Chapter 6
Introducing +3 BASIC
Subjects covered...
The editor
The edit menu
Renumbering a BASIC program
Swapping screens
Listing to the printer
Typing in a program
Moving the cursor
Running a program
Commands and instructions
Simple disk operations
Formatting a disk
Saving a program
Filenames
Disk catalog
Loading a program
Error reports
The +3 has an advanced editor to create, modify and run BASIC
programs. To enter the editor, select the option +3 BASIC from the
opening menu, using the cursor keys and ENTER. (If you don't know how
to select a menu option, refer back to chapter 2.)
The screen should now look like this...
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There are three things to notice about this screen.
Firstly, there is a flashing blue and white blob in the top left-hand
corner. This is called the cursor, and if you type any letters at the
keyboard, then they will appear on the screen at the position of the
cursor.
Secondly, there's a black bar towards the bottom of the screen. This
is called the footer bar, and tells you which part of the +3's
built-in software you're using. At the moment, it says +3 BASIC
because that's the name of the editor.
The last item of note at the moment is the small screen. This fits
between the footer bar and the bottom of the screen, and is currently
blank. It only has room for two lines of text, and is most often used
by the +3 when it detects an error and needs to print a report to say
so. It does have other uses, however, and these will be described
later.
Now press the EDIT key. You will notice two things happen - the cursor
vanishes, and a new menu appears. This is called the edit menu...
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The edit menu's options are selected in the same way as for the
opening menu (by using the cursor keys and ENTER).
Taking the options in turn...
+3 BASIC - This option simply cancels the edit menu and restores the
cursor. On the face of it - not very useful; however, if EDIT is
pressed accidentally, then this option allows you to return to your
program with no damage done.
Renumber - BASIC programs use line numbers to determine the order of
the instructions to be carried out. You enter these numbers (which can
be any whole-number from 1 to 9999) at the beginning of each program
line you type in. Selecting the Renumber option causes the BASIC
program's line numbers to start at line 10 and go up in steps of 10.
BASIC commands which include references to line numbers (such as GO
TO, GO SUB, LINE, RESTORE, RUN and LIST) also have these references
renumbered accordingly.
If for any reason it's not possible to renumber, perhaps because
there's no program in the +3, or because Renumber would generate line
numbers greater than 9999, then the +3 makes a low-pitched bleep and
the menu goes away.
A useful aid to this renumbering facility can be found in chapter 8
part 33.
Screen - This option moves the cursor into the smaller (bottom) part
of the screen, and allows BASIC to be entered and edited there. This
is most useful for working with graphics, as any editing in the bottom
screen does not disturb the top screen. To switch back to the top
screen (which you can do at any time whilst editing), select the edit
menu option Screen again.
Print - If a printer is connected, this option will print-out a
listing of the current program to it. When the listing has finished,
the menu will go away and the cursor will come back. If for some
reason the computer cannot print (e.g. the printer is not connected or
is off-line), then pressing the BREAK key twice will return you to the
editor.
Exit - This option returns you to the opening menu - the +3 retains
any program that you were working on in the memory. If you wish to go
back to the program again, select the option +3 BASIC from the opening
menu.
If you select the opening menu option 48 BASIC (or if you switch off
or reset the +3), then any program in the memory will be lost. (You
may, however, use the opening menu option Calculator without losing a
program in the memory.)
Reset the computer and select +3 BASIC. Now type in the line below. As
you type it in, the characters will appear on the screen (a character
is a letter, number, space, etc.). Note that to type in the equals
sign '=' you should hold down the SYMB SHIFT key, then press the L key
once. Try typing in the line now...
10 for f=1 to 100 step 10
...then press ENTER. Providing you have spelt everything correctly,
the +3 should have reprinted the line with the words FOR, TO and STEP
in capital letters, like this...
10 FOR f=1 TO 100 STEP 10
The +3 should have also emitted a short high-pitched bleep, and moved
the cursor to the start of the next line.
