Commodore PET 2001 Series Reference manual

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Table of Contents
CONGRATULATIONS 1
UNPACKING YOUR PET AND TURNING IT ON 1
TOURING THE KEYBOARD 2
Figure 1. The keyboard 3
Figure 2. The characters without shifting 3
Figure 3. The characters with the SHIFT key in use 4
Exercise I - Testing the keyboard 4
Exercise 2 - Using the cursor (... and introduction
to screen editing) 6
Exercise 3 - Using graphics 7
Figure 4. Rocket drawing using the graphics keys 7
Figure 5. Graphics keys used to draw rocket 8
Exercise 4 - Creating a program 8
Exercise 5 - Listing and running your program 10
Exercise 6 - A mending the program 10
Exercise 7 - Screen editing 12
Exercise 8 - Using the reverse field 15
Exercise 9 - Programming cursor movement 16
EDITING; A REVIEW 17
Direct Cursor Control 17
Programmed Cursor Control 19
Programmed Edit Functions 20
EXPLORING BASIC 21
Exercise 10 - Printing on the screen 22
Exercise I1 - Using the built-in clock 22
Exercise 12 - Setting the clock 23
Exercise 13 - Solving mathematical problems 23
Exercise 14 - Animating your PET 30
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USING YOUR CASSETTE TO SAVE A PROGRAM
31
USING YOUR CASSETTE TO LOAD A PROGRAM 33
APPENDIX
I . Error messages 36
2.
Basic commands 40
String functions 43
Arithmetic functions 44
Special symbols, commands and statements 45
I/O Commands 46
3.
Special keys 46
4.
Cleaning Your PET 47
5.
Cleaning and demagnetizing your tape deck head 48
Figure 6. Cassette deck head area 49
6.
References 49
Congratulations ...
We congratulate you on your purchase of a Commodore PET com-
puter and welcome you to the growing legion of PET users.
The number of applications for the PET is as great as your imagina-
tion and the number of programs available.
We shall attempt, in this modest booklet, to introduce you to the art
of doing your own programming and to the PET method of using
prepared programs.
We do recommend you to the brief bibliography in this booklet's
appendix. Enjoy learning about your PET and the world of com-
puters as much as this booklet's authors did.
By the time you're through with this booklet you should be on your way
to enjoying and using your PET to its fullest.
Unpacking your PET
and turning it on
Please check the carton for any special unpacking instructions.
And examine carefully
,
your P17 for any concealed damage. If there is
any, report this IMMEDIATELY to both ('commodore (or your
dealer) and to the shipping agent.
Remove your PET from its protective shipping carton, and place it on
the counter, desk or other suitable surface, then plug it into any
standard, grounded electrical outlet.
Push the rocker switch, in the lower left rear where the line cord
enters the unit, to the power-on position.
Momentarily you may be able to see that the PET television display
contains a collection of random letters and symbols. This is normal. On
"Power up," PET's electrical circuits have to wake up slowly before they
can function and clear the screen.
Almost as fast as you can blink an eye, the screen will clear and one of
the two messages below will be printed out in white letters on the black
screen. The second line of the message will vary depending on
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which memory option of PET you are using. It tells you how much
memory space is free for you to use.
4k power-on display 8k power
-
-on display
(A byte is the fundamental data element of the PET computer
and corresponds roughly to one letter or digit of information.
For the curious: the 4k model should show - in theory
-
''4096 bytes" and the 8k, "8192 bytes. "But a few hundred
are used' by the PET internally. The balance shown "3071
and "7167" are net available bytes.)
If you fail to get the power-up display the first time, try turning the
power switch slowly off then back on. Rarely will PET fail to respond
to this, but if it does. turn to the Hints if You Have a Problem section
at the back of this pamphlet.
Most noticeable in the display is a flashing white square called a
cursor. Whenever PET is waiting for some keyboard information,
the cursor will begin blinking and this is where the next charac-
ter will appear if it is typed in.
Touring the keyboard
But, before you can speak to your PET, we need to take a brief
tour of the keyboard.
Each key has a thin, transparent plastic film covering the keytop
which should be removed. This protection was left in place to
protect the keys against scratches during shipping. To remove the
film, carefully peel off with your fingernail.
Key top legends bear much resemblance to those of a standard
typewriter keyboard, but there are a few differences.
The letters are all in virtually the same place as on a standard
typewriter keyboard, but, for your convenience in numerical
computations, the numbers are separate and laid out very much
like a calculator keyboard. (See Figure 1.)
Figure 1. The keyboard
On most typewriters, if you strike a letter key, without shifting,
you will get a lower case letter. On your PET, if you press a letter key
without shifting, you will get the capital letter. (See
Figure 2.)
