Change color of the text written in PPU memory (NES-Super Mario Bros)

Question

Let's say I want to write K to the top of the status bar in SMB. I can use the following set of statements to write to PPU memory.

    memory.writebyte(0x2001, 0x00) -- Turn off rendering
    memory.readbyte(0x2002) -- PPUSTATUS (reset address latch)
    memory.writebyte(0x2006, 0x20) -- PPUADDR high byte
    memory.writebyte(0x2006, 0x2A) -- PPUADDR low byte
    memory.writebyte(0x2007, 0x14) -- PPUDATA ('K')
    memory.writebyte(0x2001, 0x1e) -- Turn on rendering

I tried changing the color of the text by changing the color palette before writing text as -

memory.writebyte(0x2001, 0x00) -- Turn off rendering
memory.readbyte(0x2002) -- PPUSTATUS (reset address latch)
memory.writebyte(0x2006, 0x3F) -- Selecting high byte color palette
memory.writebyte(0x2006, 0x11) -- Selecting low byte color palette
memory.writebyte(0x2007, 0x16) -- Red color
memory.writebyte(0x2006, 0x20) -- PPUADDR high byte
memory.writebyte(0x2006, 0x2A) -- PPUADDR low byte
memory.writebyte(0x2007, 0x14) -- PPUDATA ('K')
memory.writebyte(0x2001, 0x1e) -- Turn on rendering

However that changes the color of the background objects.

My question is how I can set the color of the displayed text?

I've never tried programming the NES, so maybe I'm completely wrong, but shouldn't you be updating the relevant block in the attribute table (https://wiki.nesdev.com/w/index.php/PPU_attribute_tables) rather than changing palette entries for this? — occipita, Jul 24 '20 at 05:12
The emulator Mesen lets you write a Lua script to interact with the game. — puppydrum64, Sep 04 '21 at 02:04
useless noob question here, what platform are you programming on ? — mh-cbon, Jun 04 '22 at 09:13

score 2 · Answer 1 · answered Aug 02 '20 at 02:25

Look at it in the Mesen emulator, under Debugger / PPU Viewer / Palette Viewer (there you can change the palette to see the effect).

For the text color, you want palette address $3F09. This is also the cloud's color. $3F11 is in the sprite palette rather than the background, and it seems to be Mario's outfit color.

Also, note that most games store the palette in RAM, and write to the PPU every vblank. Makes it easier to handle palette cycling, transitions, stuff like that.

Weird, when I try it I get the following error message: "attempt to index a nil value (global 'memory')" — puppydrum64, Sep 04 '21 at 01:57

score 0 · Answer 2 · answered Sep 04 '21 at 02:01

Instead of changing the palette itself you might find it easier to change the attributes instead.

Try this:

memory.writebyte(0x2001, 0x00) -- Turn off rendering
memory.readbyte(0x2002) -- PPUSTATUS (reset address latch)
memory.writebyte(0x2006, 0x23) -- Selecting high byte attribute table
memory.writebyte(0x2006, 0xC1) -- Selecting low byte attribute table 
; you might need to fiddle with this number, should be somewhere between 0xC0 and 0xC8
memory.writebyte(0x2007, 0x00) -- Use Palette 0
memory.writebyte(0x2006, 0x20) -- PPUADDR high byte
memory.writebyte(0x2006, 0x2A) -- PPUADDR low byte
memory.writebyte(0x2007, 0x14) -- PPUDATA ('K')
memory.writebyte(0x2001, 0x1e) -- Turn on rendering

Equivalent assembly code if you prefer:

LDA #$00
STA $2001
LDA $2002
LDA #$23
STA $2006
LDA #$C1
STA $2006
LDA #$00
STA $2007
LDA #$20
STA $2006
LDA #$2A
STA $2006
LDA #$14
STA $2007
LDA #$1E
STA $2001

Change color of the text written in PPU memory (NES-Super Mario Bros)

2 Answers2