Venetian blind crash

The Venetian blind crash may occasionally occur in Vanilla Doom. The name of the error comes from a "scanline" effect that causes every alternating horizontal line of pixels on the screen to be darkened. This makes the screen seem to fold up like venetian window blinds. In DOS this can usually be fixed by issuing the command 'mode co 80' which resets the display to the regular dos graphics mode. Although in severe cases a restart may be the only way to fix the problem.

Venetian blind crashes are caused by ordinary program errors such as integer division by zero, segmentation or page faults, and illegal opcode execution. However, Doom does not crash gracefully like some other programs because it fails to shut down its libraries and unhook all interrupts when this happens. This leaves video, audio, and input unusable. Most DOS source ports immediately fixed this by changing the exit behavior and making sure everything is shut down completely even when a crash occurs.