
The printer, besides being fast, also produces a high quality print. It can handle large volumes of printed output.
Printing takes time, and often the CPU is the idle while the printing process takes place. Techniques of spooling or printer buffer is used to optimise CPU resources.
Spool: Simultaneous Peripheral Operations On-line.
Using the spooling technique, output is not sent directly to the printer. The output is first transferred to an intermediary storage medium, such as a disk file. Outputs can be stored in separate files and printed out at a later stage as time and resources permit.
The CPU transfers print output to temporary memory called a "buffer".
The printer then accesses this information and prepares one or more lines of printed output. During this time the CPU performs other tasks. When all the information is printed, the CPU refills the buffer with more data. Printers contain built-in buffers.
Besides, there are many other types like Daisy Wheel Printer/ Band Printer etc., also.
Data Storage Devices
Data processing system often requires access to very large quantities of data. The computer memory or Immediate Access Storage (IAS) is unsuitable for this task. The amount of data needed for any commercial system is too large to be held in the processor, and secondly, normal processor memory is volatile, i.e. it does not retain its contents once power is switched off or even if there is a temporary break in power. Some form of backing storage is therefore required.
The memory unit of the CPU is a place where programmes or instructions and data are stored while processing. The device consists of a number of storage locations. Each storage location may be identified by a unique number which is called its address. During processing, data may be stored in any location which is identified by the address of the location.
The storage area may be designed to store a fixed number, of characters which are treated as a single entity or word.
Typically, semiconductor elements are used in primary storage sections or main memory. Semiconductor storage elements are small integrated circuits. The storage cell circuits and the support circuitry needed for reading and writing data are packaged on chips of silicon. A number of semiconductor storage technologies are in use. However, chips that use Metal-Oxide Semiconductor (MOS) technology are usually used in the primary storage section. The components are called Random Access Memory (RAM) chips. It is possible to read from and write to any location within RAM by specifying its location or address. New data can be written onto any location. However, while doing so the previous existing data is erased.
RAM chips may be classified as i) Dynamic RAM chips and ii) Static RAM chips.
Dynamic RAM chips: The storage cell circuits contain a transistor (functions like a mechanical on-off light switch) and a capacitor used to store an electric charge.
Depending on the switching of the transistor, the capacitor may have no charge (0 bit) or hold a charge (1 bit). The charge on the capacitor must be periodically refreshed or recharged.
In the event of power loss, dynamic RAM loses its contents. It is thus called volatile storage.
Static RAM chips: They are also volatile storage devices. However, as long as they are supplied with power, they do not require special regenerator circuits to retain the stored data. More transistors and other devices are needed to store a bit in static RAM. These chips are more complicated than dynamic RAMs. Static RAMs are used in specialised applications. Dynamic RAMs are typically used on the primary storage section.
There are certain essential functions that the computer must perform when it is switched on (e.g.: establishing connection within the various components of the computer and its peripherals). These low-level or machine-level functions are carried out through a series of programmes or microprogrammes. These microprogrammes are stored on chips. These Read-Only Memory chips (ROM) contain data which can be read randomly when required but cannot be written onto.
Data is hardwired onto these chips at the time of manufacture. They cannot be changed by the user.
While both RAM and ROM are storage devices and can be accessed randomly, they differ in that data can be written onto RAM while ROM does not permit the user to write onto it.
ROM retains the data in it even in the absence of power and is thus nonvolatile storage.
There are some types of read only memory called Programmable Read-Only Memory (PROM). Critical or lengthy operations that are slowly carried out by software can be converted into micro programs and fused into a programmable read-only memory chip. Once they are in hardware form, these tasks take a fraction of the time. Each bit can be individually programmed to a ''1" or "0" by burning out a fusible link within the selected cells. Afused link cannot be restored. Operations once written cannot be erased. PROM can be programmed only once.
The Erasable Programmable Read-Only Memory (EPROM) makes it possible for the user to repeatedly erase and reprogram the ROM. Erasing is done by exposing the EPROM to ultra-violet rays of a specific frequency.
An Electrically Erasable Programmable Read-Only Memory (EEPROM) --can be programmed through the use of special electrical pulses. It is possible to integrate the circuitry into the computer, so that the EPROM does not have to be removed from its socket for programming.
CD-ROM (Compact Disk Read-Only Memory) is a non-erasable disk used for storing computer data.
The disk is formed from a resin, such as polycarbonate and coated with a highly reflective surface, usually aluminium. Information is imprinted as a series of microscopic pits on the reflective surface. A master disk is created using a finely-focussed, high-intensity laser. The master is used to make copies. Atop coat of clear lacquer protects the pitted surface from dust and scratches. A laser shines through the clear protective coating while a motor spins the disk past it. When it encounters a pit, the intensity of the reflected light of the laser changes. The change is detected by photo sensors and converted into a digital signal.
By the use of CD writer it has become possible to use CDs as erasable storage device. Data can be written onto the CD or erased like in a floppy disc.
The size of the memory unit in the CPU is often inadequate, when large programmes need to be stored or large amounts of data need to be processed. To overcome this drawback, the technique of virtual memory is used. In this technique, which is implemented by hardware or software or both, the memory of the computer appears to have been increased. The programme is divided by the software into pages or segments. Only that portion of the programme and data that is being processed is kept in primary storage. Part of backing storage is treated as an extension of the main memory and information that is not required is swapped in and out of main store by the operating system. Thus the size of memory becomes equal to the size of primary storage plus the size of secondary storage being used. Processing time is thus automatically increased.
