Ahoy 1987 February/Ahoy_Magazine_87-02_1987_Double_L_Side_B.d64/comal article

Text File | 1987-01-01 | 8.7 KB | 322 lines

COMAL AND YOU - For Beginners Only
by David Stidolph

WHY LEARN TO PROGRAM?

The many commercial computer programs
available today are more than
sufficient for most people's needs.
If word processors, spread sheets,
and data base managers are all your
computer is used for, learning to
program would be a waste of time.

For some, however, a goal is being
able to learn something new...
something that can control that
mysterious box called a computer.
There's nothing like the feeling of
accomplishment when you type in your
first working program. Also,
familiarity with computers and
programming helps ease the sense of
helplessness most people get when
dealing with computers. (How did you
feel the first time you got a
computerized bill?) This article is
for people who have made the decision
to not only learn about computers,
but to learn programming as well.

WHAT IS A COMPUTER LANGUAGE?

Computers work with a language of
zeros and ones called machine code.
This is as difficult to use as it
sounds, and few programmers now work
directly in machine code. They choose
instead to use computer "languages".
These range from low level languages
like assembly code, where the words
merely represent individual machine
code instructions, to high level
languages which look more like
English. Compare the following two
programs:

ASSEMBLY CODE
-------------

       * =  START
       LDY  #$00
LP     LDA  STRING,Y
       BEQ  ENDLP
       JSR  OUTPUT
       INY
       BNE  LP
       BEQ  ENDLP
       .BYT 'This is a string',13,0
ENDLP  <...>  ; rest of program code

COMAL
-----

PRINT "This is a string"

As you can see, the statement written
in COMAL is shorter and much more
readable. Although the computer will
seem to understand COMAL statements
and programs, the computer itself
only understands machine code. COMAL,
the language, is a machine code
program. It must be loaded into your
computer before you can write, edit,
or run COMAL programs. Think of the
language as a translater between you
and the computer. This means you
don't have to learn machine code -
you only have to make sure that COMAL
is in the machine before you can run
your COMAL programs.

WHY LEARN COMAL?

Since BASIC comes with most personal
computers today, many people think
that it is the best computer language
to learn. Not so. BASIC is
implemented on so many computers
because it is the easiest language to
write. It has the fewest commands,
and NO definite standard to follow.
This means that a BASIC program
written on one computer may NOT run
on another computer. BASIC does,
however, have one good feature; it
will let you type in a short program
and see it execute as soon as you
type the word RUN. Text editors or
elaborate compiler commands are not
necessary (Most other high level
languages like Pascal, FORTRAN and
COBOL require them). This makes BASIC
seem like a easy-to-learn language
for everyone.

COMAL started with this idea of
interactive work with the programmer,
then added to it. COMAL has
structures such as WHILE, REPEAT, and
FOR loops, multi-line IF-THEN-ELSE
statements, a CASE statement
(similiar to a multiple choice
question), and named PROCedures and
FUNCtions complete with parameters.
These structures are similiar those
in "professional" languages like
Pascal. COMAL also has the turtle
graphics made famous by Logo. COMAL
is now the language taught in the
schools of 5 European countries.
COMAL is easier to learn than BASIC,
and teaches the idea of structured
programing necessary with modern
computer languages.

HOW DO I GET COMAL?

Congratulations, COMAL is included on
this Ahoy disk. To use COMAL, follow
these steps:

1. Type LOAD ":*",8
   Type RUN
   This puts you in Ahoy's main menu.
2. Choose the "COMAL 64 BOOT" option.
   This will give you COMAL's
   introductory screen. You will be
   given some COMAL information and
   then asked if you want error
   messages in memory. Hit the return
   key for the default answer of yes.
3. After a short wait, you will be
   put in a COMAL program displaying
   another menu. This will allow you
   to run the COMAL programs on this
   disk, or to read articles such as
   the one you are reading now. If
   you want to enter the COMAL editor
   to write your own programs, simply
   enter the "QUIT THIS MENU" option.
4. If you are already in COMAL, but
   want to re-enter the menu from
   step 3, all you need to do is type
   in:

   CHAIN "hi"

   assuming the Ahoy disk is in the
   disk drive.

