; lenser -- 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

        ; hold the picture per the display option (our IRQ chains to the KERNAL
        ; $ea31 housekeeping, so GETIN and the jiffy clock both work here).
#include "wait.i"

        ; 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