Archimedes 400 Series BBC BASIC Guide

450 Pages·1988·21.09 MB·English

by  Acorn Computers Limited

The Archimedes Series
BBC BASIC GUIDE
Copyright © Acorn Computers Limited 1988
Neither the whole nor any part of the information contained in, nor the
product described in, this guide may be adapted or reproduced in any
material form except with the prior written approval of Acorn Computers
Limited.
The product described in this guide and products for use with it are subject to
continuous development and improvement. All information of a technical
nature and particulars of the product and its use (including the information
and particulars in this guide) are given by Acorn Computers Limited in good
faith. However, Acorn Computers Limited cannot accept any liability for any
loss or damage arising from the use of any information or particulars in this
guide.
All correspondence should be addressed to
Customer Support and Service,
Acorn Computers Limited,
Fulboum Road
Cherry Hinton
Cambridge CBl 4JN.
ACORN, ARCHIMEDES and ECONET are trademarks of Acorn Computers
Limited.
Within this publication, the term 'BBC' is used as an abbreviation for 'British
Broadcasting Corporation'.
First published 1988
Issue 1
Published by Acorn Computers Limited
ISBN 1 85250 052 2
Part number 0483,00 l
ii
Contents
About the BBC BASIC Guide
Command mode 3
Simple programming 7
Variables and expressions 19
Numeric variables 21
Bases 27
String variables 33
Arrays 41
Outputting text 51
Inputting data 57
Control statements 67
Procedures and functions 83
Data and command files 97
Screen modes 101
Simple graphics 111
Complex graphics 121
Graphic patterns 131
Viewports 141
Sprites 145
Teletext mode 149
Sound 155
Accessing memory locations 161
Contents iii
Error handling and debugging 165
Keywords 171
Keywords: Numerical 173
Keywords: Trigonometric 185
Keywords: Logical 197
Keywords: Program construction 203
Keywords: Program statements 221
Keywords: Variables 249
Keywords: Error handling 259
Keywords: Input/Output 267
Keywords: Character/String handling 283
Keywords: Sound 293
Keywords: File commands 301
Keywords: Assembly language 313
Keywords: Structures 331
Keywords: Graphics 347
YOU commands 373
The BASIC screen editor 383
Appendix A-Minimum abbreviations 401
Appendix B -Error messages 407
Appendix C-lnkey values 411
Appendix D- Plot codes 415
Appendix E -VDU commands 417
Appendix F -Operating system commands 421
Appendix G- *FX commands 423
Appendix H- BBC BASIC's history 427
Index 435
Contents iv
About the BBC BASIC Guide
BBC BASIC is onl! of thl! most popular and widely-used programming
languages. It consists of special keywords from which the programmer can
create sequences of instructions, or programs, to be carried out by the computer.
Such programs might perform calculations, create graphics on the screen,
manipulate data, or carry out virtually :my action involving the computer and the
devices connected ro it. Several examples of programs written in BBC BASIC
are provided on the Applications suite.
A brief history of BBC BASIC is provided in the Appendices.
The BASIC language operates within an environment provided by the
computer's operating system. The operating system is responsible for
controlling the devices available to the computer, such as the keyboard, the
:.creen, and the filing system. For example, it is the operating system which
reads each key you press and displays the appropriate character on the screen.
Operating system commands can be entered directly from within BASIC by
prefixing them with an asterisk(*).
The first chapters of this guide explain how to program in BASIC, and
introduce many of the commands provided by the language. A complete lbt of
the BASIC keywords, grouped by function, is given in the chapters headed
Keywords:. The last two chapters, and the appendices, list the features provided
by the operating system, and the commands available to control them. The
BASIC screen editor is also described.
About the BBC BASIC Guide
Conventions used in The following conventions arc applied throughout this guide:
this guide
• Specific keys to press are denoted as <Ctrl>, <Delete> anJ so on.
• Instructions which require you to pre~ a comhmation of keys are shown thus:
<Shift> <Break> means hold down the <Shift> key and press and release
the <Break> key.
• Text you type on the keyboard anJ text that is dtsplayed on the screen