Figure 2. The characters without shifting
If, on the other hand, you simultaneously press the shift key and a
letter key, you will get the particular graphic that appears above the
letter: (See Figure 3.)
The graphics characters are a special set of symbols unique to PET.
They are used to draw pictures and lines on the screen and to perform
simple animation. The graphics can be printed on the screen just like
any other letter or digit.
For now, locate the L
key and press it a number of times to get a
row of characters-AAAAA -on the screen. (Do riot use the
2
key. If you did, you'd get L .)
I
SHIFT
3

key in use
Next, press the
I
key
labeled
on the keyboard illustration
(above). Type a different letter. Then press
see the character erase?
again. Did you
(P' to again. without shifting, you're getting DELETE. Shifting
would get you INSERT.)
Play a little game where you type in more letters and DELETE them
too.
Remember that no matter what key you press, there is no way to damage
the insides of your PET by normal keyboard operation. (Of course, PET
is not intended to survive hard falls or attacks with sharp objects
- but
with normal care it will give you years of service.) Do
not ever be afraid to
experiment.
Test out the keyboard by trying the following sequence of key-
strokes. Don't worry about making typing mistakes; you already
know how to correct them.
Exercise 1 - Testing the keyboard
the
key to erase the last character typed, as we explained
earlier. But what if you typed something wrong at the beginning of the
line? You could delete characters back to that point, then retype.
But there is an easier way: PET has a feature called screen edit
which allows you to move the cursor to any position on the line and at that
point either insert, delete, or retype.
(The movement of the cursor is non-destructive to the charac ters
over which it passes. The characters will not be deleted or changed
as you more the cursor around on the screen.)
Locate this row of keys on top of the right hand numeric keypad.
These are all double function keys, their action depending on
The RETURN] key is a special signal to PET that you have finished
typing a line and it should do something with it. This feature allows
you to edit the line and get it correctly typed before your PET can
act on it.
The important thing about this exercise is to get the following dis-
play on the screen after you've done:
whether or not a I SHIFT] key is used. Press the labeled
CLR
HOME
key top. See the cursor move to the top left corner of the screen'? This
is the "HOME" position.
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45
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6
7
The same key pressed while the key is presse d clears the
screen. Hold down
If there were any characters on the screen, then they were all erased ...
or "cleared."
The best exercise to learn the individual cursor movement keys is to
move the cursor right, down, left, and up in a sort of circle path to
return to the original starting position. You will move the cursor on
the screen in a path like this:
(If you press the last key too many times and wind up in position you
have discovered another feature called "wrap-around" which has
moved the cursor to the end of the previous line. Type
without the shift key held down to move the cursor
ways. Try moving the cursor around the screen between two arbitrary
points. Practice until you are confident you can put the cursor where
you want it on the screen.
Exercise 3 - Using graphics
If you have accomplished moving the cursor, then you can use your PET
like an electronic sketch pad. The characters on the upper half
of each keytop are called graphics. When you hold down the
key as you type, the graphics are printed instead of letters or
numbers.
Now let's draw a figure that should look very much like this by the time
we get through.
Figure 4. Rocket drawing using the graphics keys
Follow the instructions exactly as shown in the diagram that follows:
to clear the screen.
Move the cursor to the right 6 spaces: as shown in the diagram.
Press I SHIFT I and type the graphics. Now you use the cursor keys
to get the cursor in position to type the next line.
NOTE: Shaded keys are keys that must be accompanied by
pressing the I SHIFT I key.
this time and then press
Both functions of
thiskeyaffect the
screen.
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9
So let's turn each line of the picture into a program step and see
what happens.
The importance of a program to a computer can be likened to
the importance of a driver to a car. The car does nothing with-
out a driver and the computer does nothing without a program.
A program is stored as a list of steps or instructions in PET's
memory. Before we can create a program in its memory, we
should make PET forget about any previous program. This is
what we did when we typed the word NEW. Use the command
any time You want to enter a ,NEW program.
Now type
Ili
I
RETURN I
The number 1 tells PET "This is
.'VOTE: Do not press
Your PT,-Twill think you've,finished, it will not understand and
the first thing to do." The ? tells PET to print, and the quotes tell
PET to print a MESSAGE.
(If you make an error, do not try to correct it. Instead
press ( RETURN I, then move the cursor up and type the correct
number, the question mark, and the quotes. Then press
I at an t^ time in this exercise.
If this happens. first clear the screen again and start over.
When your rocket is complete, move the cursor to the left edge
of the screen at line 10.
Now type N E W
The Cursor should be in the left part of your screen when you've done
all the above.
Exercise 4 - Creating a program
When you have finished this exercise, you will have drawn a pic
ture on the screen. You probably went to a lot of work to create this
picture. You'd like to preserve it so you can view it again.
