Home Computer: What's on display colours key decisions: The quality of the picture on the screen determines the power of the central processor that the system needs. Andrew Brown explains why rams and roms create the spots before your eyes

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THE COMPUTER business is full of numbers, which is one of the great difficulties for beginners, since numbers are frightening when not bizarre and always difficult to think about.

But it is easier to choose a computer if you understand what some of the more common numbers and acronyms mean. All explanations should start right in front of your eyes: if you can decide what kind of screen you want to look at, the other numbers can all be worked out from there. The quality of the screen you need will depend on the kinds of things you want to do on it - simple word processing, or drawing and editing pictures for desk top publishing, for example.

Computer screens, like televisions and indeed the printed page, show you a picture made up of dots. If the dots that make up the letters and pictures on screen are drawn individually on to the screen the result is known as a graphical user interface. If they are sent in fixed patterns, as with a typewriter, this is known as a character-mode interface.

First generation word processors were limited to using only character-mode interfaces, but with increases in computing power graphical user interfaces now predominate.

There are two main sorts of graphical user interface available for the two main types of professionally used computer systems: Microsoft Windows 3.1 for PC-compatibles and the Apple Macintosh's integral graphics-based operating system. (There are similar systems available on Atari, Amiga, and Acorn computers).

The most common - and confusing system - is the PC-compatible. If you want a screen that will run Microsoft Windows, it needs a resolution of at least 800 dots across by 600 down and also to be able to display 256 colours.

A specification of 800 by 600 by 256 is enough to display a great many pretty and informative pictures. The only other important thing about a monitor is that it should be 'non-interlaced' - if you can afford the extra cost. Interlaced monitors achieve impressive resolutions by a piece of technical trickery which makes them flicker just subliminally until they turn the operator's mind to scrambled egg.

The smaller and closer together the dots are and the more colours they can be painted, the more realistic the screen appears and the more information it can display.

So a resolution of 1024 dots by 768 dots is really useful. It fits in nearly twice as much information as the basic resolution screens: on the screen in front of me that means about 500 words, or half this article. Or it allows me to have two documents 'open' simultaneously, and to be cutting text out of one to stick it in the other.

Because of the way in which the pictures on screen are composed in the computer's memory, there is a trade-off between the number of colours and the number of dots on screen at any one time. The computer on which I am writing will show 32,000 shades of colour at 640 dots by 480 dots. But at 1024 dots by 768 dots, which is the useful resolution, it will only show 256 colours. Sixteen colours is plenty for word processing and quicker to display.

A screen of this size and quality demands a lot of computing power behind it. There are 786,432 dots on the screen in front of me and the machine has to work out and then keep track of the colour of each one. This is where the second wave of jargon begins: power is measured in three different ways.

Computer programs need space to work in, space to be stored in, and the computer needs the power to manipulate them. Space inside a computer is rather like space inside a book: the sensible measure for space inside a book is the number of letters or words it contains rather than the amount of paper in it.

Size inside a computer, used to be measured in 'bytes': one byte is the amount of memory needed to store one letter on an early, primitive machine. On a more modern machine, it might be the memory needed to store one of the dots that make up a letter on screen, or even to record one of the colours of one of the dots of a letter on screen. It is easy to see that larger units of memory than bytes are needed.

The next one up is the 'kilobyte' or 'K', each of these being 1024 bytes. These, in turn, proved too small to be useful, and were replaced by 'megabytes', or units of 1,000 kilobytes.

Programs for Microsoft Windows can be extremely large: Windows takes up about 20 megabytes of space fully loaded and a full-scale word processor can easily take up another 15 megabytes when you add in the spell-checker, the grammar checker, the thesaurus and the pretty pictures supplied.

Fortunately there is no need to buy a computer with a memory as large as this, for all computers work rather like a library: the books are stored away when not in use. You have to ask the librarian to bring one to your alloted work-table. When you have finished, the book is returned to the shelves from whence it came.

The computer's shelves or storage space is its 'hard disk'. These get physically smaller, yet hold more information, every year. But to get the most out of a library, you need to be able to study many books at once and this needs sizeable tables to work at.

The table at which you do your work is known as 'memory' or more precisely random access memory, or ram. The 'library staff' who fetch and carry the books are the central processing unit: the chip that drives the computer. Remember when the machine is switched off - the library shuts - the table is cleared and what has not been written back to disk is lost. Only some basic information, such as where the disk drives are, is stored in memory that is not cleared when the machine shuts down: such memory chips are known as roms, for 'read only memory,'

There is really no point in buying a machine to run the latest version of Windows with less than an 80 megabyte hard disk: 100 megabytes or so is much better. Even 240 megabytes is surprisingly easy to fill. Large programs need a lot of memory if they are to work quickly - four megabytes of ram is the minimum to run Windows sensibly. Eight megabytes is better if you normally run several large programs at once, or wish to scan incoming faxes to turn them into machine-readable text.

The final figure in all this is the central processor. There are only two numbers here which matter: to run windows satisfactorily the main processor of a PC should be either a 386 or a 486. These are simple product designations and have no relation to any real figures, such as the 'speed' of the processor - the number of simple instructions it carries out every thousandth of a second. The 486 processors are newer, more efficient, although a little more expensive than 386s. They are also easier to upgrade.

The speed of a chip is measured in megahertz (MHz) and again, broadly speaking, the more of these the better. More important is whether the chip has 'SX' after it: the two letters indicate that the chip is a less powerful and cheaper version. A 486 SX processor running at 25 megahertz is powerful enough for most things a Windows computer should do.

And those are all the numbers you need to know to go shopping for a PC Windows machine. Apart from one other: 17.5 per cent, which is the rate of VAT, and should be added to almost all prices you are quoted.

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