Day 11 - Variables And Work RAM
Originally, Day 11 was going to be about playing a wav with direct sound.
I then realized one important thing that I have been leaving out of every piece of
code I've written so far. I've used them for ages, but I never wrote anything on how
to use them. This is the story of that mysterious area of memory that everyone's
wondering about Work RAM @ 0x3000000 . We use this space for little (32bits actually)
variables, they're not exactly variables, but this is as close to variables as we're
going to get as Assembly Language Programers.
Patater GBAGuy Mirror
How This Is Done.
First things first though. If you've programmed with C or QBasic or anything, you
know that variables have types. Well, I'm going to burst your bubble now. Our variables are
always a 32bit number (not always actually, they could be byte sized or 16bits, more later).
That's right, no type, consider it almost like a CPU register except that I takes 2 lines
of code to access most of the time.
To declare a variable, it goes like this:
variable_name @DCD initial_value
So if I wanted to create a variable called num1 with
and initial value of 0x0000F6A2, I'd go like tis:
num1 @DCD 0x0000F6A2
Neat, isn't it?
A Few Problems
If you're really crafty, maybe you noticed the two problems with the way we
declare variables in the previous section. They are:
1. The variable is in ROM right now.
2. Declaring a variable in ROM will cause
the CPU to execute the bytes defined
in it's initialization.
I have a confession to make. I've really aready told you this, but here's me
outright saying it: Our variables are nothing more than labeling an address in memory
possibly letting Goldroad give it an initial value. I just really want to get the point
across. Pretend I'm a hypnotist: U - N - D - E - R - S - T - A - N - D.
Hehehe. Guess I got a little silly there. Anyway, getting rid of these two highly related
problems is as easy as adding two lines of Goldroad Instructions. Goldroad instructions
are words that start in '@'. The lines of code we add to this are:
@textarea 0x3000000 and @endarea.
So now we have this:
num1 @DCD 0
Notice that this time I initialized the memory location (our variable) to zero (0).
I like to put all my variables in the same @textarea block as if you use another like so:
In addition to the last code.
num2 @DCD 0x9
Note that now you'll have two variables that are EXACTLY THE SAME as they are both located
at the same point in memory. Modify one and try to access the other and you'll find that it's
value has changed, they are both aliases to the same point in memory. I don't know if Goldroad
gives this an error or not or if it actually works and Goldroad advances the memory so that
they both don't alias the same memory location. So bottom line:
PUT ALL YOUR "VARIABLES" IN THE SAME @TEXTAREA BLOCK!
Setting A WRAM Variable's Value
Now that we've declared it, we really ought to set it's value. So here's what we
need to do line-by-line to set a WRAM variable's value:
Assume the variable's name is num1.
Also assume that r1 has the value we want to set num1 with.
ldr r0,=num1 ; get's the address of num1 in r0.
str r1,[r0] ; puts the value in r1 into memory at the address in r0.
; remember that our variables are actually aliases for memory addresses.
That's it. Setting is rather short. Getting is a bit longer.
So much for line-by-line. I didn't remember that the code is short. :)
Getting A WRAM Variable's Value
Getting, well, how 'bout I just do it and explain it with comments?
Here it is:
ldr r0,=num1 ; get's the address of num1 in r0
ldr r1,[r0] ; loads r1 with the value at the memory address
; in r0, in this case r0 has the address of our variable in WRAM
; so boom r1 has num1's value in it.
Notice that that last line of code is the EXACT opposite of the SToRe instruction as that
if we went: str r1,[r0] , we'd be putting r1 into memory at r0. Instead of taking
the value of memory at r0 and putting it into r1. So you see they are reverse instructions.
That's really all you need to know about this. Not that hard, but important.
If you need more help with this, ask me and I'll update this tutorial with
a simple (really simple small demo) to help illustrate.
This Day In Review
I'm sorry if you expected to be able to copy 'n paste some code to get some
cool output or something. But it seemed to me that using Work RAM is rather important
especially in a large game or something where even having 13 CPU registers becomes not
That's all for today, hope you likey.
-Mike H a.k.a GbaGuy
"I'll cut you in for a piece of the action."
- Kirk to a gangster. "A Piece of The Action"
- Star Trek The Original Series
Intro - Day 12