The Control Unit and the Arithmetic-Logical Unit.

The basic components of a computer system, the input, the output, the memory and the processor operate only in response to commands from the control unit. The control unit operates by reading one instruction at a time from memory and taking the action called for by each instruction. In this way it controls the flow between main storage and the arithmetic-logical unit.

A control unit has the following components:

1. A counter that selects the instructions, one at a time, from memory.

2. A register that temporarily holds the instruction read from memory while it is being executed.

3. A decoder that takes the coded instruction and breaks it down into the individual commands necessary to carry it out.

1. A clock, which while not a clock in the sense of a time keeping device, does produce marks at regular intervals. These timing marks are electronic and very rapid.

Binary arithmetic (the kind of arithmetic the computer uses), the logical operations and some special functions are performed by the arithmetic-logical unit. The primary components of the ALU are banks of bi-stable devices, which are called registers. Their purpose is to hold the numbers involved in the calculation and to hold the results temporarily until they can be transferred to memory. At the core of the arithmetic-logical unit is a very high-speed binary adder, which is used to carry out at least four basic arithmetic functions (addition, subtraction, multiplication, and division). The logical unit consists of electronic circuitry which compares information and makes decision based upon the results of the comparison. The decisions that can be made are whether a number is greater than (>), equal to (=) or less than (<) another number.

(E. C. S.)

Упражнение 24.

Tapescript

Primary and Secondary Memory

The term ‘memory’ is usually referred to the internal storage locations of a computer. It is also called real storage or primary memory, and is expressed as quantities of K. Each K is equal to 1,024 bytes, and each byte is equal to 8 bits. Modern computers measure their memory in megabytes (Mb) – a megabyte is equal to 1048576 bytes.

Primary memory is closely associated with the CPU because it stores programs and data temporarily, thus making them immediately available for processing by CPU. To facilitate processing, two things are needed: random access and speed. The former means that any part of the memory may be read, or accessed, equally quickly. This is made possible by the system of addresses in primary memory, where the storage locations are like a series of tiny compartments, each having its own address. These addresses are like the addresses of houses, in that they do not change. Because they are always fixed, the control unit knows where to find them at a very high speed. When it finds them, it puts into the compartments whatever must go there and wipes out whatever was stored there. The information present in these compartments is called the contents of the memory.

Most primary memory is costly, therefore it is used transiently (временно), which means that a program, or parts of it, is kept in internal storage while the program is being executed. This, however, is not true for mini and micro applications where the computer performs the same function, referred to as a dedicated function, all the time. But since computers must process vast quantities of data and programs, a lot of storage space is required. For this reason various secondary memory technologies have been developed.

Secondary memory devices fall into two categories: sequential devices and random-access devices. Sequential devices permit information to be written on to or read off some storage medium in a fixe etic disk, which is faster than the magnetic tape and more expensive. When disks are hooked up to the computer and used as an extension of internal storage in order to increase the capacity of primary memory, this is called virtual storage. The memory size of computers is increasing as memory chips become cheaper.

(E. C. S.)

Упражнение 43.

Tapescript

Good morning everyone! Welcome to the model office. Last week I showed you around the office and indicated some of the range of equipment we have here for students to practise using. You had just had a lecture on peripherals hadn't you? well, said I would talk to you in more detail today about printers, so here we go.

As you know there are many types of printers and which one you decide to use will depend on all sorts of things like how much you can afford, what kinds of documents you are intending to produce who will be receiving your printed material and so on. It may not always be necessary to use the finest quality or printing all the time. There will be occasions when a draft quality will suffice, for your own use or some other in-house function.

Now then, if you can move round to this work station and make sure everyone can see ... good, that's fine. Printers provide the user with hard copy (that 's? permanent copy) of information that can, for example, be posted to clients etc. I'm thinking of bank statements salary slips and so on which need to be output on a line printer. There are two types of printer: line printers and character printers. Do make a note of those two terms. Line printers can type a complete line at a time whereas character printers can only print a single character at a time. An ordinary typewriter is a character printer.

Now look closely at this. This is a dot matrix printer. Some of these are line printers but the majority are character printers. They are mainly used with micro computers because they are fairly cheap. They consist of seven or nine hammers that can be struck individually under computer control against an inked ribbon to make a dot on the paper. By striking the right hammers at the right times they print numbers and letters. By going over each character twice and moving the print head slightly it is possible to produce near letter-quality print. Obviously this takes longer.

The daisy wheel printer consists of a wheel with lots of arms attached to it, rather in the way petals are attached to a daisy. At the ends of these arms are two characters one above the other. The daisy wheel rotates and a hammer presses the carbon paper against the arm. Daisy wheel printers are slow but produce high quality print. They are used mainly for producing business letters in conjunction with word processors and aren't expensive.

Laser printers are extremely fast and are used for producing Giro cheques and gas and electricity bills. Quiet, of course, because the system is non-impact. Laser printers are quite expensive but are ideal where large quantities of bills need to be sent out in a short space of time.

ink jet printers use a technique of spraying tiny drops of ink onto paper to form characters. The new printers are able to print graphics as well as use several coloured inks. They are expensive but have the advantage of being quiet. Goodness, we're almost running out of time. Does anyone have any questions? I'll try to...

(C.S.)

Литература:

Основная:

1. INFOTECH. ENGLISH FOR COMPUTER USERS.Remacha Esteras, Cambridge University Press. 1996 (E.C.U.);

2. Oxford English for COMPUTING. Keith Boeckner, P. Charles Brown, Oxford University Press.1993 (O.E.C);

3. English for Computer Science. P. Charles Brown, Norma D. Mullen, Oxford University Press. 1991 (E.C.S.);

4. Computer Science. Tricia Walker, Published 1995 by Phoenix ELT (C.S.).

5. PC Review. 1996 (PC R.).

Дополнительная:

Периодические издания (специальные журналы по информатике и вычислительной технике).

Материалы сети Интернет.

1 качаться, дрожать, «вихлять»

2 покачивание, «виляние»

3 засыпать, заваливать, загружать

4 сшивать, брошюровать

2