Application in VDM May Receive Overrun Error

• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5

SYMPTOMS

MS-DOS-based applications that use serial communications may receive a serial overrun error when running in a Windows NT version 3.5 virtual DOS machine (VDM), even though the application ran correctly under MS-DOS. In a Basic application, this would be seen as Error 57.

This has generally been seen after several hours of heavy communications at 9600 bits per second (bps).

CAUSE

Because MS-DOS-based applications running under Windows NT are subject to CPU time-slicing; they may not execute for extended periods of time (in Windows NT, the highest priority task that is able to run always runs). To prevent lost characters from serial communications ports, Windows NT virtualizes the port and presents the characters to the MS-DOS-based application as that application receives CPU time. The virtualization includes code to detect that an application is ignoring the serial port; a problem in this code sometimes incorrectly reports a serial overrun error.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 3.5. This problem was corrected in the latest U.S. Service Pack for Windows NT version 3.5. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K