Working Python version for Commodore.

c64view/viewer/interlace.s

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+; c64view -- multicolor interlace viewer (self-contained)
+;
+; Shows two multicolor frames on alternating fields by flipping the VIC bank in a
+; once-per-frame raster IRQ (no cycle-exact timing needed, so it is robust).
+; Frame A lives in bank 0 (bitmap $2000, screen $0400); frame B in bank 1 (bitmap
+; $6000, screen $4400).  Both use $D018=$18; only the $DD00 bank bit toggles.
+;
+; Memory layout of appended data (loads from $2000):
+;   $2000 bitmap A 8000
+;   $3F40 screen A 1000   (copied to $0400)
+;   $4400 screen B 1000   (in place, bank 1 video matrix)
+;   $6000 bitmap B 8000
+;   $8000 colour RAM 1000 (copied to $D800)
+;   $83E8 background 1
+;
+; assembled by viewer/assemble.py via xa
+
+        ; BASIC autostart, SYS 2061
+        * = $0801
+        .word basicend
+        .word 10
+        .byte $9e
+        .byte "2061"
+        .byte 0
+basicend:
+        .word 0
+
+SRC = $fb
+DST = $fd
+
+start:
+        sei
+
+        ; screen A $3F40 -> $0400
+        lda #$40
+        sta SRC
+        lda #$3f
+        sta SRC+1
+        lda #$00
+        sta DST
+        lda #$04
+        sta DST+1
+        jsr copy1024
+
+        ; colour RAM $8000 -> $D800
+        lda #$00
+        sta SRC
+        lda #$80
+        sta SRC+1
+        lda #$00
+        sta DST
+        lda #$d8
+        sta DST+1
+        jsr copy1024
+
+        lda $83e8
+        sta $d021                 ; background
+        lda #$00
+        sta $d020                 ; border black
+        lda $dd00
+        and #$fc
+        ora #$03
+        sta $dd00                 ; start on VIC bank 0 (frame A)
+        lda #$18
+        sta $d018                 ; screen $x400, bitmap $x000+$2000
+        lda #$d8
+        sta $d016                 ; multicolor on
+        lda #$3b
+        sta $d011                 ; bitmap mode, display on
+
+        ; frame-flip raster IRQ near the bottom border
+        lda #<irq
+        sta $0314
+        lda #>irq
+        sta $0315
+        lda #$7f
+        sta $dc0d
+        sta $dd0d
+        lda $dc0d
+        lda $dd0d
+        lda #$01
+        sta $d01a
+        lda #$fa
+        sta $d012                 ; line 250
+        lda $d011
+        and #$7f
+        sta $d011
+        asl $d019
+        cli
+
+waitkey:
+        jsr $ffe4                 ; GETIN (scanned via our IRQ -> $ea31)
+        beq waitkey
+
+        ; restore text mode + KERNAL IRQ
+        sei
+        lda #$00
+        sta $d01a                 ; disable raster IRQ
+        lda #$81
+        sta $dc0d                 ; re-enable CIA timer IRQ
+        lda #$31
+        sta $0314
+        lda #$ea
+        sta $0315
+        lda #$1b
+        sta $d011
+        lda #$c8
+        sta $d016
+        lda #$15
+        sta $d018
+        lda $dd00
+        ora #$03
+        sta $dd00
+        asl $d019
+        cli
+        jsr $e544                 ; clear screen
+        rts
+
+; once per frame, flip bank 0 <-> bank 1, then let the KERNAL IRQ finish
+irq:
+        lda $dd00
+        eor #$01
+        sta $dd00
+        asl $d019                 ; ack raster IRQ
+        jmp $ea31                 ; KERNAL housekeeping (keyboard) + RTI
+
+copy1024:
+        ldx #4
+        ldy #0
+cploop:
+        lda (SRC),y
+        sta (DST),y
+        iny
+        bne cploop
+        inc SRC+1
+        inc DST+1
+        dex
+        bne cploop
+        rts