; NES background image viewer (6502 / 2C02 PPU).  NROM, 16K PRG at $C000.
; Waits for PPU warm-up, loads the palette ($3F00, 32 bytes), nametable +
; attribute table ($2000, 1024 bytes), enables background, then idles.  The
; image-specific data lives at fixed PRG addresses written by the builder-
;   $F000  32-byte palette        $F020  1024-byte nametable + attributes
; Tiles come from CHR-ROM pattern table 0.

        * = $C000

PPUCTRL   = $2000
PPUMASK   = $2001
PPUSTATUS = $2002
PPUSCROLL = $2005
PPUADDR   = $2006
PPUDATA   = $2007

start:
        sei
        cld
        ldx #$ff
        txs
        lda #$00
        sta PPUCTRL             ; NMI off
        sta PPUMASK             ; rendering off during setup

        bit PPUSTATUS           ; clear latch
w1:     bit PPUSTATUS
        bpl w1                  ; wait for first vblank
w2:     bit PPUSTATUS
        bpl w2                  ; wait for second vblank (PPU warmed up)

        ; ---- palette- $3F00..$3F1F from $F000 ----
        lda #$3f
        sta PPUADDR
        lda #$00
        sta PPUADDR
        ldx #$00
pal:    lda $f000,x
        sta PPUDATA
        inx
        cpx #$20
        bne pal

        ; ---- nametable + attributes- $2000..$23FF (1024) from $F020 ----
        lda #$20
        sta PPUADDR
        lda #$00
        sta PPUADDR
        ldx #$00
nt0:    lda $f020,x
        sta PPUDATA
        inx
        bne nt0
nt1:    lda $f120,x
        sta PPUDATA
        inx
        bne nt1
nt2:    lda $f220,x
        sta PPUDATA
        inx
        bne nt2
nt3:    lda $f320,x
        sta PPUDATA
        inx
        bne nt3

        ; ---- enable background ----
        lda #$00
        sta PPUSCROLL
        sta PPUSCROLL
        lda #$00
        sta PPUCTRL            ; nametable $2000, bg pattern table $0000, NMI off
        lda #$0a
        sta PPUMASK            ; show background, no left-column clip
loop:
        jmp loop
        ; the builder writes the $FFFA-$FFFF vectors (all -> start = $C000)