Mistake

or

No such variable

Ignore these messages for the time being and merely type the line in again;

their significance is explained in later chapters. One of the most likely mistakes

at this stage is to type the letter O instead of the number 0, which are denoted

by 0 and Ø respectively. If things appear to have gone irretrievably wrong, try

pressing ESCAPE and, if that has no effect, press BREAK.

The computer is able to display output on the screen in a variety of different

modes, each of which has its own characteristics, in terms of the number and

length of its lines of text, the size and shape of the characters displayed and its

ability to present graphics (points, lines and areas of colour). Each screen

mode is identified by a number, which may be in the range 0 -- 7 or 128 -- 135.

These two sets of modes are identical in terms of what is actually displayed on

the screen; they differ only in the size and location of the area of memory set

aside for storing the current content of the screen. Modes 0 -- 7 are identical to

the eight modes available on the BBC Model B microcomputer; modes 128 --

135 are referred to as the shadow screen modes (identical to those available on

other BBC Master Series microcomputers) which provide the maximum

amount of user memory for a given type of display. We shall use modes 128 --

135 in all the examples in this guide.

You have a means of instructing the computer to start up in any of the

available modes (see page B10) but the standard setting is mode 7, which

provides:

-- 25 lines of text, each 40 characters in length;

-- the teletext character set (see below);

-- limited graphics in the form of small blocks of colour.

The > symbol immediately to the left of the flashing cursor is an example of a

prompt and its appearance indicates that the computer is waiting for you to

type something. Try typing these lines to see the effect; in each case the

computer will respond by displaying the characters inside the quotation marks:

PRINT"White on black'' RETURN

PRINT''SHIFT+f0 Red on black" RETURN

In mode 7, SHIFT+ f1 and SHIFT+ f2 etc. generate what are known as

teletext control codes which affect the way in which the remaining

characters on a particular line are displayed. Examples of this type of screen

display can be seen on pages from either the BBCs CEEFAX or the IBA's

Oracle services and further information is provided In the section of this

manual on the BASIC language.

A 9