Notice that the only thing that changed has been the number (1 ,2,
3 and 4) that tells PET "this is the th thing to do." So now tell PET
the 5th, 6th. 7th, 8th, and 9th things to do, just like we've done with
the first four. Be sure to keep the numbers in the right
sequence.
Stop when you reach the line containing the word "NEW," because you
don't want that word included in your program. Using the
key, move the cursor down the screen until it is below
the word "READY." Would you believe you've just created a
computer program?

Exercise 5 - Listing and running your
program
Clear the screen and type:
L I S T
LIST is a command to your PET to print the lines of program stored in
memory onto the screen so that you can look at them. You should see
something like this on your screen
followed by
LIST your program again if you wish. When everything is just as you
want it to be type R U N
There! Your picture will appear on the screen. RUN tells PET to
execute the BASIC program you have entered, starting at the lowest
line number step and proceeding with subsequent steps in ascending
line number order.
Exercise 6-Amending the program
RUN your program again. If you did not clear the screen first, you may
have seen the old rocket disappear at the top of the screen and
the new
rocket roll up from the bottom of the screen.
This phenomenon is called "scrolling." When PET is printing in the
bottom-most line of the screen, everything moves up rather than the
cursor moving to a lower line. PET cannot scroll the other way,
however. Information that scrolls off the top of the screen is lost.
We can use this scrolling effect to our advantage to produce an ani-
mation in which it appears as though a stream of rockets are blasting
off from the bottom of the screen and are streaking off the top. To
do this we will learn a new BASIC language command.
Type this line in
The line number (100) was chosen so that it would be greater than any
you had used previously and thus would be the last step of your
program
to execute.
GOTO is a BASIC command to break the sequential execution of
statements and "go to" the line number specified. If you entered the
rocket picture with line numbers exactly as shown, line 1 is the first
line of the program which prints the rocket picture. Change the target
line of the "go to" to correspond to your first line number if it is not
line 1. The effect of line 100 is to repeatedly print the rocket and scroll it
off the screen.
But, because we don't want the rockets to be touching nose to tail, we'
d like to add some space between them. When we typed LIST, we
noticed that the last line number was 9. (We've since added line 100).
Any numbers greater than 9 and smaller than
100
will be positioned
correctly in line number sequence by your PET. So let's add the
statements:
The ? that you have typed in as a shorthand for PRINT has been
expanded out in the listing. Other than that, everything should be as
you typed it in. If there
is an extra line which should not be there, it
may be deleted by typing just the number of the line
1
1
1

Now type RUN. As soon as you press
your program.
PET will execute You can change a single character or you can add characters to lines you
already have. You can see exactly what you are changing because the
changes are visible as you enter them.
Let's try it.
(But before we start a new program, let's type ne w and press
I tits is important: it clears aft previous programs in
Rockets should be flashing on the screen so fast that it may be
difficult to see them. The speed at which characters are printed on
the screen can be controlled while the program is running by
pushing the key
screen. Now, release the key. Use of this key reduces the printing
speed to about 2 lines per second.
The program you have created contains what is called an "infinite
loop." Statement 100 does not contain a condition to stop
running
the program and cease printing the rocket, but
unconditionally goes to the start of the program over and over. It will
continue like this forever unless you pull the plug.
(Pulling the plug or shutting off the power-on switch not only
stops the program, it also destroys the program statements. You'
ve put in a lot of time typing theta and stay not want them
destroyed.)
do not press the SHIFT key. This will effectively "pull the plug on this
program, without losing the program statements.
PET will respond with something like:
BREAK IN LINE 8
This message means that execution of your program was stopped when
it reached line R (or
whatever line it was in your case) because
you pressed the
your PET and thus avoids any confusion.)
Type in: 10 PRINT "HELLO, HOW HOW ARE YOU?"
We have one too many HOWs in the line. Let's type LIST so we can
see the line on the screen.
This will move the cursor tip two lines from its lower position to the
first position of line 10.
is over the space after the W in either one of the HOWs.
You see how easy that was.
Do the same thing w i ll have eliminated the HOW:
RETURN
OFF
RVS
. Hold this key down while you watch the
This is a STOP function when you
key
At this point you may want to save your program. See page 31, "
Using your Cassette to Save a Program. "
Exercise 7 - Screen editing
One of the handiest features of PET is the ability to modify easily
the program you have entered, as we have just seen.
12
13

Now let's insert the missing HOW.
You'll read:
Now position the cursor over the A in ARE by pressing
several times.
then
You
With editing that easy you need have no fear of making typing
errors. Agreed?
Let's try another interesting screen edit. First, type NEW to clear
out the old program.
