38 lines
1.0 KiB
C
38 lines
1.0 KiB
C
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/*
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The Atari 7800 uses display lists and interrupts to
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partition the video frame, but the 6502 can also be
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synchronized with each scanline. When the CPU strobes
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the WSYNC register, the CPU is halted until the end
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of the current scanline. This is similar to the WSYNC
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register on the Atari 2600, except that VBLANK and
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VSYNC are automatically applied.
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*/
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#include "atari7800.h"
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void main() {
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byte y1 = 110; // Y start of bar
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byte y2 = 10; // height of bar
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byte i;
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while (1) {
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// wait for end of frame / start of vblank
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while ((MARIA.MSTAT & MSTAT_VBLANK) == 0) ;
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// wait for end of vblank / start of frame
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while ((MARIA.MSTAT & MSTAT_VBLANK) != 0) ;
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// skip scanlines until at top of bar
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for (i=0; i<y1; i++) {
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WSYNC();
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}
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// set the background color on each scanline
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for (i=0; i<y2; i++) {
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MARIA.BACKGRND = P6532.SWCHA - i;
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WSYNC();
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}
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// clear the background after the final scanline
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MARIA.BACKGRND = 0;
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// move bar slowly downward
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y1++;
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}
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}
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