Microsoft KB Archive/58747

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Do Not Use DOS 3.x Box with OS/2 on LAN Man 2.0 Server

Article ID: 58747

Article Last Modified on 9/30/2003


  • Microsoft LAN Manager 2.0 Standard Edition
  • Microsoft LAN Manager 2.1 Standard Edition
  • Microsoft LAN Manager 2.1a
  • Microsoft LAN Manager 2.2 Standard Edition

This article was previously published under Q58747


Due to timing delays associated with MS-DOS 3.x compatibility box support under OS/2 versions 1.1, 1.2 and 1.3, OS/2 LAN Manager version 2.0, 2.1, 2.1a, and 2.2 servers should not be configured with an MS-DOS 3.x box.

While MS-DOS 3.x box usage on lightly loaded OS/2 LAN Manager servers may not cause any problems, the MS-DOS 3.x compatibility box is not supported. This is true especially for heavily loaded OS/2 LAN Manager servers, as well as servers that are configured with SQL Server or Communications Server products.


As a multitasking operating system, OS/2 performs "context switching" to allow multiple applications to run at once, where each application receives its own time slice. OS/2 and OS/2-based applications operate in Intel 80x86 protected mode, as opposed to real mode used by MS-DOS.

Under OS/2 1.1, 1.2, and 1.3, the MS-DOS 3.x compatibility box is supported using real-mode operation when it is in the foreground, while protected-mode applications that are in the background continue to receive time slices. However, the converse is not true: when the MS-DOS 3.x box screen group is in the background, all 3.x box execution is suspended, including interrupts.

Because mode switching between protected and real modes is a slow process and leaves interrupts disabled for almost 1 millisecond (on the Intel 80286 chip), mode switching can cause interrupt overruns on fast devices such as serial ports and communication boards. This timing delay is much more pronounced on an 80286, since the 80286 chip doesn't support an instruction to switch from protected to real mode, but does support a real-to-protected-mode switch. The 80386 chip supports both of these instructions, which OS/2 uses, so the delay is much smaller on the 80386 chip.

Additional query words: 2.00 2.10 2.10a 2.20 dos box

Keywords: KB58747