Enter:
1 0 P R I N T "ANYTHING"
(This time we won't type LIST each time we make a correction.)
Interesting? Think of the applications. If you want to repeat a com-
plex statement several tines in the same program ... or if you want to
change just a part of a statement on one line and enter that
amended statement on another line.
Exercise 8 - Using the reverse field
Every key on the keyboard, with the exception of a few which we
shall note, prints almost exactly what you see onto the screen. We
say "almost" because the screen displays characters in white
Your PET displays 128 unique symbols which, with the addition of
reverse field, really adds up to a total of 256 different characters
that can be displayed.
Reverse field remains in effect until a) you type RETURN or b)
you
to get past the READY in the display. Type LIST.
on a black background. There is a key which, when pressed.
causes all subsequent characters to be displayed in reverse
field - black on white - on that line.
Type A B C
A B C and you'll see:
Press
in line # 10.
and so that you position the cursor over 1
Press 2 then press
Now type EVERYTHING" and press
until it reaches the A in ANYTHING"
As an example, type:
Now press
together so that you position the
cursor over the 2 in line #20. Press 3 then
over to the
E in EVERYTHING and type NOTHING". Press SPACE three times. (
Because EVERYTHING is three letters longer than NOTHING.)
Press
Now type LIST and press
14
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5

Exercise 9 - Programming cursor movement
Cursor control characters may be programmed into PRINT state-
ments. It is often desirable to clear the PET display under program
control. We will do it in a direct statement.
Note that you did not clear the screen by typing these keys, but that
the sc reen
When you have typed an odds number of quote marks you are in this
special cursor control character insertion mode.
( •• I Represents a single quote mark, for this discussion. And one is
an odd number.)
is a representation of a CLEAR SCREEN control character.
Do not type RETURN yet. Instead type
Editing: a Review
When you press one of the PET's cursor control keys, you may be in one
of two editing modes, as you have already
seen.
I. DIRECT CURSOR CONTROL
The cursor is moved as soon as you press the cursor control key.
In DIRECT mode, the User is creating program code. The cursor
control keys allow the User to insert or delete characters at will
unless he specifically indicates (by typing a quotation mark) that
the cursor movement is to be a part of the created code.
When entering program code, the User can correct typographical
errors in one of four ways.
A. Delete all characters back to the error, then retype.
B. If no quotation marks have been used, backspace (cursor left) over
the intervening characters until the cursor is positioned over
the
error, retype the character, then forward space (cursor right) to
the next desired character position to be typed.
Tilt
C. If a quotation mark has been used, press
program line. Then move the cursor up
to leave the
and over to one space
These prin t out
which are cursor co ntrol characters for CURSOR RIGHT and
CURSOR DOWN.
If you now type a second
of quote marks and you will leave the special mode. Typing
I
will again move the cursor, but this time. without printing anything.
Any time you want to enter or leave the control character insertion
mode you may do one of two things:
then forward space to the next desired character position to be
typed. Programmed cursor control is no longer in effect.
D. Another method is to close the quotes (type the ending quotation
mark) then backspace to the offending character and retype.
Again, programmed cursor control is no longer in effect.
There may be occasions when it is appropriate to lengthen a state-
ment line. If the cursor is moved to the end of an existing line, the
additional characters may be typed in. The cursor will wrap around to
the next lower line if more than 40 positions are used. If the
lower line
contains a program statement, it can be over-typed. Extra characters
remaining from that previously typed line must be deleted or they will
be incorporated into the line being edited.
Original Program
you will have entered an even number
key. Then use
your cursor keys
to return to
the point on
the preceding
line ... or
2. Delete the first
quotation
1. Enter a second
Q . press
the
1
6
Move the cursor until it is, positioned over the closing quotes in
statement 10, and type GOOD MEN TO".
17

the I INSERT function to open up spaces into which he can then type
If you wish to insert characters within a statement line, position the
cursor over the first character to be shifted to the right.
press
greater than 40 spaces, a space will open up between the line being
edited and the next program line, and the characters to the right of
the insertion will move into the opened space. This is difficult to
show on paper, so just follow the instructions and watch the
result on your screen.
I . Type this program
10 PRINT "NOW IS THE TIME TO"
20 PRINT "THE END"
2. List the program
3. Move the cursor to the letter T in the word TO in statement 10.
key 17 times
5. Type FOR ALL GOOD MEN T
6. Press
7. LIST the program again
Using Direct Cursor Control while coding a string literal:
To edit a string literal, such as a print message or a data statement, the
user must press the RETURN key and leave the statement line. A
literal cannot be edited (except for character deletion and retyping)
while it is being originated, because all cursor controls
except delete
and insert are programmable. The user must leave the statement line via
a carriage return, then move the cursor back to the offending
character and retype. Furthermore, to program cursor controls within
the string after having left the line, the user must use
the appropriate control character.
