In addition, the Teletext character set contains the elementary graphics

characters and Teletext control codes mentioned on page C61, full details of

which are given in Appendix 2.

Matters are made somewhat more complicated by the fact that your computer

allows the ASCII character set to be redefined and extended, thereby enabling

foreign, italic and a variety of user-defined characters to be displayed. The

example below redefines the key so that it displays the mathematical

symbol used to denote pi:

M0DE134

VDU23,9,Ø,2,124, 168,4Ø,4Ø,4Ø,Ø

A utility to help you design your own characters is provided as part of the

Welcome software.

Communicating with the computer

You have now spent a short time typing things at the computer's keyboard and

witnessing the result. Initially, it does not seem particularly surprising that

when you press, say, A, the computer displays an A on the screen -- this is

exactly what you would expect. In fact, one part of the computer, called the

machine operating system (MOS) works incredibly hard to produce this

simple result and it is in action for every instant that the computer is switched

on. Even when the computer appears to be idle, waiting for you to type

something at the keyboard, the MOS is busy maintaining the screen display

and carrying out other vital functions.

The MOS is also responsible for calling up each of the other systems provided in

your computer e.g- VIEW. Only one system may be operational at a given time

and, unless you tell it otherwise, the MOS will automatically select the BASIC

language system for you when the computer is switched on - hence the

appearance of the word BASIC in the screen display shown on page A3.

Thereafter, all input from the keyboard is collected by the MOS and passed to

the system you have chosen -- you have (perhaps without realising it) been

typing BASIC instructions in the previous section. Any messages you received,

such as Mistake or Missing " , were produced by the BASIC system to indicate

that it was unable to make sense of the line it received from the MOS. Needless

to say, it was the MOS which actually did the job of putting the characters on

the screen.

There are, however, occasions when it is neccessary to communicate directly

with the MOS, regardless of the system currently in use. Direct operating

system commands have an asterisk (*) as their first character and this

symbol is used to tell the MOS that it must deal with the remainder of the line

itself.

A 11