Get the COMPLETE source code HERE.

Now it is time to get your hands dirty. You will be making your first snes asm
program today. It's about time, huh. So let's get on with it!

What this program will do:

Well it will basically fill then entire screen with one color. MAN is
that EXciting OR WHAT! Now, before we begin, I want you to know what
you are writing. So some very Quick (actually long) theory. let's go!

The CGRAM is where we store the palette data. Now, there's 2 registers that we
need to deal with in order to modify the palette. They are $2121, and $2122.
Let's see what Yoshi has to say about these.

| w   |$2121   |Color # (or palette) selection register [CGDATA]
|     |        |xxxxxxxx             x: address (color #).
|     |        |
|     |        |
| wd  |$2122   |Color data register [CGDATA]
|     |        |xxxxxxxx             x: Value of color. 
|     |        |
|     |        |Snes color is 15 bit; 5 bits for red, green, and blue. The
|     |        |order isn't RGB though: it's BGR (RGB reversed!).

Ok so now we know what we need to do to modify the palette. We need to write
the number of the color we wish to modify to $2121, and then write the new
color data to $2122. And Look, it says you need to write to $2122 twice
(wd - d = double-write required). But what's the format of these 2 bytes we need
to write? Let's look in Snes2.txt

The format (in binary) is the following:

    b: blue                               ?bbbbbgg gggrrrrr
    g: green
    r: red
    ?: "The infamous bit-of-confusion." (just leave it 0 ;)

Yoshi even provides you with some sample colors. Well let's use one of his
sample colors, Red ;). We'll fill the screen with red.

Since InitSNES turns off the screen, we need to turn it back on. Look up
register $2100 in Yoshi's doc to see what I did to turn on the screen.

Note that the screen is turned off in order to force a vertical blank.(VBLANK).
When the screen is being drawn, you cannot edit data in VRAM (or at least not
accurately). You need to either wait for the tracer to be in the stage where
it's not drawing, or just shut off the entire screen and "force VBLANK."
Just thought you should know that if you didn't already.

;-------------------------- Begin SCREENFILL.ASM ----------------------------

;== Include memorymap, header info, and SNES initialization routines
.INCLUDE "header.inc"
.INCLUDE "SnesINIT.asm"

;========================
; Start
;========================

.BANK 0 SLOT 0
.ORG 0
.SECTION "MainCode"

Start:
        InitSNES            ; Init Snes :)

        stz $2121           ; Edit color 0 - snes' screen color
                            ; you can write it in binary or hex
        lda #%00011111      ; binary is more visual, but if you
        sta $2122           ; wanna be cool, use hex ;) 
        stz $2122           ; second byte has no data, so we write a 0

        lda #$0F            ; = 00001111
        sta $2100           ; Turn on screen, full brightness
        

forever:
        jmp forever

.ENDS

;------------------------------------ END ------------------------------------

Ok, hows does it fill the whole screen?

Well, the big important part is that we edited color 0. From what I
understand, the SNES uses Color 0 as the "transparent" color and fills the
screen with it. When we edit it, the screen color changes.
The loop just loops.. It keeps the program running, nothing more. Although
nothing is updated later, the loop is not really necessary, but it's good
practice.

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