In an instruction cycle, the CPU accesses main memory to fetch the instruction. It also accesses memory one or more times to fetch operands or store results. The rate at which the CPU can execute an instruction is, therefore, limited by the speed of main memory. To build main memory with the same technology required for CPU registers, -so that memory cycle times are comparable to processor cycle times, is very expensive. As a solution, a small, fast memory is provided between the CPU and main memory. This is called cache memory.
A copy of portions of main memory is maintained in cache. When the CPU reads a word of memory, it first reads cache. If found, the word is delivered to the CPU.
Gallium Arsenide Chips
Chips made of gallium arsenide are expected to be five times faster, consume less power and operate at higher temperatures than silicon. These chips are going to replace the presently used integrated chips.
Magnetic tapes are convenient, inexpensive devices which can be used to store large volumes of data. The magnetic tape is similar to the commonly used audio-tape recorders. The access time in the case of magnetic tapes is quite high.
* Large volumes of data can be stored.
* Data can be transported easily.
* Cost effective.
* Re-usable medium.
* For data that has to be loaded sequentially.
Used mainly with micro computers, they are also called "floppy tapes" since it is easy to "flop" the cartridge tape in and out of the recording device. Unlike commercial tape, it does not have to be manually threaded while inserting it onto the tape drive.
They are used with low-speed micro computers and for small-scale applications.
Streamer Tapes Magnetic Disks
A magnetic disk is a circular platter that may be made of smooth metal or mylar plastic. This is coated with magnetisable material. Data is stored or retrieved from the disk using a conducting coil called the head. During a read/ write, the head is stationary while the platter rotates beneath it.
Floppies are made of mylar plastic coated with magnetic oxide. The flexible material is cut into circular pieces 5 1/4 " in diameter. There are mini floppy disks 3 1/2" in diameter.
Since they are made of flexible tape unlike the hard disk, they are called "floppy disks."
The circular pieces are packaged in 5 1/4 " square plastic covers. The 3 1/2 " floppy is covered by a rigid plastic case.
A long slit is provided for the read/ write head to access the disk. A hub in the centre is used for mounting the disk drive. A hole is used to sense index marking.
Most storage devices are based on the principle of magnetism. Some storage devices, however, are based on the use of light. Optical disks use this technology. Streams of digital data, in the form of tiny pits, are burned onto a thin coating of metal or other material deposited on a disk. A beam of laser light is used to read these pit patterns. These pits once burned onto the disk cannot be erased. The disk cannot be used for re-recording.
Erasable optical disk
The erasable optical disk is a recent development. Data can be repeatedly written and overwritten as done with a magnetic disk. The disk is coated with a magnetic material. A laser beam heats a specific spot on the medium and a magnetic field changes the orientation of that spot while its temperature is elevated.
For reading, the direction of magnetism can be detected by polarised laser light.
The Write Once Read Many (WORM) disk is prepared in such a way that it can be written once with a laser beam of modest intensity. It is possible to read the disk a number of times.
A high powered laser is used to produce a series of blisters of the disk. When the pre-formatted medium is placed in a WORM drive, a low-powered laser can produce just enough heat to burst the prerecorded blisters. During a disk read operation, a laser in WORM drive illuminates the disks surface. Since the burst blisters provide higher contrast than the surrounding area, these are easily recognised by simple electronics.
RAM disk storage of "Virtual disk" storage is a facility offered by some versions of operating software. A section of the main memory is treated as a disk, with data being organised in files and the same commands being used to access or control it as for a backing storage device requiring Input/ Output. Instead of writing to backing storage, however the data is written onto RAM chips in memory.
All digital computers store numbers, letters, and other characters in coded form. The code used to represent characters is the binary code, i.e., a code made up of binary digits or bits. Every character is represented by a string of "0s" and "1s" - the only digits found in the binary numbering system.
A sequence of 8 bits is called a byte. 1024 bytes make a kilobyte. 1024 kilobytes make a megabyte (MB). 1024 megabytes make a gigabyte. A word consists of several bytes. Most computers have words that consists of 8 or 16 bits. However in larger computers the number of bits could be 16, 32, 36 or 40 bits.
When data is typed into a computer, the keyboard converts each keystroke into a binary character code. This code is then transmitted to the computer. When the computer transmits the data to the printer, or to the screen or to the disk, each individual character is communicated in binary code. It is then converted back to the specific character while displaying or printing the data.
Unit of Information
Most computers however do not represent characters as pure binary numbers.
They use a coded version of true binary to represent letters and special symbols as well as decimal numbers.
In the English language there are 26 characters. If we also take into consideration the uppercase, special symbols like *, %, +, - etc, the ten decimal digits and non-printable control characters like the carriage, return etc. We have 128 characters. We would. require 7 digits to uniquely represent all the 128 characters uniquely. Coding of characters has been standardised to enable transfer of data between computers.
The most popular and common standard is the American Standard Code for Information Interchange (ASCII). ASCII uses 7 bits per character. With 7 bits it is possible to provide 128 (27) different arrangements.
Besides codes for character, codes are also defined to convey information such as end of file, end of page etc., to the computer. These codes are called non-printable control characters. The ASCII code is used to represent data internally in personal computers.
Another code also exists called External Binary Coded Decimal Interchange Code (EBCDIC). EBCDIC uses 8 bits per character. Thus 256 characters can be represented using EBCDIC. The EBCDIC code is used in IBM mainframe models and other similar machines.
Electronic circuits are available to transform characters from ASCII to EBCDIC and vice versa. We can also achieve the same results using a computer program.