8bitworkshop/presets/atari7800/wsync.c

/*
The Atari 7800 uses display lists and interrupts to
partition the video frame, but the 6502 can also be
synchronized with each scanline. When the CPU strobes
the WSYNC register, the CPU is halted until the end
of the current scanline. This is similar to the WSYNC
register on the Atari 2600, except that VBLANK and
VSYNC are automatically applied.
*/

#include "atari7800.h"

void main() {
  byte y1 = 110;	// Y start of bar
  byte y2 = 10;		// height of bar
  byte i;
  while (1) {
    // wait for end of frame / start of vblank
    while ((MARIA.MSTAT & MSTAT_VBLANK) == 0) ;
    // wait for end of vblank / start of frame
    while ((MARIA.MSTAT & MSTAT_VBLANK) != 0) ;
    // skip scanlines until at top of bar
    for (i=0; i<y1; i++) {
      WSYNC();
    }
    // set the background color on each scanline
    for (i=0; i<y2; i++) {
      MARIA.BACKGRND = P6532.SWCHA - i;
      WSYNC();
    }
    // clear the background after the final scanline
    MARIA.BACKGRND = 0;
    // move bar slowly downward
    y1++;
  }
}