search

joncampbell123 / dosbox-x

DOSBox-X fork of the DOSBox project
GNU General Public License v2.0
2.64k stars 374 forks source link

Flash productions "monstra" extra white line at top of screen (when machine=svga_s3) #40

Open joncampbell123 opened 9 years ago

joncampbell123 commented 9 years ago

During the silent copperbars part an extra white line is visible at the top of the screen that doesn't happen on real hardware.

joncampbell123 commented 9 years ago

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; }

joncampbell123 commented 9 years ago

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?