If the line remains in small letters and you hear a low-pitched bleep,
then this indicates that you have typed in something wrong. Note also
that the colour of the cursor changes to red when a mistake is
detected, and you must correct the line before it will be accepted by
the +3. To do this, use the cursor keys to move to the part of the
line that you wish to correct, then type in any characters you wish to
insert (or use the DELETE key to remove any characters you wish to get
rid of). When you have finally corrected the line, press ENTER.
Now type in the line below...
(The colon ':' is obtained by SYMB SHIFT and Z, and the minus sign '-'
is obtained by SYMB SHIFT and J.)
20 plot 0,0:draw f,175:plot 255,0:draw -f,175
...then press ENTER. On the screen you will see...
10 FOR f=1 TO 100 STEP 10
20 PLOT 0,0: DRAW f,175: PLOT
255,0: DRAW -f,175
Don't worry about line 20 spilling over onto the next line of the
screen - the computer will take care of this and align the text so
that it is easier to read. Unlike a typewriter, there's no need for
you to do anything when you approach the end of a screen line because
the +3 detects this automatically and moves the cursor to the
beginning of a new line.
The final line of this program to type in is...
30 next f
...again, press ENTER.
The numbers at the beginning of each line are called line numbers and
are used to identify each line. The line you just typed in is line 30,
and the cursor should be positioned just below it. As an exercise, we
will now edit line 10 (to change the number '100' to '255'). Press the
cursor up key (four times) until the cursor has moved up to line 10.
Now press the cursor right key until the cursor has moved to the right
of '100'. Press DELETE three times and you will see the '100'
disappear. Now type in '255' and press ENTER. Line 10 of the program
has now been edited...
10 FOR f=1 TO 255 STEP 10
The computer has opened up a new line in preparation for some new
text. Type...
run
Press ENTER and watch what happens. Firstly, the footer bar and the
program lines are cleared off the screen as the +3 BASIC editor
prepares to hand over control to the program you've just typed in.
Then the program starts, draws a pattern, and stops with the report...
0 OK, 30:1
Don't worry about what this report means.
Press ENTER. The screen will clear and the footer bar will come back,
as will the program listing. This takes about a second or so, during
which time the +3 won't be taking input from the keyboard, so don't
try and type anything while it's all happening.
You've just done most of the major operations necessary to program and
use a computer! First, you've given the +3 a list of instructions.
Instructions tell the +3 what to do (like the instruction '30 NEXT
f'). Instructions have a line number and are 'stored away' rather
than used immediately you type them in. Then you gave the +3 the
command run to execute the stored program.
Commands are just like instructions, only they don't have line numbers
and the +3 carries them out immediately (as soon as ENTER is pressed).
In general, any instruction can be used as a command, and vice versa -
it all depends on the circumstances. Every instruction or command must
have at least one keyword. Keywords make up the vocabulary of the
computer, and many of them require parameters. In the command 'DRAW
40,200' for example, 'DRAW' is the keyword, while '40' and '200' are
the parameters (telling the computer exactly where to do the drawing).
Everything the computer does in BASIC will follow these rules.
Now press EDIT and select the Screen option. The editor moves the
program down into the bottom screen, and gets rid of the footer bar.
You can only see line 10 of the program as the rest is 'hiding'
off-screen (you can prove this by moving the cursor up and down).
Press ENTER then type...
run
Press ENTER again, and the program will run exactly the same as
before. But this time, if you press ENTER afterwards, the screen
doesn't clear, and you can move up and down the program listing (using
the cursor keys) without disturbing the top screen. If you press EDIT
to get the edit menu, you might think that this would mess up the top
screen. However, the +3 remembers whatever's behind the edit menu and
restores it when the menu is removed.
To prove that the editor really is working in the bottom screen, press
ENTER and change line 10 to...
10 FOR f=1 TO 255 STEP 7
...by moving the cursor to the end of line 10 (just to the right of
'STEP 10'), then pressing DELETE twice, and typing '7' (press ENTER).
Now type...
go to 10
(Press ENTER.) The keywords 'go to' tell the +3 not to cleat the screen
before starting the program. The modified program draws a slightly
different pattern on top of the old one. You may continue editing the
program to add further patterns, if you wish.
