First public commit.

lenser/a5200/viewer/viewer.s

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+; 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"