CHR-RAM Writing/Swapping Thread

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Main - ROM Hacking - CHR-RAM Writing/Swapping Thread

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RetroRain

Posted on 08-06-12 10:24 PM

Link | Quote | ID: 151830

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I had a painful day yesterday. I spent literally the whole day trying to figure out how to deal with CHR-RAM. I have had tons of trouble with CHR-RAM in the past, really getting turned off with games that have it, and not CHR-ROM, since CHR-ROM is easier to deal with. But after all of my trouble yesterday, I finally figured out how to deal with CHR-RAM. A couple of years of mystery has now been unveiled. And yes, it has been a couple of years, but bear in mind, that I haven't hacked in a long time. So, it simply was a question that I never got an answer to, until yesterday when I was finally determined to get to the bottom of this once and for all.

Now, why am I making this thread? Very simple.

Through ALL of the searching I have done on the Internet, and you can search too if you want, there is not one decent document that covers how to deal with CHR-RAM, AT ALL.

Which is bullshit considering how long the ROM hacking scene has been around.

So, I will unveil the mystery, and post it here, for anyone who needs to deal with CHR-RAM, but doesn't know how to.

So, here it goes:

1. Put the wait for Vblank code:

wait_vblank:
LDA $2002
BPL wait_vblank

2. Set $2006 to where you want to write to. So if tile $00 on the left side of the PPU, then

LDA #$00
STA $2006
STA $2006

If $00 on the right side of the PPU, then

LDA #$10
STA $2006
LDA #$00
STA $2006

You have to write to $2006 twice. If it is tile $20 on the right side, you want to write to, it's:

LDA #$12
STA $2006
LDA #$00
STA $2006

The numbers are different when it comes to PPU tiles, because of the size.

3. Run a loop 16 times, since 16 bytes make up one 8x8 tile.

During that loop, write your tiles from a table close-by, to $2007. That's it! See below.



$B758:AD 02 20  LDA $2002 = #$10  ; Wait for vblank

$B75B:10 FB     BPL $B758



$B75D:A9 00     LDA #$00           ; Set $2006 to tile $00 on the left side of the PPU, we're writing to that tile

$B75F:8D 06 20  STA $2006 = #$C1

$B762:A9 00     LDA #$00

$B764:8D 06 20  STA $2006 = #$C1



; Loop code

$B767:A2 00     LDX #$00

$B769: BD 78 B7 LDA $B778,X @ $B779 = #$10

$B76C:8D 07 20  STA $2007 = #$00

$B76F:E8        INX

$B770:E0 10     CPX #$10

$B772: D0 F5    BNE $B769



; Graphics table.  16 bytes make up one 8x8 tile.

$B778:00        BRK

$B779:10 1B     BPL $B796

$B77B:1F        UNDEFINED

$B77C:1F        UNDEFINED

$B77D:1F        UNDEFINED

$B77E:1F        UNDEFINED

$B77F:0F        UNDEFINED

$B780:00        BRK

$B781:00        BRK

$B782:00        BRK

$B783:00        BRK

$B784:00        BRK

$B785:00        BRK

$B786:00        BRK

$B787:00        BRK

So, whenever you need to write one tile to the PPU for a game that uses CHR-RAM, you can copy this code, and use it for your own purposes.

For multiple tiles, just use another loop for multiple cycles.

Note: I was able to write $90 bytes without problems. If you try to write too much, this will create problems in the PPU and on the screen. I believe it is because you are trying to write too much during vblank. It can only take so much.

I believe the way around that, is to wait for vblank, then shut the screen off, write everything you need to write, and then turn the screen back on. I haven't tested that yet, but I will soon. If anyone can confirm that this works, feel free to reply.

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Hamtaro126

Posted on 08-07-12 12:20 AM

Link | Quote | ID: 151832

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Here is another, Better to read one, Using your base code:


;Original code by RetroRain!

; Wait for vblank
wait_vblank: 
                LDA $2002
                BPL wait_vblank
                RTS

; Set $2006 to tile $00 on the left side of the PPU, we're writing to that tile+
SmileCHR:
                JSR wait_vblank
                LDA #$00 
                STA $2006 
                LDA #$00
                STA $2006

; Loop code
                LDX #$00
TileLoop:
                LDA Tile0,X
                STA $2007
                INX
                CPX #$10
                BNE TileLoop
                RTS

; Graphics table.  16 bytes make up one 8x8 tile.
; (Did Words instead of Bytes make this easy)

Tile0:
;Layers:  1111111122222222
     .dw %0000000000111100
     .dw %0000000001011010
     .dw %0000000011011011
     .dw %0000000011111111
     .dw %0000000010111101
     .dw %0000000011000011
     .dw %0000000001100111
     .dw %0000000000111100

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