Editor's note: The information contained in this article may be outdated and/or inaccurate.
Well, here I am again. Every time I look at my calculator then look at my
computer, I wonder, "Phil, exactly WHY are you making this column?" Then I remember
how much fun it is to convert BASIC programs into ASM, then see how much smaller I
can make them, or how fast they can run (trust me, it's fun).
When we last left off, I had just told you how to make a very simple program and
assemble it, right? Now that Ive told you how to cut and paste your way into making a
program, I believe that it will benefit you to know what some of the simple commands do.
BASIC COMMANDS AND VARIABLES
I believe that in the previous log, I told you that the following lines:
#define equ .equ
#define EQU .equ
#define end .end
were called the head of the program. Just to review, these commands tell the assembler
that the following program is a TI-83 program, and it also defines most of the commands.
Now, it you were to make a BASIC program or game, one of the main parts of it would
be the use of Lbl and Goto commands (Well, this only applies to the really GOOD or
complex programs). In ASM, you use the same style of body structure. The only
problem is, there are no If ... Then statements in ASM. Programmers have to find clever
ways around using these statements in their ASM codes.
For starters, lets look at the most basic command (REMEMBER, all commands in
an ASM program cant be put in the first column of text, so TAB over at each
command!). This command is:
Thats all, the letters L and D next to each other. This is short for the command
LOAD. Now, I hear you asking "Phil, what does LOAD do?". LOAD is just like the
Sto command in basic, or ->. Now, just like you cant put X -> 5 in BASIC, there is
a form to follow in ASM too. The way the BASIC command looks is this:
(NUMBER OR VARIABLE) -> (VARIABLE)
But in basic, the -> is change to a <-. The way the ASM command looks is like so:
ld (VARIABLE),(NUMBER OR VARIABLE)
or, if you were to compare it to BASIC:
ld (VARIABLE) <- (NUMBER OR VARIABLE)
So if I wanted to Load 5 into a, I would do so like this:
Thats it. You can also store the value of one variable into another. All you have to do is
replace the 5 above with b or something.
Now, there is something I must tell you before I go on. ASM variables and BASIC
variables are NOT the same. At the end of the program, the calculator deletes the ASM
variables. Also, all ASM variable must be defined before using them in and equation or in
a command with no numbers. If you wanted to load b into a, you cannot just put:
Youd have to tell the calculator what b is first, like so:
Thats enough for the ld command for now. Its time to learn a command called
cp. The cp is short for COMPARE. Now you dont have to ask me "Phil, whats
COMPARE do?" because Im just getting into that. In a definition, cp subtracts a
minus the variable or number that follows the command. If this is too confusing, I will
show you a sample use of cp:
As we all (should) know, a minus 5, or 5 minus 5, equals zero. This is
important!!! The cp command is most commonly used as an If ... Then statement when
followed by other commands.
Now, real fast, I'm going to try to explain to you the different variables that ASM
uses. Remember that ASM variables and BASIC variables are not the same. The names
of the variables are: a b c d e h l . These can all be up any 8 bit number (0 to 255, or
in binary, if it helps, 00000000 to 11111111). 'a' is the most important of these variables.
It is called the accumulator. Almost ALL commands requiring one 8 bit variable use the
accumulator. If you wanted a larger number than 255, you can combine two 8 bit
variables to get one 16 bit variable. These variables are: bc de hl , and they can be any
number between 0 and 65535 (or in binary, if it helps, 0000000000000000 to
1111111111111111). 'hl' is the most important of these variables. In almost all
commands which use a 16 bit number, the variable 'hl' is used. There are some tricks
where you can combine two 8 bit variables to get one 16 bit variable, but they are
unimportant now, because you can just as easily load a 16 bit number into a 16 bit
variable. In other words, instead of doing this:
to get 'hl=259', you can do this:
The reason that 'hl=259' when 'h=1' and 'l=3' is simple binary math. 1 in binary is
00000001, and 3 in binary is 00000011. So when you put the two next to each other, you
get 0000000100000011, or 259.
This is all you need to know for now, though, until I get into talking about labels.
Remember all that you have learned in this log and the previous log, for I will be referring
to them many times in my next logs. Things will start speeding up from here, so heart
patients should be advised, and children under the age of 12 should be 50 inches or taller
Thanks again go out to Ahmed, Bill, Alex, Jareth, Jeff, Andrew, Andy, and Joe.
Extraspecialsuper thanks go out to Jamie, for her inspiration (though she doesnt know it).
Thanks for your time. See you in TI-83 ASM Logs vol. 3!