maskmovdqu instruction performs a write to VGA memory. If one of the doublewords in the mask has the high bit set in every byte but the third, the first byte gets written, but the second and fourth do not.
I've written a test program to check for this defect. It should display a continuous line in the middle of the screen; if a dotted line appears, your chipset has the bug.
Download ihdbug.zip (1 kB)
The test program uses mode 13h; however, this issue affects all CGA, EGA and VGA graphics modes. VBE modes are unaffected.