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