The user can, of course, close the quotes, and thereby signal PET
that he is through with the literal message. However, once the second
quote mark has been typed, PET will no longer recognize cursor
movement as a part of created code, and the cursor will move accord-
ing to the Function represented by the key pressed.
II. PROGRAMMED CURSOR CONTROL
The cursor movement is executed during a program run. It is
part of a PRINT statement and has been enclosed within
quota tion marks.
Function
CURSOR UP CURSOR
DOWN CURSOR LEFT
CURSOR RIGHT CLEAR
SCREEN HOME
CURSOR INSERT
CHARACTER*
DELETE CHARACTER*
REVERSE FIELD
RESET REVERSE'
with the
key. If the new spaces increase line length to
4. Hold the
key and press the
(Here's where the screen will show a space being opened between
statement lines)
18
*The INSERT and DELETE functions are not programmable. Use
CHRS (20) to delete during program run and CHRS (148) to
insert during program run.
PET uses the quotation mark to signal the beginning of a string literal,
as in a DATA or PRINT statement. When attempting to edit a program
line, the User should be aware that if PET sees an opening quote, it
will consider all cursor movement instructions as part of the
string.
19

13
23
The first BASIC command we shall explore will tell your PET to
PRINT something on the screen. This is one of the more useful
commands, for with it you can make your PET display data, draw
pictures, or play games.
Now enter the following by pressing this sequence of keys. (We'll
call it "typing" from this point on.)
Exercise 10- Printing on the screen
Did your PET print HELLO PET on the screen? If it did not, then try it
again. PRINT is a command which tells your PET what to do
with the
rest of the line. This example has a message between quotes. The
quotes tell your PET to print out the message exactly as it appears
within the quotes without any further processing.
Exercise 11 - Using the built-in clock
Now, let us speak to your PET in BASIC and get it to tell you what time
it is. Your PET has a built-in clock that starts from 0 the
moment
you turn on the computer.
To discover the elapsed time, type:
The ? is a shorthand which you may use instead of always typing
PRINT when you want your PET to print something. The $ at the
end of the word TIME tells PET to print the time in hours/minutes/
seconds. Though the elapsed time may he different, you should See
The first two digits .ire elapsed hours, the second two digits arc elapsed
minutes, and the last two digits are elapsed seconds -
which, in the above
example, means that PET has been running for 00 hours,
1 1 minutes,
and 30 seconds. The time you see on your PET, however,
will depend
entirely on how long you have had it on thus far. PET's
clock is crystal-
controlled and very accurate. It is also a 24 hour
clock which means it
will count up to 23:59:59 then roll over to
00:00:00.
Exercise 12 - Setting the clock
It is very easy to set your PET clock. Assume it will be 12:30 p.m.. in a
few seconds. Press the following sequence of keys:
T I M E$=" 1 2 3 0 0 0"
When the designated time ( 12:30 p.m..) comes up on your watch,
and PET will set the time.
Substitute your current local time and try setting the clock as in the
previous example. (If the time is 9:30, be sure to type "093000"
you need 6 digits.)
Now, whenever you type:
your PET will tell you the correct time. Remember that if you turn the
power off, the clock will stop running and you will have to reset it when
you turn the power on again. Once you have reset it, though, you have a
highly accurate built-in clock available at all times. Just
Exercise 13- Solving mathematical problems
BASIC is essentially an algebraic language which means that you
can use your PET much as you would a pocket calculator. Though it
pros'
type in ? T I M E S and there it is.
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14 25
packs the power of several programmable calculators put together,
it is as easy to use as a simple four-function calculator. Furthermore,
everything you type into it is instantly displayed on the screen, and
that makes it easier to keep track of what you are doing.
To perform arithmetic on your PET, simply tell it in BASIC to print
your answer. Note how much it looks like a direct question:
? 2 + 2
When you press the RETURN key. PET prints the result on the
screen.
PET arithmetic is not complex or mysterious. But, like human arith-
metic, it does have rules. One of the most important rules is
the order of operation:
I . exponentiation t (above RETURN key)
2. multiplication and division * /
3. addition and subtraction +-
In direct mode, type:
?9*3+7*6-5*4/3*2 PET
will respond: 100.
666667
You (because you're the one in charge here) decide how to group the
expression so the result will be correct. You do this with parentheses.
