Chapter 8
The +3 BASIC programmer's guide

Part 1
Introduction

Whether you read chapter 6 first, or came straight here, you should be
aware that...

	Commands are obeyed straight away.

	Instructions begin with a line number and are stored away for
	later use.

This guide to BASIC starts by repeating some of the information given
in chapter 6 (Introducing +3 BASIC), but in greater detail. You may
also find exercises at the end of some sections - don't ignore these,
as many of them illustrate points that are hinted at in the text. Look
through them, and do any that interest you or that seem to cover
ground that you don't understand properly.


The Keyboard

+-----------------------------------------------------------------------------+
|TRUE |INV. |     |     |     |     |     |     |     |     |     |     |     |
|VIDEO|VIDEO|    1|    2|    3|    4|    5|    6|    7|    8|    9|    0|SPACE|
|-----------------------------------------------------------------------------|
|       |     |     |     |     |     |     |     |     |     |     |     |   |
| DELETE|GRAPH|    Q|    W|    E|    R|    T|    Y|    U|    I|    O|    P|   |
|-------------------------------------------------------------------------+   |
| EXTEND  |     |     |     |     |     |     |     |     |     |     |       |
|  MODE   | EDIT|    A|    S|    D|    F|    G|    H|    J|    K|    L| ENTER |
|-----------------------------------------------------------------------------|
|            |CAPS |     |     |     |     |     |     |     |     |          |
| CAPS SHIFT | LOCK|    Z|    X|    C|    V|    B|    N|    M|    .|CAPS SHIFT|
|-----------------------------------------------------------------------------|
|SYMB |     |     | /-- | --\ |                       | ||  | /\  |     |SYMB |
|SHIFT|    ;|    "| \-- | --/ |                 SPACE | \/  | ||  |    ,|SHIFT|
+-----------------------------------------------------------------------------+


The characters used on the +3 comprise not only single symbols
(letters, digits, etc.) but also compound tokens (keywords, function
names, etc.). Everything must be typed in full, and in most cases it
doesn't matter whether capital letters (known as UPPER CASE) or small
letters (lower case) are used. There are three sorts of keys on the
keyboard: letter and number keys (called alphanumeric keys); symbol
keys (punctuation marks); and control keys (things like CAPS SHIFT,
DELETE and so on).

The most commonly used keys for BASIC are the alphanumeric keys. When
a letter key is pressed, a lower case letter will appear on the screen
with a flashing blue and white blob called the cursor. To get an upper
case letter, the CAPS SHIFT key should be held down while the letter
is typed.

If you wish to continuously type upper case letters, then pressing the
CAPS LOCK key once will make all subsequent letters typed upper case.
To return to lower case letters, simply press CAPS LOCK again.

To type the symbols which appear on the alphanumeric keys on the
keyboard, i.e....

	! @ # $ % & ' ( ) _ < > ^ - + = : <pound sign> ? / *

...simply hold down the SYMB SHIFT key while the alphanumeric key with
the required symbol on it is pressed (see the following diagram)...

+-----------------------------------------------------------------------------+
|     |     | !   | @   | #   | $   | %   | &   | '   | (   | )   | _   |     |
|     |     |    1|    2|    3|    4|    5|    6|    7|    8|    9|    0|     |
|-----------------------------------------------------------------------------|
|       |     |     |     |     | <   | >   |     |     |     |     |     |   |
|       |     |     |     |     |    R|    T|     |     |     |     |     |   |
|-------------------------------------------------------------------------+   |
|         |     |     |     |     |     |     | ^   | -   | +   | =   |       |
|         |     |     |     |     |     |     |    H|    J|    K|    L|       |
|-----------------------------------------------------------------------------|
|            |     | :   |pound| ?   | /   | *   |     |     |     |          |
|            |     |    Z|signX|    C|    V|    B|     |     |     |          |
=======-----------------------------------------------------------------=======
=SYMB==     |     |     |     |                       |     |     |     =SYMB==
=SHIFT=     |     |     |     |                       |     |     |     =SHIFT=
=======-----------------------------------------------------------------=======

                      Symbols available using SYMB SHIFT


Additionally, the symbols...

	 [ ] (C) ~ | \ { }

...can be obtained by first pressing the EXTEND MODE key once, then
holding down SYMB SHIFT while pressing the appropriate alphanumeric
key (see the following diagram)...

