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8bitworkshop/presets/vcs/examples/vsync.a

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2016-12-16 01:21:51 +00:00
processor 6502
include "vcs.h"
include "macro.h"
org $f000
; Now we're going to drive the TV signal properly.
; Assuming NTSC standards, we need the following:
; - 3 scanlines of VSYNC
; - 37 blank lines
; - 192 visible scanlines
; - 30 blank lines
; We'll use the VSYNC register to generate the VSYNC signal,
; and the VBLANK register to force a blank screen above
; and below the visible frame (it'll look letterboxed on
; the emulator, but not on a real TV)
; Let's define a variable to hold the starting color
; at memory address $81
BGColor equ $81
; The CLEAN_START macro zeroes RAM and registers
Start CLEAN_START
NextFrame
; Enable VBLANK (disable output)
lda #2
sta VBLANK
; At the beginning of the frame we set the VSYNC bit...
lda #2
sta VSYNC
; And hold it on for 3 scanlines...
sta WSYNC
sta WSYNC
sta WSYNC
; Now we turn VSYNC off.
lda #0
sta VSYNC
; Now we need 37 lines of VBLANK...
ldx #37
LVBlank sta WSYNC ; accessing WSYNC stops the CPU until next scanline
dex ; decrement X
bne LVBlank ; loop until X == 0
; Re-enable output (disable VBLANK)
lda #0
sta VBLANK
; 192 scanlines are visible
; We'll draw some rainbows
ldx #192
lda BGColor ; load the background color out of RAM
ScanLoop
adc #1 ; add 1 to the current background color in A
sta COLUBK ; set the background color
sta WSYNC ; WSYNC doesn't care what value is stored
dex
bne ScanLoop
; Enable VBLANK again
lda #2
sta VBLANK
; 30 lines of overscan to complete the frame
ldx #30
LVOver sta WSYNC
dex
bne LVOver
; The next frame will start with current color value - 1
; to get a downwards scrolling effect
dec BGColor
; Go back and do another frame
jmp NextFrame
org $fffc
.word Start
.word Start