Full screen background = 1K bytes?

[casprog]

Reading along the Nerdy Nights tutorial (awesome guide) I came across this:

When we are using nested loops to copy entire backgrounds we want 256 x 4 = 1KB

http://nintendoage.com/forum/messagevie ... adid=33274

It then shows 4 passes of 256 reads/writes.

Isn't a full screen 960 bytes? 32 x 30 tiles?

I ended up writing something that loops 256 3 times, then finishes off the stragglers for the remaining 192 bytes.

Code: Select all

    LDA $2002             ; read PPU status to reset the high/low latch
    LDA #$20
    STA $2006             ; write the high byte of $2000 address
    LDA #$00
    STA $2006             ; write the low byte of $2000 address

    LDA #LOW(StartNametable)
    STA pointerLow
    LDA #HIGH(StartNametable)
    STA pointerHigh

    LDX #$00
    LDY #$00

LoadStartNametableLoop:
        LDA [pointerLow], y
        STA $2007
        INY
        ; nametable data will hold one full screen, or 32*30 bytes
        ; we need to check our x counter to determine when to stop reading "chunks" of 256 bytes
        ; if x is 3 we have already read 3 * 256 bytes, and only need the straggler 192 bytes
        ; otherwise we keep reading until y flips again (another 256 bytes read)
        CPX #$03
        BNE StartNametableChunk
        CPY #$C0
        BNE LoadStartNametableLoop
        JMP LoadStartNametableDone
StartNametableChunk:
        CPY #$00
        BNE LoadStartNametableLoop

        INC pointerHigh
        INX
        JMP LoadStartNametableLoop
LoadStartNametableDone:

My code is not as clean looking but it seems to load it all in. Curious if I'm doing this properly. Looking at the nametable viewer seems correct.

[rainwarrior]

Following the 960 tiles, there is 64 bytes of "attribute" data that tells it which colour palettes to use for the tiles.

https://wiki.nesdev.com/w/index.php/PPU ... ute_tables

[casprog]

Thanks, just to help me get my head around this, I thought originally the attribute table would hold 60 bytes since there is one byte to color a 4x4 area at 960 tiles.

Where does the extra 4 bytes come from?

[rainwarrior]

The page I linked should show how it's laid out, but the last "row" of 8 bytes wastes half its data. So, yes, there's only 60 bytes of attribute information, but it's still spread across 64 physical bytes.

[casprog]

doh, I understand now. Thank you : )