appear~ as follows:
PRINT "Hello"
• Classe~ of item arc shown in italics: For example, tn the descriptions of
BASIC keywords, you might see something like:
LET var = expression
where var and expression would he replaced with actunl instances, eg:
LET a$="hello"
• Items within square brackets [] are optional. For example,
GCOL [expr,] expr
means that the first expr and the comma after it do not have to be included.
• After entering any text, press <Return> to tell the computer that you have
completed the line and that you want the computer to act upon it.
• Extra spaces are inserted into program listings to aid clarity, but need not
be typed in.
• Program ltstings are indented to illustrate the structure of the program:..
If at any time you wish to interrupt a program the computer is executing you
can do so safely by pressing <E~c> .
Do not he afraid to experiment. Try modifying the programs liMed in this
book and writing new ones of your own.
About the BBC BASIC Guide 2
Command mode
Entering BASIC To activate BASIC, display the Task Manager menu (click the centre mouse
button over the 'A' icon at the hottom righthand corner of the desktop), and
type
BASIC
in the New Task submenu box. Press <Return>, and a RASIC window will
open on the screen.
Alternatively, press <F12> from the desktop, then type BASIC from the
command line.
Infonnation about the *BASIC command may be found in the chapter
Keywords: Program construction.
You can configure your computer to enter BASIC automaucally when it is
switched on. To do this, type the command:
*configure language 4
then press <Ctrl> <Break> .
Leaving BASIC To leave BASIC, type *QUIT, the press <Return> twice to get back to the
desktop.
When you enter BASIC it •~ 10 command or interactive mode (sometimes this is
rermed immediate mode). Th1s means that you can type commands and the
computer responds straight away. For example, if you type
PRINT "Hello"
the computer displays the following on the screen:
Hello
Command mode 3
PRINT is an example of a keyword which the computer recognises. It instructs
the computer to display on the screen whatever follows the PRINT statement
enclosed in quotation marks. Keywords arc always written in upper case
letters (capitals).
If you make a mistake, the computer may not be able to make sense of what
you have typed. For example, if you type:
PRINT "Hello
the computer responds with the message:
Missing "
Thi~ is an error message. It indicates that the computer cannot obey your
command because it does not follow the rules of BASIC (in this case because
the computer could not find a second quotation mark).
If PRINT is followed by any series of characters enclosed in quotation marks,
then these characters arc displayed on the screen exactly as you typed them.
Thus:
PRINT "12 - 3"
produces the output:
12 - 3
PRINT, however, can abo be used to give the result of a calculation. For
example, typing
PRINT 12 - 3
produces the output:
9
In th1s case, because the sum was not enclosed in quotation marks, the
computer performed the calculation and displayed the result.
Similarly, multiplication and division can be performed using the symbols *
and /. For example:
PRINT 12 * 13
PRINT 111 I 11
Command mode 4
Some commands, although they have an effect on the computer, do not give
evidence that anything has changed. If, for example, you type
LET FRED = 12
nothing obvious happens. Nevertheless, the computer now knows about the
existence of a variable called FRED which has the value 12. A variable is a
name which can have different values assigned to it. It is described in more
detail later in this manual.
Now if you type
PRINT FRED / 3
the computer responds by displaying the number 4.
The program below illustrates how you can give commands to produce some
graphics on the screen:
MODE 12
CIRCLE FILL 600,500,100
The MODE command sets up the computer to produce high resolution graphics
(640 by 256 dots in 16 colours). It also clears the screen.
The CIRCLE FILL command tells the computer to draw a circle at a position
600 dots across from the left of the screen and 500 dots up from the bottom.
This is near the centre of the screen because the screen is 1280 units across and
I 024 units high. The third number tells the computer how big the circle should
be, in this case giving a radius of 100 dots.
>CIRCLE FILL 600,500.100
>_
Command mode 5