A word of warning - while editing in the bottom screen, don't try to
edit instructions which are more than two screen lines long.
Otherwise, when the editor comes across an instruction which has its
beginning or its end off-screen, it may become 'confused'. (The same
is true of the top screen, but of course, this is unlikely to cause
any problems as the screen is so much larger.)
One thing you may notice while you're typing away is that CAPS SHIFT
and the number keys used together do strange things. CAPS SHIFT with
5, 6, 7 and 8 move the cursor about, CAPS SHIFT with 1 calls up the
edit menu, CAPS SHIFT with 0 deletes a character, CAPS SHIFT and 2 is
equivalent to CAPS LOCK, and finally CAPS SHIFT with 9 selects
graphics mode. All of these functions are available using the
dedicated keys on the +3, and so there is no reason why you should
ever want to use the above CAPS SHIFT and number key alternatives.
Simple disk operations
You have seen how to place a program into the computer's memory by
typing it in. This is all very well the first time you write a
particular program, but what about if you switch off the computer and
want to use the same program the next day? Surely you don't have to
type it all in again from scratch - the answer, of course, is no
[anyone with the slightest grasp of grammar will rightly disagree] -
the disk drive section of the +3 allows you to save a program from the
computer's memory onto a disk, and to load a program from a disk into
the computer's memory. This means that you can type in a program, save
it to disk, then happily switch off the +3 knowing that next time you
switch it on, you'll be able to load that same program back into the
memory.
The final part of this chapter, therefore, deals with these two very
important operations (saving and loading). However, before you can do
either of these, you will be shown how to prepare a brand new disk so
that it is ready for saving programs onto. This preparation process is
called formatting.
You will need to have a brand new blank disk to hand as you work
through this chapter. Whatever you do - don't use a disk with any
valuable software, games, etc. on it.
Disks and tapes
Even if you are familiar with saving and loading, it is worth pointing
out two important points which must be remembered when dealing with
disks.
Firstly, a brand new blank disk cannot be simply taken out of its
wrapper and recorded onto (as is the case with a tape) - instead, each
side of a disk must be formatted first. Note that the formatting
process completely erases that side of the disk.
Secondly, it is important that disk files are correctly 'named'.
Filenames on tape may vary greatly in length and may at times be
omitted. Not so with disks - disk filenames must conform to very
strict standards (and you will read about these shortly in the section
ahead called 'Filenames'.
Formatting a disk
Formatting can be likened to building a series of shelves and
pigeonholes on a disk [!? never tried that one myself] prior to the
storage of information on those shelves. In other words, formatting
lays down an organised framework around which data can be put in or
taken out.
The formatting divides the disk into 360 distinctly separate areas...
[Standard diagram of a disk. Track 39 is innermost, track 0 outermost.
Shows what a sector is, too.]
There are 40 concentric tracks from the outside of the disk (Track 0)
to the inside (Track 39), and each track is divided into 9 sectors.
Each portion of track in a sector can store up to 512 bytes of data,
hence the total available space on each side of a disk is 180
kilobytes (180K). Note that 7K of the 180K is reserved for the
computer's own use, this leaves 173K per side for your programs.
We will now format a new blank disk, and save the program below...
10 FOR f=1 TO 255 STEP 7
20 PLOT 0,0: DRAW f,175: PLOT 255,0: DRAW -f,175
30 NEXT f
...which should still be in the memory from the previous exercise
(check that the above program is currently in the memory by pressing
ENTER then typing...
list
(Press ENTER again.) If the program isn't in the memory (or you have
since switched off the +3), then switch it on, select +3 BASIC and
type in the above program).
Insert side 1 of a new blank disk into the disk drive and type...
format "a:"
(Press ENTER.) The read/write indicator lamp on the disk drive will
start to flash on and off. About 30 seconds later, you will see the
report...
0 OK, 0:1
You have now formatted side 1 of the disk. Once you have done this,
you should not need to format side 1 of that disk ever again.
(If you don't receive the above report (and some other message appears
instead), check the section entitled 'Error reports' at the end of
this chapter.)