Type: ?(9*8+7)*6-5*4/3*2
PET says: 460.666667
Type: ?(9*8+7*6-5)*(4/3*2)
PET says: 290.666667
Type: ?((9*8+7*6-5*4)/3)*2
PET says: 62.6666667
Type: ?(9*8+7*6-5*(4/3)1*2
PET says: 214.666667
PET does all the work inside the parentheses first, before it does any-
thing outside of them. Once inside parentheses, PET does
multiplica tion and division, then addition and subtraction. After all
the work inside the parentheses is done, PET moves outside and does
everyt hing" there.
Let's look now at some of the arithmetic functions and find out how to
use them. First, type NEW, to delete the program we were using. Now,
type these program lines:
100 FOR I=1to10 120
PRINT I,I*I,SQR(I) 130
NEXT I
and RUN.
The result represents I (the count or index), I squared (the * means
multiply), and the square root of I. SQR(X ) is the Square Root func-
tion, and finds the square root of whatever number you put inside the
parentheses. Like all BASIC functions, SQR(X) requires an argument
any number you choose, enclosed in parentheses. Arithmetic expres sions
can be used if you like instead of numbers, like SQR(5*20)
All the arithmetic functions are used in a similar fashion. Turn to the
BASIC keywords (Appendix II) and find the section on Arithmetic
functions. You can check out the way each one works by substituting it
for one of the functions in line 1 2Q of the program. The brief
description of the function, along with actually trying it yourself, will
show you how to use it.
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Try using the string functions, too. You'll need a new program to do
that (be sure to type in NEW first):
NEW
10 FOR I = 1 TO 10
20 INPUT"ENTER A STRING";A$ 30
PRINT LEN (A$)
40 NEXT I
Lines 10 and 40 you already know about. In Line 30, the new item is
the dollar sign. This tells PET you are entering letters or graphics instead
of numbers. Line 30 prints the function we selected for this example.
Since you know your name best, use your name as the data you INPUT
in line 20. PET should print the number of letters and spaces in your
name when you RUN the program. PET does count spaces in strings-
even if that's almost the only place it wants them!
Now let's go a step beyond.
First type these lines. Note the comments in italics; they explain
each niece of the program lines.
You, as the programmer, can put any number you like in statements
1 10 and 120. You don't have to use 5 and 10.
It would be cumbersome to have to retype lines 1 10 and 130 every
time you wanted to change the numbers. So, PET's BASIC allows you to
change the numbers during program execution, using the
INPUT
keyword. Let's change the program to show you how to INPUT
data:
100 PRINT "ENTER A NUMBER"
110 INPUT X
115 PRINT "ANOTHER NUMBER"
120 INPUT Y
Since lines 130 and 140 stay the same, we won't retype them. Here,
line 100 prints a message. When you run the program, PET will print
the message so you'll know what you're supposed to do. By
using this
"prompt," anyone can use your program, because he'll
be told what to do and won't have to guess. Line 1 10 will force PET to
wait until you type in a number and press RETURN.
If you press RETURN without entering any number, PET will think
you don't want to continue running the program. It will jump out of
the program and tell you it's READY for whatever you want it to do
next. Line 115, like line 100, prints a message prompting you to enter
another number, and line 120 makes PET wait for you to do so.
Now RUN the program.
PET will show:
Line 1 10, 120 and 130 are called "assignment statements", because
they are used to assign values to letters of the alphabet. Line
140
is a PRINT statement and causes PET to print the value of the letter Z.
(PET knows the value of Z because statement 130 told it that Z is
the sum of X and Y.)
Of course, this is a trivial program. You've already learned how to type
`' 5 + 10 and have PET print the answer. But it gets more inter-
esting as we go along.
Now, if you RUN this program, PET will print:
Type a number-say 23 -and press RETURN. PET prints:
the cursor will flash here'.

Type another number--perhaps I " and press RI l URN. PET will print
the answer 35 and say READY. Your screen should look
like this:
numbers. But it you have 10 pairs of numbers, typing RUN each time
can STILL he tedious!
So, BASIC has two features which allows you to do an operation
(in
this case, add a pair of numbers) as many times as you like with
out
typing RUN. One of these features is the GOTO statement. It forces
PET to GO TO a line you specify. instead of doing whatever it
would normally do. (In this case, PET would normally stop and
print
READY when it finishes the sum.) You can do that with this
statement:
150 GOTO 100
Now PET will print the sum, then go hack to line 100 and print the "E
NTE R A NUMBER" message. Try it. Add up several pairs of numbers.
When you're ready to go on, just press RETURN without
having given
PI :T any data (numbers) and it will jump out of your
program and alt
patiently for you to tell it what to do next. (You'll
see PET's READY
message and the flashing cursor.)
The second feature is called a FOR-NE
X
T loop. It allows PET to
perform an operation (or a sequence of operations) FOR as many
times as you like. The word NEXT is the last line of the sequence,
and tells PET that it has completed all the repeatable operations.
