Colour Lives in a Palette

Last unit the block came out white, and we never said white — we just wrote two mystery bytes to $3F00 and called them the stage light. Time to open it up.

Here's the idea that catches every newcomer to the NES: a tile carries no colour of its own. The byte you wrote to the nametable was a tile number — a shape. The shape's colour is decided somewhere else entirely, in a separate little table called the palette. The tile's pixels don't say "white"; they say "colour 1", and colour 1 is whatever the palette currently says it is. Change the palette, and every pixel pointing at that slot changes with it — though you never touched the screen's memory.

This is the opposite of how it feels in BASIC, where a thing you draw is a colour. On the NES, shape and colour are two separate maps, and the tile is just a pointer from one into the other.

What you'll see by the end

A red block on a light-blue screen. The tile write is identical to Unit 5 — same number, same cell. The only thing that changed is the palette, and that changed both the block's colour and the colour of everything behind it.

A background palette is four colours

The PPU's palette table starts at $3F00. A background palette is four entries:

Every background pixel is two bits — a number from 0 to 3 — picking one of these four. Our solid block is made entirely of "colour 1" pixels, so it draws in whatever sits at $3F01. Last unit that was white; this unit it's red ($16). The blank tiles everywhere else are all "colour 0", so they draw in the backdrop at $3F00 — light blue ($21) this time. Same four pour-throughs of the window you already know:

    lda #$21                ; $3F00 backdrop      = light blue
    sta $2007
    lda #$16                ; $3F01 colour 1      = red  (the block)
    sta $2007
    lda #$27                ; $3F02 colour 2      = orange
    sta $2007
    lda #$30                ; $3F03 colour 3      = white
    sta $2007

(Colours 2 and 3 aren't used by our block — it's all colour 1 — but a palette is always four entries, so we fill them in. We'll use the other slots once a tile has more than one colour in it, next unit.)

The program

The tile write in the middle is exactly Unit 5's. Only the palette at the top has changed:

; ============================================================================
; Meet the Machine (NES) - Unit 6: Colour Lives in a Palette
;
; The block's colour does not live in the tile. It lives in a separate little
; table - the palette - and the tile only points at it. Change the palette and
; the same tile changes colour, with no change to the screen's memory at all.
; ============================================================================

.segment "HEADER"
    .byte "NES", $1a
    .byte 2
    .byte 1
    .byte $00, $00

.segment "CODE"

reset:
    sei
    cld
    ldx #$40
    stx $4017
    ldx #$ff
    txs
    inx
    stx $2000
    stx $2001
    stx $4010
warm1:
    bit $2002
    bpl warm1
warm2:
    bit $2002
    bpl warm2

    ; --- the palette: a small table of colours the tiles point at ---
    bit $2002
    lda #$3f
    sta $2006
    lda #$00
    sta $2006               ; aim at palette address $3F00
    lda #$21                ; $3F00 backdrop      = light blue
    sta $2007
    lda #$16                ; $3F01 palette-0 col 1 = red   (the block's colour)
    sta $2007
    lda #$27                ; $3F02 palette-0 col 2 = orange (unused here)
    sta $2007
    lda #$30                ; $3F03 palette-0 col 3 = white  (unused here)
    sta $2007

    ; --- the same block as last unit: tile 1 at the top-left cell ---
    bit $2002
    lda #$20
    sta $2006
    lda #$00
    sta $2006
    lda #$01
    sta $2007

    ; --- square the picture up and turn it on ---
    bit $2002
    lda #$00
    sta $2006
    sta $2006
    sta $2005
    sta $2005
    lda #$1e
    sta $2001

forever:
    jmp forever

nmi:
    rti
irq:
    rti

.segment "VECTORS"
    .word nmi
    .word reset
    .word irq

.segment "CHARS"
    .byte $00,$00,$00,$00,$00,$00,$00,$00
    .byte $00,$00,$00,$00,$00,$00,$00,$00
    .byte $ff,$ff,$ff,$ff,$ff,$ff,$ff,$ff
    .byte $00,$00,$00,$00,$00,$00,$00,$00
    .res 8192 - 32, $00

That's the whole point this unit makes: colour and shape are independent. You recoloured the block by editing a table that has nothing to do with the screen's memory.

Assemble and run

ca65 palette.asm -o palette.o && ld65 -C nes.cfg palette.o -o palette.nes

A red block on a light-blue field. Prove the independence to yourself: the nametable write is unchanged from last unit, yet the colour is completely different.

Try this: recolour without touching the screen

Change the $3F01 value — lda #$16 — to another NES colour and run: $2a green, $30 white, $28 yellow, $12 blue. The block changes colour each time, and you never went near the nametable. Shape stays; colour moves.

Try this: change the world behind it

Now change the $3F00 backdrop — lda #$21 — instead. The whole field changes colour, because every blank cell is colour 0, which is the backdrop. Set it to $0f and the background goes black, throwing the block into sharp relief. One byte repaints everything behind your tiles.

Try this: four palettes, and the catch

The NES has four background palettes ($3F00, $3F04, $3F08, $3F0C), so different parts of the screen can use different four-colour sets. Which palette a cell uses isn't stored in the cell, though — it's chosen in coarse 16×16-pixel blocks by a separate attribute table. That's why neighbouring tiles often have to share colours: a famous NES constraint, and a cousin of the Spectrum's attribute clash. We lean on just palette 0 in the Primer; the attribute table is a game's concern.

If it doesn't work

• The block is invisible. $3F01 probably matches $3F00 — colour 1 is the same as the backdrop, so the block blends in. Give them different values.
• The colours look wrong or washed. Check you wrote the four palette bytes in order through $2007 after aiming at $3F00, and that you didn't swap a digit — NES colour codes run $00–$3F.
• Nothing shows at all. As ever, check the picture is turned on (lda #$1e / sta $2001) at the end.

What you've learnt

Colour lives in the palette, separate from the screen. A background palette is four entries from $3F00; a tile's pixels point at one of them by number, so the same tile draws in whatever colour the palette currently holds. Change the palette and the picture recolours without a single write to the nametable.

What's next

We keep calling tile 1 "a solid block" — but where does that shape come from? Next — Where the Shapes Come From — we open the last mystery in the harness: the pattern table, the 8×8 shapes the PPU draws from. And you'll draw your own.