From 53293e824380176d8f203eed978d8d03b421c69e Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Fri, 25 Nov 2022 19:17:34 -0800
Subject: [PATCH] closer

---
 dither4.s | 96 +++++++++++++++++++++++++++++++++----------------------
 1 file changed, 57 insertions(+), 39 deletions(-)

diff --git a/dither4.s b/dither4.s
index 4f49dd7..067544c 100644
--- a/dither4.s
+++ b/dither4.s
@@ -20,12 +20,21 @@ NMIEN = $D40E
 .import framebuffer
 
 displaylist:
+    ; @fixme resolve the line counts
     .repeat 3
         .byte $70 ; 8 blank lines
     .endrep
-    .byte $4e ; ANTIC mode e (160px 2bpp, 1 scan line per line)
+    ; Note the framebuffer cannot cross a 4KiB boundary
+    ; so this is split in half
+    ; ANTIC mode e (160px 2bpp, 1 scan line per line)
+    .byte $4e
     .addr framebuffer
-    .repeat 191
+    .repeat 95
+        .byte $0e
+    .endrep
+    .byte $4e
+    .addr framebuffer + 96*40
+    .repeat 95
         .byte $0e
     .endrep
     .byte $41 ; jump and blank
@@ -36,8 +45,39 @@ displaylist:
 
 .export start
 
-; Loop through scan lines updating the palette
-.proc update_palette
+.proc start
+    ; Disable display DMA
+    lda #$00
+    sta SDMCTL
+    ; Set up the display list
+    lda #.lobyte(displaylist)
+    sta SDLSTL
+    lda #.hibyte(displaylist)
+    sta SDLSTH
+
+    ; Turn off interrupts except reset button
+    ;lda #$20
+    ;sta NMIEN
+
+    ; Manually wait for first scan line
+wait_vblank:
+    lda VCOUNT
+    bne wait_vblank
+
+    ; Re-enable display DMA
+    lda #$22
+    sta SDMCTL
+
+wait_start:
+    lda VCOUNT
+    cmp #12
+    bne wait_start
+    ldy #0
+
+each_scanline:
+    tya
+    pha
+
     lda palette1,y
     pha
     ldx palette2,y
@@ -52,40 +92,18 @@ displaylist:
     sta COLPF0
     stx COLPF1
     sty COLPF2
+
+    pla
+    tay
+    iny
+    cpy #192
+    bne each_scanline
+
+    jmp wait_start    
+
+.endproc
+
+; Loop through scan lines updating the palette
+.proc update_palette
     rts
 .endproc
-
-.proc start
-    ; Disable display DMA
-    lda #$00
-    sta SDMCTL
-    ; Set up the display list
-    lda #.lobyte(displaylist)
-    sta SDLSTL
-    lda #.hibyte(displaylist)
-    sta SDLSTH
-
-    ; Turn off interrupts except reset button
-    lda #$20
-    sta NMIEN
-
-    ; Manually wait for first scan line
-wait_vblank:
-    lda VCOUNT
-    bne wait_vblank
-
-    ; Re-enable display DMA
-    lda #$22
-    sta SDMCTL
-
-even_scanline:
-    ldy VCOUNT
-    jsr update_palette
-
-odd_scanline:
-    ldy VCOUNT
-    iny
-    jsr update_palette
-    jmp even_scanline
-
-.endproc