A computer's memory is a storehouse of information and instructions
Till 1970 memory elements made of magnetic cores were very popular. The cores are tiny doughnut shaped objects of ferro- magnetic material. Fine wires are threaded through the hole at the centre of each core. The property of the core is such that it can be magnetized in two possible directions depending upon the sense of direction of current passed through the wires. Each core is said to store one bit of information, either a 0 or a 1.
The CPU gets new information and instruction from the memory at almost every stage of computation. Hence, the memory has to function very rapidly. The speed of response of the memory built out of magnetic cores is often of the order of one or two million words per second from a large array of a few million words. With the advancement of semiconductor technology, magnetic core memory was replaced by semiconductor memory. The memory available today is made out of integrated circuit chips each of which houses hundreds of thousands of Memory elements in a physical area of a quarter square inch. The speed of response of these memories is at least five to six times that of the core memory. The power requirements are also substantially less. But it suffers from the disadvantage that it requires uninterrupted power supply for retaining information, whereas the magnetic core holds information even if the power is switched off, the reason being the inherent property of magnetic core. The typical size of the memory of a modern computer is of the order of a few million bytes.
The Control unit receives instruction from the memory and issues commands after it is interpreted by the electronic circuits. The commands in the form of pulses trigger the circuit and set up appropriate pathways through the system so that instruction is carried out. The Alu performs computation as instructed by the control unit - adding, subtracting, multiplying, dividing and making logical decisions. During calculation, numbers are stored in areas of ALU called registers, a series of memory elements made out of semiconductor chips.
The number of registers in the ALU, is one of the indices of the speed of computer. Information can be moved from one register to another. A simple 'add' instruction will, for example, take a number from its register and add it to another register. The result of the addition may be put into the memory to await further computation, passed to an output device such as a printer or a display screen, or recorded in magnetic medium like a tape or disks.
Digital computers have two main storage areas, the main storage which we have already discussed, and the backup storage, which can be compared with a filing unit for storing papers. For reasons of economy, it is impractical to store all the programmes and data in the main memory. Files of data and programmes are kept on magnetic disks and magnetic tapes. These are the most common form of backup storage. The data is recorded (written) on the magnetic coating of the devices and played back (read) from them by means of a set of read-write heads. These operate, in principle, in a way similar that of the record and playback heads on a tape recorder. The heads contain the electromagnetic coils which perform the reading and writing functions. Signal pulses from the CPU are fed to the recording coils and the magnetic fields produced in the coils, magnetise the areas of the recording medium next to them. The data is thus recorded as a series of magnetic spots (bits) on the magnetic coating of the medium. When the recorded medium moves past the coils of the read head, electric current is induced in the coil of the head. The magnetic spots represent either a 0 or a 1, the two symbols used in the binary code.
The magnetic tape is used on computers resembles that of a sound recorder, but is usually 2,400 ft long and a half inch wide, wound on 10-inch diameter spools. Magnetic tape unit
Magnetic disks are even faster and higher capacity devices. Disks are used in sets or packs which look like a stack of gramophone records. They are made of metal with a magnetic coating on both surfaces and a pack, typically of 11 disks, is placed on the central spindle of the drive unit, which runs at a speed of about 3,000 revolutions per minute. All the surfaces except the top and bottom of the stack are used for data storage and there is a read-write head for each. The capacity of a modern disk pack is of the order of 400 million bytes - i.e. 400 million symbols consisting of letters, numerals, punctuation marks and other characters can be stored in a pack. The transfer rate between a disk unit and CPU is 70 to 80 percent faster than that of tape units.
Hard disc unit
Let us now look into the software aspects of the computer. Since the digital computer uses only 0s and 1s, the instructions of a programme can contain only these two symbols. For the earliest computers, a programmer had to write his instructions as a series of 1s and 0s. This was not only a very slow but also an extremely monotonous job. Besides, it was subject to frequent mistakes as it is almost impossible to remember the long strings of symbols that represent various operations.
