ID Number: Q58747
2.00
OS/2
Summary:
Due to timing delays associated with MS-DOS 3.x compatibility box
support under OS/2 versions 1.1 and 1.2, OS/2 LAN Manager version 2.0
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.
More Information:
As a multitasking operating system, OS/2 performs "context switching"
to allow multiple applications to run at one time, 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 and 1.2, 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.