Type these lines.
90 FOR I = 1 TO 10
150 NEXT I
In this example. I is called the "index." PET keeps track of the
number of times it performs the sequence of operations (lines 100
through 140) and keeps the count in the index. You set the index to l in
statement 90 and tell PITT to count to 10. Everytime PET
reaches
statement 150 it increments, or adds to, the count, and goes hack to
statement 90. Then PET checks to see if the number in the index is
greater than the number of repeats you wanted. If it is, then
PET looks
for something else to do. If the count is less than or equal
to the
number of repeats (in this case, 10), it performs the whole sequence
again.
In brief, PET will let you enter a pair of numbers FOR as many times
as you request. And PET will do as much as you ask to each
pair of
numbers before going after the NEXT pair.
29
Note that your PET automatically places the line numbers in ascending
order, too.
Using INPUT makes it easier to check the numbers, doesn't it'? Now
you just type RUN, and each time you do so. yon can use different
28
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Exercise 14 - Animating your PET
It's easy to move an object smoothly across the CRT, thanks to PET's
programmable cursor controls. The listings below give you the fun-
damental right-left-up-down motions. Later we will show you how to
program an unidentified flying object.
Move a ball right & left across the screen. (Note: all symbols are shown
as they appear on screen.)
Note: While "cursor up" character and "ball" character (the shifted "
Q" character) look alike, they obviously perform quite different
functions as you will perceive ... provided you press the right key.
70 NEXT
80 GOTO 20
And, of course, moving a ball diagonally across the screen (top left
to bottom right)
Using your cassette
to save a program
After the effort of entering and debugging your BASIC program,
you may want to save it on cassette tape for later use.
Place a blank cassette in the recorder unit, and press
"REW" to be
sure the tape is fully rewound. When rewinding is complete, press "
STOP" on the cassette unit. You are ready now to save your
program.
It's a good idea to save your program by name. Having completed the
rocketship drawing and converted it to a program, let's save it,
using the name "ROCKETSHIP."
Type SAVE "ROCKETSHIP" and pass RETURN . PET will display:
Press the correct buttons, and the display reads:
Now, when PET is ready, and the cursor flashes, you should rewind
the tare and verify that your program really did get saved. When the
tan, is fully rewound tune the word VERIFY and press
PET will tell you which cassette unit button to press. Do so, and
PET will assure you it is VERIFYING. Then, when PET is READY, you
now can be sure your tape has an accurate copy of your program.
30
'filter you type, in sequence, a q uotation mark and a cursor
key, you'll see a symbol appear to indicate the cursor movement
or action. This is a guide to these symbols:
31

If you see VERIFY ERROR, rewind your tape and try it again. If you still
get VERIFY ERROR, save your program and start over. (Note: be sure
to use music-quality low noise cassette tapes.)
You can use any name you like for your program. Because your PET
will only display 16 of the characters used in a program name, you'll
probably want to keep the names short.
When you name your programs, remember that PET doesn't need to
have the whole name typed on the keyboard in order to find it, when
you ask PET to load it. If you name your program "ROCKETSHIP", you
may ask PET to LOAD
-
ROCK" and it will find the right pro gram and load
it. This means you must he careful to avoid naming one program "ROCK"
and another program on the same tape "ROCKET". PET won't know
which is which, and will load the first program with the letters ROCK in
its name.
You do not need to incorporate the program name into the program
itself. PET saves the program name in a file header when it saves the
program, and that the only identification it looks for.
This means you can save your program in small pieces as you write it.
You may find it worthwhile to SAVE your work every 20 to 30 lines
or so, to avoid losing it in event of a power failure. But, since you may
not want to type the full program name each time, PET is designed not
to REQUIRE: a name:
even a third SAVE may prevent loss of your program and will prove to he
worth the extra few minutes it takes to save and verify the
extra
copies.
Using your cassette
to load a program
The built-in cassette drive in your PET computer is your easy access
to a library of BASIC programs, either created by you or purchased
from the extensive COMMODORE library.
PET is like a pocket calculator in that it forgets everything when you
turn the power off. (Remember what happens to time) That is why
PET has a built-in cassette drive. Programs can be saved on tape
before power is turned off. They may he restored to PET's memory
when power is turned on again.
Take a cassette, open the cover, and place the cassette in just as
you
would a normal audio cassette. Do not push any cassette keys at
[Ii is time.
Type SAVE and press
. PET will display
Now, hold down the
key. If you see:
key and touch and release the
and_ when you do so. PET will show
And as soon as the program is saved, PET will add the word READY
to the display.
