; Atari 5200 image viewer -- self-contained, no OS.
;
; The 5200 has no operating system (only a small BIOS that jumps straight to the
; cartridge), so this code programmes ANTIC and GTIA hardware registers directly.
; ANTIC DMAs the bitmap and display list straight out of cartridge ROM, so nothing
; needs copying to RAM -- the viewer only applies the GTIA colour registers, points
; ANTIC at the display list, enables DMA, and holds.
;
; GTIA is at $C000 on the 5200 (not $D000 as on the 400/800); ANTIC is at $D400.
;
; #defines from viewer/assemble.py --
;   DLIST    cartridge address of the display list
;   SCRIPT   cartridge address of the GTIA register script -- (offset, value)
;            byte pairs applied as $C000+offset = value, terminated by $FF

        * = $4000

start:
        sei
        cld
        ldx #$ff
        txs
        lda #$00
        sta $d40e               ; NMIEN = 0 -- no interrupts
        sta $d400               ; DMACTL = 0 while we set up

        ; ---- apply the GTIA register script (offset, value pairs) ----
        ldx #$00
sloop:
        lda SCRIPT,x
        cmp #$ff
        beq sdone
        tay                     ; Y = register offset within GTIA
        inx
        lda SCRIPT,x
        sta $c000,y             ; GTIA register = value
        inx
        jmp sloop
sdone:
        ; ---- point ANTIC at the display list ----
        lda #<DLIST
        sta $d402               ; DLISTL
        lda #>DLIST
        sta $d403               ; DLISTH

        ; ---- enable DMA -- normal playfield + display-list DMA ----
        lda #$22
        sta $d400               ; DMACTL

        ; ---- hold the picture (forever / until a button / N seconds) ----
#include "awyt5200.i"