+-----------------------------------------------------------------------------+
|     |     |     |     |     |     |     |     |     |     |     |     |     |
|     |     |     |     |     |     |     |     |     |     |     |     |     |
|-----------------------------------------------------------------------------|
|       |     |     |     |     |     |     | [   | ]   |     |     |(C)  |   |
|       |     |     |     |     |     |     |    Y|    U|     |     |    P|   |
===========---------------------------------------------------------------+   |
= EXTEND ==     | ~   | |   | \   | {   | }   |     |     |     |     |       |
== MODE ===     |    A|    S|    D|    F|    G|     |     |     |     |       |
===========-------------------------------------------------------------------|
|            |     |     |     |     |     |     |     |     |     |          |
|            |     |     |     |     |     |     |     |     |     |          |
=======-----------------------------------------------------------------=======
=SYMB==     |     |     |     |                       |     |     |     =SYMB==
=SHIFT=     |     |     |     |                       |     |     |     =SHIFT=
=======-----------------------------------------------------------------=======

              Symbols available using SYMB SHIFT in EXTEND MODE


To enter graphics mode, the GRAPH key is pressed once. Mosaic graphics
(see the following diagram) can then be produced by pressing the
number keys (except 9 and 0). Pressing the letter keys (except T, U, V,
W, X, Y and Z) produce user-defined graphics (if set up).

+-----------------------------------------------------------------------------+
|     |     |..## |##.. |#### |.... |..## |##.. |#### |.... |graph|     |     |
|     |     |....1|....2|....3|..##4|..##5|..##6|..##7|....8|off 9|     |     |
|-----------------------------------------------------------------------------|
|       |     |     |     |     |     |     |     |     |     |     |     |   |
|       |GRAPH|     |     |     |     |     |     |     |     |     |     |   |
|-------------------------------------------------------------------------+   |
|         |     |     |     |     |     |     |     |     |     |     |       |
|         |     |     |     |     |     |     |     |     |     |     |       |
|-----------------------------------------------------------------------------|
|            |     |     |     |     |     |     |     |     |     |          |
|            |     |     |     |     |     |     |     |     |     |          |
|-----------------------------------------------------------------------------|
|     |     |     |     |     |                       |     |     |     |     |
|     |     |     |     |     |                       |     |     |     |     |
+-----------------------------------------------------------------------------+

                    Mosaic graphics available using GRAPH


To obtain inverted mosaic graphics, press the above number keys while
holding down CAPS SHIFT. [See (near the end of) chapter 7 for a
diagram showing them.]


General keyboard notes

If any key is held down for more than 2 or 3 seconds, it will start
repeating. As keys are pressed, a line will be built up on the screen.
A line, by the way, means a line of BASIC, and may easily be several
lines long on the screen. The cursor keys can be used to move about
the line, and if the part of the line that the cursor is moved to is
off screen, then the text on screen will scroll up or down to display
it. Any characters typed will be inserted at the cursor, and pressing
DELETE causes the character to the left of the cursor to be removed.
As soon as ENTER is pressed or any attempt is made to move the cursor
off the line, the +3 checks to see if the line makes sense. If it
does, then there is a high-pitched bleep, and the line is either acted
upon immediately or stored away as part of a program. If the line
contains an error, then the +3 generates a low-pitched bleep and moves
the cursor to the area where it thinks the error is (the colour of the
cursor also changes to red to indicate the error). It is impossible to
move off a line which contains an error [or even some which contain no
error - see imc's ROM bugs list] - the +3 will always move the cursor
back.


The monitor screen

This has 24 lines (each being 32 characters long) and is divided into
two parts. The larger (top) part of the screen is at most 22 lines and
displays either a listing or program output. It is the one used most
often for editing. When printing in the top part has reached its
bottom limit, the contents scroll up by one line. If, however,
scrolling would mean losing a line that you haven't yet had a chance
to see, then the +3 stops with the message...

	scroll?

Pressing any key (except N, BREAK or the space bar) will let scrolling
continue.

Pressing one of the keys N, BREAK or the space bar will make the
program stop with the report...

	D BREAK - CONT repeats

The smaller (bottom) part of the screen is used for editing short
programs, entering input data, entering direct commands (where the
main screen must not be used, e.g. graphics programs), and also for
displaying reports.


Program entry

If the program being entered gets bigger than the screen size, then
the +3 attempts to display the area of most interest (usually the last
line entered together with its surrounding lines). You may, however,
specify a different area of the program to be displayed using the
command...

	LIST xxx

...where xxx is a line number, telling the +3 to bring a specified
area of the program into view.

When a command is executed or a program is run, output is displayed in
the top part of the screen and remains there when the program
finishes (until a key is pressed). If the program is being edited in
the bottom part of the screen, then any output in the top screen will
stay there until it is either overwritten, scrolled off, or a CLS
command is issued. The bottom screen may display a report giving a
code (digit or letter) referred to in part 29 of this chapter. This
report remains in the bottom screen until a key is pressed.

While the +3 is running a BASIC program, the BREAK key is checked
every so often. This happens at the end of a statement, during use of
the cassette unit (if connected) or printer (if connected), or while
music is being played. If the +3 finds that the BREAK key is pressed,
then program execution stops and displays a report. The program may
then be edited.