First public commit.

lenser/a7800/viewer/viewer.s

@@ -0,0 +1,47 @@
+; Atari 7800 image viewer -- programs MARIA, then holds.
+;
+; MARIA DMAs the display-list list (DLL), the per-line display lists (DLs) and the
+; bitmap straight out of cartridge ROM, so the viewer only loads MARIA's colour
+; registers, points it at the DLL, turns DMA on, and loops.  MARIA registers live
+; in zero page ($20-$3F).
+;
+; #defines from viewer/assemble.py --
+;   SCRIPT   address of the colour register script -- (reg, value) byte pairs
+;            written as $0000+reg = value, terminated by $FF
+;   DLL      address of the display-list list
+
+        * = $4000
+
+reset:
+        sei
+        cld
+        ldx #$ff
+        txs
+        lda #$00
+        sta $3c                 ; CTRL = 0 -- DMA off while we set up
+
+        ; ---- load MARIA colour registers from the script ----
+        ldx #$00
+sloop:
+        lda SCRIPT,x
+        cmp #$ff
+        beq sdone
+        tay                     ; Y = MARIA register ($20..$3F)
+        inx
+        lda SCRIPT,x
+        sta $0000,y
+        inx
+        jmp sloop
+sdone:
+        ; ---- point MARIA at the display-list list ----
+        lda #>DLL
+        sta $2c                 ; DPPH
+        lda #<DLL
+        sta $30                 ; DPPL
+
+        ; ---- enable DMA (160A, no DLI) ----
+        lda #$40
+        sta $3c                 ; CTRL
+
+loop:
+        jmp loop                ; the packer fills the reset vectors at $FFFA