Let us reproduce a typical stored word in the memory of a computer with16 bits word-length: 1001110101010101. It may mean add X and Y and feed the result in a certain place in the memory. To simplify the problem of coding it was necessary to invent Languages that were easily understandable and more or less like English. Fortran, Cobol, Basic, Pascal, etc. are languages available to the users of written programmes. A typical Fortran instruction is : Z = X + Y +5, meaning add X and Y and 5 to produce Z. The same instruction in Cobol reads, add X to Y giving Z. There is no 1, nor 0, only English words and numerals. The computer is No more unapproachable. These languages are known as higher level languages. Each high level language has its Compiler, a larger programme that translates high-level language to strings of 0s and 1s acceptable to a digital computer.
Every component of a computer system is expensive. The cost ranges from tens of thousands to hundreds of millions of rupees, depending on the power of a computer. The power is measured by the speed of CPU which varies from about 300,000 instructions per second for a single chip simple Microprocessor to 10 million and more instructions per second for a Supercomputer. A medium speed computer executes about two million instructions per second, which means it performs about 0.5 million decimal multiplication involving 16 to 18 digits with the highest precision.
Just as there is a wide range of hardware prices, there is also a wide range of software prices. In some applications, software packages have been developed and optimized over many years, so competition between software houses has often led to reduced cost.
The running cost of a computer centre is very expensive. Besides the cost of hardware, software and maintenance of the system by a team of engineers, the computer system has to be kept in controlled environmental conditions throughout its operational life. The typical temperature and humidity maintained in a computer hall are 20°C- 22°C and 45 to 55 percent, respectively.
For a commercial computer centre, housing a modern and medium speed computer, the cost of computation is around Rs.4000 per hour use of CPU plus the cost of stationery. For the full utilization of such a system, many people use the computer simultaneously. The railway or airlines reservation systems are the best examples of this with booking clerks, all using the single computer. The computation is done in real time - that means computations of input data and display of results follow almost immediately after the original enquiry. Other computing operations such as weekly accounting, where the data is accumulated over a period of time before processing, are called batch processing. Since many users' programmes are inside the system and the requirement of each programme is different from the other, there must be some means to ensure that when one programme is printing its results on the printer, another should not attempt to print too. It should wait till the first printing is done.
The system supervisor, or Operating System (O.S), large programme, with varieties of features, monitors these activities. It supervises the status of each of the resources like CPU, memory, printer, disk units, tape units, etc. and optimizes their use. When one programme is outputting results, the other programme may compute, or if it is finished, it should wait for previous printing to get over and yet a third programme may be transferred from disk or tape unit to the main memory for computation. The O. S is like an efficient manager of an office who coordinates jobs of different people working in various capacities so as to ensure that no one is idle and the total work is executed in a most efficient manner. The operating system, the compilers or language translators and a few utility programmes are usually supplied by the manufacturer of the computer.
The article is not complete unless we describe how terminals establish communication with the computer which is located quite far away. In fact, Computer Terminals are scattered in diverse places. The distance of terminals may range from a few meters to hundreds of meters. There are various ways of establishing communication between the terminals and the computer. Besides private lines, directly connecting the computer and the terminals, already laid telephone lines are often used.
The latest aid in long distance, overseas communication is the Geostationary Communication Satellite. The satellite is equipped with radio receivers and transmitters and acts as a relay station. Engineering techniques have allowed users in one continent on the earth to bounce signals off it and the signals come down to receivers in another continent. Signals are blocked by mountains, by the earth's curvature or sometimes even even by tall buildings ; therefore, they have to be relayed from one transmitter to another in order to travel long distances.
It is time we adapted the computer to meet the needs of our country. It is an invention in which the creativity and innovative essence of man have blended to create precision, power and endurance, to enable it to cope with a wide range of problems. Computer development is a very fast evolving technology.
The two part article is concluded.
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