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Day 7 - Using Memory

Mike Huber

Published: 2005-Jul-21; Updated: 2010-Dec-18

Where?

Before we do anything else it would probably be best to show how to use memory for our own data. This data could be anything from just a number you need to store or X/Y values for sprites. For this data we'll use free memory at $0000 (CPU).

How?

We'll ORG at $0000 in our Code Bank 0 and label some memory before ORGing to $8000 where our code starts. To label some memory (a.k.a making a variable) we use the assembler operation .db like so:

; note this is only a small code piece to demonstrate memory labeling syntax.

.bank 0
.org $0000

label1: .db 0 byte1: .db $A5 ; notice that with Define Byte (.db) we don’t need the ‘#’ .

.org $8000

Start: ; from here on would be code as usual </code>

Since label1 and byte1 are infact labels, they do need to start at the first colon (not tabbed in).

Loading Value of Our "Variables"

To load the value of one of our "Variables", just use a regular Load instruction with the label's name. Like so:

;assume label1 and byte1 have been declared like shown above.

lda label1  ; load A with label1's value
ldx label1  ; load X with label1's value
ldy label1  ; I hope you get the idea now

lda byte1  ; load A with byte1's value </code>

That's it for getting the value. Storing is just as easy. Even though you can figure it out yourself, I'll go through the same steps with storing.

Storing Into Our Variables

Storing is just like loading except the other way around (putting, not getting). It's done like this:

;again assume label1 was declared like shown before

sta label1  ; store A's value into label1
stx label1  ; store X's value into label1
sty label1 ; store Y's value into label1

stx byte1  ; you should get it by now, I hope. </code>

Simple, isn't it?

Some Important Facts

That I Waited Until the End to Mention

The main thing I want to mention here is that we can define more than just bytes. We can also define words, but since the NES's CPU works in only 8bits, this is a little more than I want to talk about right now.

Also, I haven't tried it, but if you want to increment or decrement (Add or Sub 1 respectfully), you could probably go something like this:

inc label1 ; add 1 to label1

dec label1  ; sub 1 from label1 </code>

This Day In Review

I have no idea what's in store for tomorrow, since I need to complete Day 13 of the GBA tutorial. I hope you had fun today. Why don't you try moving a sprite with the keypad?

Until Next Time,
-Mike H a.k.a GbaGuy