You may want to save your program more than once on the same tape
to be sure you have a good
copy. As cassette tapes age, they
frequently stretch or wrinkle, especially at the ends. A second, or
32
33
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34 35
I f you've done all
this correctly you should
see:
Pressing the and the
keys caused the command LOAD
to be typed on the screen and PET responded by asking you to
operate the cassette.
(If you wish, you can also tell your PET to load the program by
Disclaimer on Software:
The complex and extensive software of the PET computer
has been thoroughly tested and is believed to be quite reliable.
However, no responsibility is assumed by Commodore or your
sales agent for inaccuracies.
typing in LOAD and [ instead of the above procedure.)
NO
This means your pressing of the key is acknowledged and PET is now
searching for data on the tape. In a few seconds (about 5-10) you will
see:
PET has found a program on the tape and is transferring it from the
cassette into its memory. This is the operation referred to as "loading.'
When loading is complete, the program will automatically begin
executing. Also, the cassette motor will be turned off.
(We're assuming that your PET's program is labeled "
PROGRAM" for the sake of this example. It could be
labeled virtually anything else.)
Press the key labeled "PLAY" on the cassette unit.
Your screen should now display:
Commodore Invites Y ou
To Submit Your Best Programs
Commodore intends to publish a library of programs
for your PET computer. This endeavor has already
begun.
We'd like to encourage PET users to submit original
programs to us. Those programs that are accepted will
earn a royalty for their developers.
If you're interested in writing programs for distribu-
tion, write to:
Software Manager
Commodore Business Machines
901 California Avenue Palo
Alto, CA 94304
We'll be glad to send you suggestions on programming
techniques, procedures for submitting programs, and
some guidance in selecting programs.
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Free for personal use but you must have written permission to reproduce

36
Appendix
1. Error messages
When an error occurs. PET returns to Command level and displays
READY on its TV screen. Variable values and the program text remain
intact, but the program cannot he continued using the CONT command.
GOSUB and all FOR ... NEXT context is lost, insofar as the current run
is concerned.
When an error occurs in a program statement, the error message dis-
play will indicate the line number in which the error occurred.
When the error occurs in a direct, or command level, statement, no
Tine number is displayed with the error message.
Error Message What caused the error and how to fix it
CAN'T CONTINUE Attempt to continue a program when none
exists. an error occurred, after a new line was typed into the
program, or a correction was made to an existing line.
Correct the error, then use a directed GOTO to get back into
the program, or type RUN and start over.
DIVISION BY ZERO Dividing by zero is an error. Check the expression
used for the denominator in the offending arithmetic statement.
then correct it so it can never he evaluated as
0
.
ILLEGAL DIRECT Use of an INPUT, GET, or DEF statement as a
direct command.
Avoid using these statements as direct commands.
ILLEGAL QUANTITY The parameter passed to a math or string
function was out of range. "ILLEGAL QUANTITY" errors can
occur due to:
a. a negative matrix subscript, such as LET A (-I ) = 0* b.
an unreasonably large matrix subscript: X=A(65536)* c.
LOG-negative or zero argument, as LOG(-X)** d. SQR-
negative argument, as SQR(-4) e. At B if A is a negative
value and B is not an integer.
(It works if a constant is used instead of a variable, i.e.,
-4t B, because exponentiation is performed before unary
minus.)
f. A call to USR before the address of the machine language
subroutine has been patched in.
Error Message What caused the error and how to fix it
*Subscripts must be equal to or greater than 0, and less than or equal to 65535. **
Be sure the argument is within the range of the function being used.
NEXT WITHOUT FOR The variable in a NEXT statement corres-
ponds to no previously executed FOR statement.
The FOR part of a FOR ... NEXT loop must be inserted or the
offending NEXT part of the loop must he deleted. Be sure the index
variables are the sunk at both ends of the loop.
Example: FOR I = I TO 10
NEXT I
OUT OF DATA A READ statement was executed but all of the
DATA statements in the program have already been read. The
program tried to read too much data or insufficient data was
included in the program.
Use the RESTORE: statement to restore the data so PET can
read it again, or restrict the number of READs to the correct
number of DATA elements, or add more DATA elements, or
use a flag at end of data list check for it before reading.
OVERFLOW The result of a calculation was too large to he repre sented
in BASIC's number format. (If an underflow occurs, zero is given as
the result and execution continues without any error message being
printed.)
You requested a number greater than even PET can remember. Try
asking for a smaller number. The largest possible number is 1.
70141 183E+38. Change the order of your calculations.
REDIMENSIONED ARRAY After a matrix was dimensioned, another
dimension statement for the same matrix was encountered. This
error often occurs if a matrix has been given the default
dimension 10 because a statement like A( I)=3 is encountered and
then later in the program, a l)IM A( 100)
is
found.
37
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