79 lines
1.8 KiB
Plaintext
79 lines
1.8 KiB
Plaintext
|
|
processor 6502
|
|
include "vcs.h"
|
|
include "macro.h"
|
|
include "xmacro.h"
|
|
|
|
org $f000
|
|
|
|
; To output a PAL signal, you need the following:
|
|
;
|
|
; - 3 scanlines of VSYNC
|
|
; - 45 blank lines
|
|
; - 228 visible scanlines
|
|
; - 36 blank lines
|
|
;
|
|
; Total = 312 lines (vs 262 for NTSC)
|
|
|
|
; PAL runs at 50 Hz (vs 60 Hz for NTSC)
|
|
; so your game will run more slowly.
|
|
; Since you have extra cycles to play with, you could just
|
|
; call your position update routine twice every five frames.
|
|
|
|
; Note also that PAL has different colors than NTSC.
|
|
; (See http://www.randomterrain.com/atari-2600-memories-tia-color-charts.html)
|
|
|
|
; The TIMER_SETUP macro only goes up to 215 lines,
|
|
; so for the PAL visible frame you would need to use
|
|
; multiple sections (say 215 + 13 = 228) or count manually
|
|
; like we do in this example.
|
|
|
|
; Many VCS games use conditional defines
|
|
; (IFCONST and IFNCONST in DASM)
|
|
; to switch between NTSC and PAL constants.
|
|
|
|
; background color
|
|
BGColor equ $81
|
|
|
|
; Start of program, init registers and clear memory
|
|
Start CLEAN_START
|
|
|
|
NextFrame
|
|
; 01 + 3 lines of VSYNC
|
|
VERTICAL_SYNC
|
|
; Now we need 44 lines of VBLANK...
|
|
TIMER_SETUP 44
|
|
TIMER_WAIT
|
|
; Re-enable output (disable VBLANK)
|
|
lda #0
|
|
sta VBLANK
|
|
; 228 PAL scanlines are visible
|
|
; We'll draw some rainbows
|
|
ldx #228
|
|
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
|
|
|
|
; 36 lines of overscan to complete the frame
|
|
TIMER_SETUP 36
|
|
; Enable VBLANK again
|
|
lda #2
|
|
sta VBLANK
|
|
TIMER_WAIT
|
|
|
|
; 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
|
|
|
|
; 6502 vectors
|
|
org $fffc
|
|
.word Start
|
|
.word Start
|