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81 lines
1.7 KiB
Plaintext
81 lines
1.7 KiB
Plaintext
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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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; Now we're going to drive the TV signal properly.
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; Assuming NTSC standards, we need the following:
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; - 3 scanlines of VSYNC
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; - 37 blank lines
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; - 192 visible scanlines
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; - 30 blank lines
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; We'll use the VSYNC register to generate the VSYNC signal,
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; and the VBLANK register to force a blank screen above
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; and below the visible frame (it'll look letterboxed on
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; the emulator, but not on a real TV)
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; Let's define a variable to hold the starting color
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; at memory address $81
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BGColor equ $81
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; The CLEAN_START macro zeroes RAM and registers
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Start CLEAN_START
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NextFrame
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; Enable VBLANK (disable output)
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lda #2
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sta VBLANK
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; At the beginning of the frame we set the VSYNC bit...
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lda #2
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sta VSYNC
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; And hold it on for 3 scanlines...
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sta WSYNC
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sta WSYNC
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sta WSYNC
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; Now we turn VSYNC off.
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lda #0
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sta VSYNC
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; Now we need 37 lines of VBLANK...
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ldx #37
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LVBlank sta WSYNC ; accessing WSYNC stops the CPU until next scanline
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dex ; decrement X
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bne LVBlank ; loop until X == 0
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; Re-enable output (disable VBLANK)
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lda #0
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sta VBLANK
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; 192 scanlines are visible
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; We'll draw some rainbows
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ldx #192
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lda BGColor ; load the background color out of RAM
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ScanLoop
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adc #1 ; add 1 to the current background color in A
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sta COLUBK ; set the background color
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sta WSYNC ; WSYNC doesn't care what value is stored
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dex
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bne ScanLoop
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; Enable VBLANK again
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lda #2
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sta VBLANK
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; 30 lines of overscan to complete the frame
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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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; The next frame will start with current color value - 1
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; to get a downwards scrolling effect
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dec BGColor
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; Go back and do another frame
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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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