NOTE: COMAL and the COMAL programs on
this disk are copyrighted, but we
give you permission to give copies
away. This permission does not apply
to the BASIC programs from Ahoy.

WHAT DO I DO WITH COMAL?

Since COMAL is on this Ahoy disk, you
can learn to write readable programs.
One way to learn is to first look at
other peoples' work. I will detail
certain commands now so that you can
do just that. The commands will be
listed in UPPERCASE, but type them in
with unshifted letters.

CAT

This command will show you what files
are on the disk in the disk drive.
The disk drive sends the disk
DIRECTORY to the computer, and COMAL
prints it on the screen. The actual
listing shows more than just file
names. It shows how big they are,
their names, and the file type. Each
entry in the directory is called a
file, and there are four types of
files - PRG (program), SEQ
(sequential - data files), REL
(random - also data files), and USR
(special files). Unlike the BASIC
commands:

  LOAD "$",8
  LIST

COMAL will not erase the program in
memory while showing a directory of a
disk. You can slow the scrolling
lines by holding down the CTRL key on
the upper left hand side of the
keyboard, or stop it by pressing the
RUN/STOP key (right below the CTRL
key). If you happen to have a dual
drive (a two drive unit) you can add
a '0' or a '1' after the command:

  cat 0    (This is for drive 0)
  cat 1    (This is for drive 1)

LOAD

Once you know what is on a disk, you
can load COMAL programs into memory
with this command. It is similar to
the BASIC LOAD command, except you no
longer need to type the comma 8. The
following is an example of loading a
program called "filename" from the
disk drive:

  load "filename"

Only PRG type files can be loaded. Be
careful, because other languages,
like BASIC, also store their programs
as PRG files. COMAL 0.14 will attempt
to load any PRG type file you ask it
to. If you are not sure whether or
not a program was written in COMAL
0.14, load the program and LIST it.
Only COMAL 0.14 programs can be
listed, any other type of program
(BASIC, COMAL 2.0, etc) will not
list. DO NOT RUN PROGRAMS WHICH WON'T
LIST. If you do, COMAL will become
confused and stop functioning. The
only thing to do after this has
happened is to turn the computer off
and reload COMAL.

LIST

Once a COMAL program is in memory,
you will want to be able to see it.
The command LIST will do just that,
it will list the program to the
screen. The first thing you will
notice is you will want to slow or
stop the listing (so you can study
it). Just as with the catalog
command, you can use the CTRL key to
slow the listing, or the RUN/STOP key
to stop the listing. When LISTing a
PROGRAM, the space bar will pause the
listing.

You will notice that each line has a
number in front of it. COMAL uses
them to keep track of the order of
the program lines. The order goes
from low (1) to high (9999). You can
use any line number between.

The LIST command can also be used to
show just part of a program. The
following are some examples to do
just that:

 list         (all lines)
 list 100-500 (from line 100 to 500)
 list 100-    (from line 100 to end)
 list -500    (from beginning to 500)

RUN

When the program you want has been
loaded into memory, you start the
program with the command RUN. The
computer does a quick scan of the
program to make sure it seems
correct, and starts executing with
the first line of the program. If an
error occurs while the program is
running, the program will stop
executing. COMAL will print what the
problem is and the line number it
occured on.

MAKING ERRORS

There is a very good chance that you
will make typing errors while trying
these commands. COMAL checks each
line you type for errors, and if it
cannot understand what you typed, it
will stop and give you an error
message. It might go out to the disk
drive and get the error message, or
it might just print the message
itself (that depends on your choice
to have error messages in memory or
not). If you get an error, COMAL will
put the cursor on the part of the
line it is having trouble with so
DON'T PANIC. Just make the correction
and press the RETURN key again.

STATUS

If the red light on the disk drive
starts blinking on and off while
COMAL just sits there waiting for you
to type something, try typing in the
STATUS command. This will print disk
error message to the screen. Check
your disk drive manual for more
information if necessary.