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Q121008: Microsoft Diagnostic Utility: Definition of SXMS

Article: Q121008
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:5.x,6.0,6.2,6.21,6.22
Operating System(s): 
Keyword(s): msdos
Last Modified: 17-DEC-2000

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The information in this article applies to:

- Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 
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SUMMARY
=======

When you use the Microsoft Diagnostic Utility (MSD) to examine memory when
Windows is not active, you may notice a reference to "Available SXMS" or
"Largest Free SXMS." SXMS is also known as Super XMS or Super Extended Memory.

MORE INFORMATION
================

MS-DOS-based programs running under Microsoft Windows or Microsoft Windows for
Workgroups can access XMS version 2.0 memory only, even if an extended memory
manager (XMM) is providing XMS version 3.0 memory at the MS-DOS level. This is
because WIN386.EXE is responsible for allocating extended memory to programs
running in Windows, and it is designed to implement XMS version 2.0 only.

However, when Windows is not running, these same MS-DOS-based programs can access
XMS version 3.0 memory as provided by an XMM. This version of the Extended
Memory Specification implements changes that are intended to provide support for
extended memory pools of up to 4 gigabytes in size. (The current application
programming interface [API] uses 16-bit values to specify block sizes and
therefore is limited to a maximum block size of 64 megabytes.) This extended
memory provided under XMS version 3.0 is known as SXMS, regardless of whether it
is above or below the previous limit of 64 megabytes.

Support for XMS 3.0 is implemented through extensions to those same functions
that were available under version 2.0. One such extension is to the function
Query Any Free Extended Memory (Function 88h).

This function now uses 32-bit values to return the size of available memory, thus
allowing returns of up to 4 gigabytes. As with XMS 2.0, this function also
returns the highest known physical memory address, or the physical address of
the last byte of memory. Two important points must be made while considering the
values returned by Function 88h:

- If less than 64 MB of RAM is present on the system, Function 88h returns the
  same value for available memory when run under XMS version 2.0 or version
  3.0. As a result, MSD reports the same values for XMS memory in Windows and
  SXMS memory at the MS-DOS level on such a system.

- There may be discontinuities in the memory map below the highest known
  physical memory address returned by Function 88h.

Additional query words:

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Keywords          : msdos 
Technology        : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a
Version           : MS-DOS:5.x,6.0,6.2,6.21,6.22

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