; ATRACT seed demo: a rainbow that scrolls down the screen, and POKE 77,0
; on every scan line, so the machine never falls into attract mode.
;
; ATRACT ($4D, location 77) is the OS's idle counter. The vertical blank adds
; one to it every four seconds; past 127 the OS starts cycling the screen
; colours to save the TV from burn-in. A program that does not read the
; keyboard writes 0 there to stay awake. This one does it constantly.
;
; The rainbow is the classic trick: VCOUNT says which scan line ANTIC is on,
; RTCLOK moves on every frame, and their sum, written to the colour
; registers right after WSYNC, gives every pair of lines its own colour.
;
; MIT licence. Copyright (c) 2026 the ATRACT authors.
; Built by seed/attract/build.js with the workbench's assembler.

ATRACT = $4D
RTCLOK = $14
COLPF2 = $D018
COLBK  = $D01A
WSYNC  = $D40A
VCOUNT = $D40B

        *= $2000
START   LDA #0
        STA ATRACT
LOOP    LDA VCOUNT
        CLC
        ADC RTCLOK
        STA WSYNC
        STA COLBK
        STA COLPF2
        LDX #0
        STX ATRACT
        JMP LOOP