Saving a program
Having formatted side 1, it is now ready for saving programs onto.
In order that each program file on a disk can be identified, you must
give the program a filename when you save it. For example, as the
program that you are about to save draws a patterned picture, save the
program using the name 'pattern.pic', i.e. type in...
save "pattern.pic"
(Press ENTER.) After a few seconds, you will see the report...
0 OK, 0:1
The program is now saved onto disk.
(If you don't receive the above report (and some other message appears
instead), check the section entitled 'Error reports' at the end of
this chapter.)
Filenames
Note that a filename on disk consists of two parts (fields). The first
field is obligatory and can contain up to 8 characters (letters and
numbers may be used but no spaces and punctuation marks). In the above
example filename, 'pattern' is the first field.
The second field is optional. You can use up to 3 characters (but
again no spaces or punctuation). In the above example filename, 'pic'
is the second field.
If you use two fields in a filename, they must be separated by a dot
(e.g. 'pattern.pic').
Disk catalog
A catalog of the disk (in alphabetical order) can be displayed by
typing in...
cat
(Press ENTER.) The filenames of all the programs on that side of the
disk will be displayed, together with each file's length (to the
nearest higher kilobyte). The amount of free space will also be
indicated.
PATTERN .PIC 1K
172K free
Loading a program
Imagine that you have switched off the +3 and later want to load the
program you have just saved. Do this now by resetting the +3 (using
the RESET button) and selecting the option +3 BASIC from the opening
menu. Type in...
load "pattern.pic"
(Press ENTER.) After a few seconds, you will see the report...
0 OK, 0:1
The program is now loaded from disk. Press ENTER and you will see the
program listing displayed.
(If you don't receive the above report (and some other message appears
instead), check the next section entitled 'Error reports'.)
Once loaded, you may run the program by simply typing...
run
...and pressing ENTER, as before.
Error reports
If you don't correctly carry out the instructions in this section, you
may receive various error reports. If so, identify the report (from
those shown below), read the explanation given, and then take the
necessary corrective action.
Drive not ready
The above report means that you have probably forgotten to insert a
disk into the disk drive. If there is a disk inserted in the disk
drive, then eject it, re-insert it and try again.
Disk is write protected
The above report means that you are trying to format, or save a
program to, a disk which has its write protect hole open. Eject the
disk, close its write protect hole, re-insert the disk and try again.
File not found
The above report means that you are trying to load a program which
doesn't exist on that side of the disk. Eject the disk, make sure that
the correct disk is inserted (the right way up) and try again. Take
care to ensure that you accurately type in the filename to load.
Bad filename
...or...
Invalid filename
The above reports mean that you are trying to load or save a program
using an illegal filename (or no filename at all). Read the section
entitled 'Filenames' earlier in this chapter, and try again.
Disk is already formatted
A to abandon, other key continue
The above report means that you are trying to format a disk that has
already been formatted. In general, a disk should need formatting only
once (at the beginning of its life). In rare cases, a disk may become
corrupted and there will be no alternative other than to format it
again. However, unless this is the case, you should always type A (to
abandon) when you see the above report.
NOTE - If you don't type A, then the formatting process will go ahead
and completely erase that side of the disk (as soon as you press a
key).
If you find that one particular disk (or side of a disk) keeps
requiring formatting, then it is likely that the disk itself is
damaged and you should avoid using it in future.
Some commands that fail produce reports that offer you the options...
- Retry, Ignore or Cancel?
If you receive the above options, then:
...typing R (after taking the necessary corrective action) makes the
computer retry the command;
...typing I makes the computer ignore the reason that the command
failed in the first place and continue regardlessly (typing I is
therefore not recommended unless you know exactly what you're doing);
...typing C abandons the command (this may be followed by the
appearance of another report).
Further information
Further information on disk operations (together with details of how
to use the +3's RAMdisk and how to use an external cassette unit) can
be found in chapter 8 part 20. A guide to +3DOS (the +3 Disk Operating
System) will be found in chapter 8 part 27.