Venetian blind crash


The crash has occured in MAP30: Last Call due to a bug with spawn spots.

A Venetian blind crash may occasionally occur in vanilla Doom for DOS. It is named for the effect where periodically repeating columns of pixels on the screen become darkened; this makes the screen seem to fold up like Venetian window blinds. This effect is caused by the fact that the Doom executable does not reset the video mode upon crashing, and that the video BIOS routines that attempt to print out text to the screen are not prepared to do so in the graphics mode set up by Doom. The text being printed is usually the crash dump generated by the DOS/4GW extender, followed by the DOS command prompt.

If the user had previously used the ctty command to change their terminal device to something other than the screen (con), the crash dump and subsequent command prompt will be printed to that device while the screen will instead simply freeze, without producing the Venetian blind effect. When launching the Doom executable, the user can also use the > operator to redirect standard output to a text file, in which case the crash dump (and Doom’s initialization messages) will be written to that file instead of the screen.

After a Venetian crash occurs, the video mode can usually be reset manually by issuing the command mode co80. Particularly severe crashes may, however, freeze the system entirely and require a reboot. This command will not work in Windows XP or 2000; it is also not available out-of-the-box in DOSBox, although it can be separately installed by extracting it from a DOS installation.

Venetian crashes are not caused by any specific bug or error; they are a generic response to any unhandled exception such as integer division by zero, segmentation or page faults, and illegal opcode execution. Doom does not crash as gracefully as 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.

A video showing this error is available at YouTube. In this case, it is the fatal result from retriggering an open door after loading a saved game, an error that was only present in the 1.1 version of Doom.