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