; 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