Mystify Screen Saver Shows Screen Snow

• Microsoft Windows NT operating system, version 3.1
• Microsoft Windows NT Advanced Server, version 3.1

SYMPTOMS

During a color palette change, you may notice snow on the screen. This is most noticeable when running the Mystify screen saver, because it changes palettes constantly.

CAUSE

Older VGA adapters have single-ported digital to analog converters (DACs). When DAC registers are written to, they cannot be read from simultaneously. If you are realizing a palette, for example, as DAC registers are written to, pixel color cannot be read and snow appears on the screen. This problem is limited to old VGA adapters (five years or older) running in 256-color mode (16-color mode is not palletized). Since most machines that are capable of running Windows NT are not five years old, this problem should not be prevalent.

This problem occurs on Video 7 VRAM I or II video adapters with BIOS revisions of 8.05 or earlier.

Newer VGA adapters do not have this problem because of dual-ported DACs and built-in anti-sparkle circuitry.

RESOLUTION

With Windows and MS-DOS, this problem was handled by waiting for a vertical retrace (when no screen drawing was occurring) to write to the DAC registers. This was an acceptable solution because nothing is happening during vertical retrace in Windows and MS-DOS. However, with Windows NT, because several threads may be running and competing for processor time, waiting for a vertical retrace would reduce performance severely.

To avoid this problem, obtain a newer video adapter.