151 lines
3.7 KiB
ArmAsm
151 lines
3.7 KiB
ArmAsm
; c64view -- FLI multicolor viewer (self-contained)
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;
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; Re-points the VIC video matrix ($D018) and forces a badline ($D011 yscroll)
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; on every visible raster line via a cycle-timed loop, giving per-line colour.
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;
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; Memory layout of appended data (loads from $4000):
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; $4000+L*$400 screen RAM for line L (L=0..7), 1000 bytes each
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; $6000 bitmap 8000 (offset $2000 in VIC bank 1)
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; $8000 colour RAM 1000 (copied to $D800)
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; $83E8 background 1
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;
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; assembled by viewer/assemble.py via xa
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; BASIC autostart, SYS 2061
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* = $0801
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.word basicend
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.word 10
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.byte $9e
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.byte "2061"
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.byte 0
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basicend:
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.word 0 ; ML begins at $080D
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start:
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sei
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; copy colour RAM $8000 -> $D800 (1024 bytes)
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ldx #0
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ccopy:
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lda $8000,x
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sta $d800,x
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lda $8100,x
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sta $d900,x
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lda $8200,x
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sta $da00,x
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lda $8300,x
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sta $db00,x
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inx
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bne ccopy
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; build per-line tables. d018tab = ((line AND 7) << 4) OR $08
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; d011tab = $38 OR (line AND 7) = BMM+DEN+RSEL plus yscroll
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ldx #0
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btab:
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txa
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and #$07
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asl
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asl
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asl
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asl
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ora #$08
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sta d018tab,x
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; yscroll must equal (raster AND 7) to force a badline; the first
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; displayed raster is 51, so line x maps to raster 51+x.
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txa
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clc
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adc #$03
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and #$07
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ora #$38
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sta d011tab,x
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inx
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bne btab
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; VIC bank 1 ($4000-$7FFF)
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lda $dd00
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and #$fc
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ora #$02
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sta $dd00
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lda $83e8
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sta $d021 ; background
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lda #$00
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sta $d020 ; border black
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lda #$d8
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sta $d016 ; multicolor on
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; raster IRQ setup
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lda #<irq1
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sta $0314
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lda #>irq1
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sta $0315
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lda #$7f
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sta $dc0d ; disable CIA timer IRQs
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sta $dd0d
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lda $dc0d
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lda $dd0d
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lda #$01
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sta $d01a ; enable raster IRQ
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lda #$30
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sta $d012 ; line 48
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lda $d011
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and #$7f
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sta $d011
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asl $d019 ; ack
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cli
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hang:
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jmp hang
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; first IRQ, arrives with jitter on line 48, sets up the stabilised one
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irq1:
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lda #<irq2
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sta $0314
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lda #>irq2
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sta $0315
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inc $d012 ; fire again next line (49)
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asl $d019 ; ack
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tsx
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cli
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; burn the rest of the line so irq2 fires inside this NOP slide
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.dsb 40,$ea ; 40 NOPs
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; stabilised IRQ on line 49, runs the FLI loop
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irq2:
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txs ; restore sp from irq1
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; cancel the remaining 0/1-cycle jitter: read the raster twice; the
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; branch is 2 or 3 cycles depending on alignment, normalising entry.
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lda $d012
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cmp $d012
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beq jl
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jl:
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; fixed delay so the first store lands before the c-access window
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ldx #$0d
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d0: dex
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bne d0
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nop
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nop
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ldx #$00
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fliloop:
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lda d011tab,x ; force badline (yscroll = line&7)
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sta $d011
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lda d018tab,x ; point video matrix at screen[line]
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sta $d018
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inx
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cpx #200
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bne fliloop
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; bottom border: leave bitmap on but stop forcing badlines
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lda #<irq1
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sta $0314
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lda #>irq1
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sta $0315
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lda #$30
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sta $d012
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asl $d019
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jmp $ea81 ; restore regs + RTI
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d018tab:
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.dsb 256,0
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d011tab:
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.dsb 256,0
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