60 lines
1.2 KiB
Plaintext
60 lines
1.2 KiB
Plaintext
processor 6502
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include "vcs.h"
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include "macro.h"
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org $f000
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; We're going to mess with the playfield registers, PF0, PF1 and PF2.
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; Between them, they represent 20 bits of bitmap information
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; which are replicated over 40 wide pixels for each scanline.
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; By changing the registers before each scanline, we can draw bitmaps.
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Counter equ $81
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Start CLEAN_START
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NextFrame
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; This macro efficiently gives us 3 lines of VSYNC
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VERTICAL_SYNC
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; 37 lines of VBLANK
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ldx #37
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LVBlank sta WSYNC
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dex
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bne LVBlank
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; Disable VBLANK
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stx VBLANK
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; Set foreground color
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lda #$82
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sta COLUPF
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; Draw the 192 scanlines
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ldx #192
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lda #0 ; changes every scanline
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;lda Counter ; uncomment to scroll!
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ScanLoop
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sta WSYNC ; wait for next scanline
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sta PF0 ; set the PF1 playfield pattern register
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sta PF1 ; set the PF1 playfield pattern register
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sta PF2 ; set the PF2 playfield pattern register
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stx COLUBK ; set the background color
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adc #1 ; increment A
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dex
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bne ScanLoop
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; Reenable VBLANK for bottom (and top of next frame)
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lda #2
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sta VBLANK
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; 30 lines of overscan
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ldx #30
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LVOver sta WSYNC
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dex
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bne LVOver
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; Go back and do another frame
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inc Counter
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jmp NextFrame
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org $fffc
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.word Start
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.word Start
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