a E i - Commodore Is Awesome

So if your screen is blank and
nothing is happening what should you
do?
On a Cl28 you could fi rst try
typing SLOW just in case the computer
has been put into fast mode. On
other computers, and the Cl28 if the
above fails, try changing the ink colour
to see if the cursor re-appears.
CONTROL and a number will do this.
If the cursor does re-appear then you
know that your program has finished
and printed some sort of message on
the screen, Find out where the colour
of the ink is changed to that of the
background within the program,
change it to something else and re-run
the program. You will now see the
message when the program stops.
Illegal What?
Quite often programs will be
published that contain a lot of DATA
statements, Somewhere within the
program this DATA will be READ
and the value then POKEd into
memory. Your Commodore uses this
type of program quite a lot as a way of
getting machine code into memory,
One of the most common problems
that people get with these is that, when
the program is RUN, the computer
responds with an ILLEGAL
QUANTITY ERROR in a certain line.
If you enter the following example I
will show you, as I promised, that the
computer is not always right:
10 FOR COUNT=0 TO 7
20 READ NUM
30 SUM = SUM + NUM
40 POKE 49152 + COUNT NUM
50 NEXT COUNT
60 READ CHECK
70 IF SUMCHECK THEN PRINT
"ERROR" STOP
80 PRINT "FINISHED"
100 DATA 102,101,103,104,105,106,
107,732
This sort of loader is used in Your
Commodore every month. Every line
has a checksum on the end, the total of
the number's that have been read being
worked out in line 30. This is then
compared with the checksum, READ
in line 60 and compared with SUM in
line 70, If they aren't the same then the
computer reports the error and
STOPs.
Let's RUN this program and see
what happens. You should get an error
message 'ILLEGAL QUANTITY
ERROR IN LINE 40', Your immedi-
PROGRAMMING
ate reaction is to check line 40. But
there's nothing wrong with this so
what is the problem?
An ILLEGAL QUANTITY
ERROR means that a number has
been used that is too large. So let's
have a look at line 40. In this line was
POKE 49152 + COUNT with the
number that has been READ from the
data statement. The maximum value
that 49152 + COUNT can reach is
49152 + 7 (see the For Next loop in the
first line) which is 49159. There is
nothing wrong with this so what about
the number that has been read? You
can only POKE a number up to 255
into memory'. What must have
happened is that you are trying to
POKE a number greater than 255 into
the computer's memory. If we tell the
computer to print this number out
when it stops with an error message
our fears will be confirmed, RUN the
program again and then type the
following line when the program stops:
PRINT NUM
The computer should respond with
732. Hence our illegal quantity error.
Yu can only POKE a number up to 255
into memory, remember? So what has
gone wrong? Let's follow the program
through.
Line 10 is a loop that repeats eight
times count going through
0,1,2,3,4,5,6 and 7.
Line 20 READS the next number from
the data statements.
Line 30 adds this number to the
current value of sum.
Line 40 POK Es the value that has been
read into memeory,
line 50 goes back round the loop eight
times,
Line 60 the checksum is read.
Line 70 the sum is compared with the
checksum and the program stops if
they are not the same.
Line 80 is the end of the program.
So what is our problem? Eight pieces
of DATA are read, the sum is then
read and the program should end. If
you take a close look at line 100 you
will see that there are only seven pieces
of data and the check sum. This means
that instead of the last number being
read and POK Ed into memory the
program READS the checks= and
tries to POKE this into memory giving
you your illegal quantity error. The
number 108 is missing from line MO. In
this ease the fact that a number was
missing from line 100 was easy to spot.
YOUR COMMODORE january 1987
110
After all we only have one line of
DATA in the program. How would
you cope if there were 600. 700 or more
lines of DATA? How could you find
the line where the DATA was missing?
System Variables
The designers of the Commodore
series of computers use certain areas of
the computers memory to hold
information about certain things in the
computer. The following memory
locations hold the line number of the
last DATA statement;
63 & 64 on the C64, C16 and Plus/4
65 & 66 on the C128
The line number of the last DATA
statement can be found by entering the
following line:
PRINT PEEK (63) + 256* PEEK(64)
or
PRINT PEEK (65) + 256* PEEK (66)
on the C128
Re-Run the above program, when the
computer stops with the error
message try the PEEK and you will
find that the number 100 is printed -
the line where the error is. So now you
can fi nd where in your program the
computer is READing DATA from
even if there are hundreds of lines.
Correct Version?
Let's correct the above program and
see if it works. Re-enter line 100 to
read:
100 DATA 102,101.103,104,105,106.
107,108,732
RUN the program again and all is well.
or is it? The computer comes up with
FINISHED and no error messages are
printed so everything shoudl be OK.
Well I'm afraid that it isn't.
Adding Up
The main failing of this type of
program is that the check on each line
is simply a sum of all of the numbers.
Therefore 3+2+1 would give you a
checksum of six. However 2+3+1
would also give you six even though
the numbers are not in the correct
order. If these numbers were part of a
machine code program, the fact that
they could be in the incorrect order
may cause the program not to work
correctly or even crash, The golden