Flash productions "monstra" extra white line at top of screen (when machine=svga_s3) about dosbox-x HOT 2 OPEN

White line does not appear unless machine=svga_s3.

Which means the blame lies with this code in src/hardware/vga_draw.cpp:

void VGA_Update_SplitLineCompare() {
vga.draw.split_line = vga.config.line_compare+1;
if (svgaCard==SVGA_S3Trio) {
if (vga.config.line_compare==0) vga.draw.split_line=0;
if (vga.s3.reg_42 & 0x20) { // interlaced mode
vga.draw.split_line *= 2;
}
}
vga.draw.split_line -= vga.draw.vblank_skip;
}

from dosbox-x.

I wonder if the guy who added that code simply wanted to double the line count for interlaced. We could fix this bug and possibly emulate the S3 interlaced line count correctly if we replaced it with:

if (svgaCard==SVGA_S3Trio) {
if (vga.s3.reg_42 & 0x20) {
vga.draw.split_line--;
vga.draw.split_line *= 2;
vga.draw.split_line++;
}
}

The question then comes down to: Do S3 chipsets emulate line compare such that line_compare == 0 means the first scanline is repeated? Or do S3 chipsets special-case against that, and do not repeat the first scanline?

from dosbox-x.