First public commit.

lenser/vic20/viewer/viewer.s

@@ -0,0 +1,95 @@
+; VIC-20 image viewer (autostart 8K cartridge at $A000).
+;
+; The KERNAL does NOT initialise the VIC before launching an autostart cart, so
+; this code programs every VIC register itself, copies the character set, screen
+; and colour data from the cartridge ROM into RAM, then loops forever showing the
+; picture.
+;
+; Layout in unexpanded RAM -- char set $1400-$1BFF (256 chars), screen $1E00,
+; colour RAM $9600 (fixed).  $9005 = $FD selects screen $1E00 + char base $1400.
+;
+; The build (assemble.py) appends the data blocks at the fixed ROM addresses
+; CHARSRC / SCRSRC / COLSRC and fills in the BG / BORDER / AUX colour defines.
+
+        .word cold              ; $A000 cold-start vector
+        .word cold              ; $A002 warm-start vector
+        .byte $41,$30,$C3,$C2,$CD   ; "A0CBM" autostart signature
+
+; zero-page scratch (KERNAL-safe temporaries)
+src     = $fb                   ; $fb/$fc copy source pointer
+dst     = $fd                   ; $fd/$fe copy destination pointer
+
+cold:
+        sei
+        cld
+        ldx #$ff
+        txs
+
+        ; --- copy the 256-char set (2048 bytes = 8 pages) ROM -> $1400 ---
+        lda #<CHARSRC
+        sta src
+        lda #>CHARSRC
+        sta src+1
+        lda #$00
+        sta dst
+        lda #$14
+        sta dst+1
+        ldx #8                  ; pages to copy
+        jsr copypages
+
+        ; --- copy the screen (506 bytes -> 2 pages) ROM -> $1E00 ---
+        lda #<SCRSRC
+        sta src
+        lda #>SCRSRC
+        sta src+1
+        lda #$00
+        sta dst
+        lda #$1e
+        sta dst+1
+        ldx #2
+        jsr copypages
+
+        ; --- copy colour RAM (506 bytes -> 2 pages) ROM -> $9600 ---
+        lda #<COLSRC
+        sta src
+        lda #>COLSRC
+        sta src+1
+        lda #$00
+        sta dst
+        lda #$96
+        sta dst+1
+        ldx #2
+        jsr copypages
+
+        ; --- program the VIC ---
+        lda #$05
+        sta $9000               ; horizontal origin
+        lda #$19
+        sta $9001               ; vertical origin
+        lda #$96
+        sta $9002               ; 22 columns + screen address bit 9
+        lda #$2e
+        sta $9003               ; 23 rows, 8x8 chars
+        lda #$fd
+        sta $9005               ; screen $1E00 + char base $1400
+        lda #(AUX<<4)
+        sta $900e               ; auxiliary colour (bits 4-7), volume 0
+        lda #((BG<<4)|$08|BORDER)
+        sta $900f               ; bg (bits 4-7) | normal mode (bit3) | border (0-2)
+
+loop:
+        jmp loop                ; hold the picture forever
+
+; copy X pages of 256 bytes from (src) to (dst)
+copypages:
+        ldy #$00
+cp1:
+        lda (src),y
+        sta (dst),y
+        iny
+        bne cp1
+        inc src+1
+        inc dst+1
+        dex
+        bne cp1
+        rts