Q84388: Windows 3.1 Memory Limits
Article: Q84388
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.1,3.11
Operating System(s):
Keyword(s): win31
Last Modified: 21-SEP-1999
3.10 3.11
WINDOWS
kbusage
-------------------------------------------------------------------------------
The information in this article applies to:
- Microsoft Windows versions 3.1, 3.11
-------------------------------------------------------------------------------
SUMMARY
=======
This article provides a brief explanation and overview of memory allocation and
limits in Microsoft Windows version 3.1. This article is not intended to be a
exhaustive explanation of the subject; it is intended to be an overview only.
This information applies to Microsoft Windows version 3.1 and may not apply to
future versions of Windows.
MORE INFORMATION
================
Microsoft Windows version 3.1, in both standard and 386 enhanced mode, allocates
the memory that it uses from the XMS driver. The only exception occurs when you
run Windows in standard mode because standard mode Windows can allocate memory
from a Virtual Control Program Interface (VCPI) provider (server) or a DOS
Protected Mode Interface (DPMI) provider. Please note that standard mode has
this capability, but 386 enhanced mode does not.
Memory Limits Imposed by Memory Managers
----------------------------------------
The XMS driver shipped with Windows 3.1 is HIMEM.SYS version 3.07, but any other
XMS driver that conforms to the XMS 2.x or 3.x XMS provider specifications
should work just as well. HIMEM.SYS XMS 2.x drivers shipped with Windows 3.0 and
MS-DOS 5.0 impose a 16 MB memory limit and do not make memory above the 16 MB
bus address available through XMS memory calls.
If Windows 3.1 is run with an earlier HIMEM.SYS XMS driver from Windows 3.0 or
MS-DOS 5.0, then Windows 3.1 cannot use memory above 16 MB. You must upgrade the
XMS driver to the new one shipped with Windows 3.1 to eliminate this 16 MB
barrier.
Windows 3.0 does not support memory above 16 MB limitation in both standard and
enhanced mode, and relies on the XMS driver to enforce this limit. This 16 MB
limit does not apply to Windows 3.1.
Memory Limits Imposed by Windows 3.1
------------------------------------
The memory limit for Windows 3.1 has been reported as 512 MB. This limit, while
technically correct, does need some qualification. This limit applies only to
Windows running in standard mode. The limit for 386 enhanced mode is 256 MB.
This number is a sum of both physical and virtual memory. The stated 512 MB
limit of standard mode Windows is only theoretically possible and is not
practical. In practice, the limit on standard mode Windows is the same as the
386 enhanced mode limit of 256 MB. This information does not apply to computers
using a 80286 processor. This processor is physically limited to accessing only
16 MB.
Windows 3.1 does not use the extended XMS 3.0 capabilities of an XMS 3.0 driver.
This does not place any limit on the ability of Windows to allocate available
XMS memory. The only additional capability provided by an XMS 3.0 memory manager
is the ability to allocate and manipulate a single XMS memory block larger than
64 MB. This does NOT place a 64 MB limit on operating systems or applications
that use XMS 2.x calls to allocate memory from an XMS 3.0 driver. The result is,
an XMS 2.x allocator has to allocate multiple blocks of XMS memory, each not
greater than 64 MB, to access all of the memory on machines with more than 64 MB
of memory. Windows does this in both standard and 386 enhanced mode.
The reason the 512 MB limit for standard mode is not practical can best be
explained by looking at where this number actually comes from. On the 286 and
386 CPUs there is a system object that is called the local descriptor table
(LDT). This data structure is used to set up the addressing of almost all of the
Windows usable memory in the system. This table has room for 8096 descriptors.
Each of these descriptors, in turn, can address up to 64K of memory. The 512 MB
value is determined by multiplying the size of the number of possible
descriptors by each descriptors maximum size.
8096k * 64K = 512 MB
To reach 512 MB, every descriptor in the LDT has to be the maximum possible size
of 64K. In a typical Windows system, the average descriptor size is much
smaller. The average descriptor size is probably 32K. This yields a limit half
as big: 256 MB.
The 256 MB limit of 386 enhanced mode Windows 3.1 is a limit of total physical
memory and total virtual memory. Normally the total amount of virtual memory is
four times as much as the total amount of physical memory that Windows can
access. Because both of these items are limited to 256 MB, the following is
generally true:
Total Physical Usual Virtual Actual Virtual
Memory in MB Memory in MB Memory in MB
-------------- ------------- --------------
4 16 16
8 32 32
16 64 64
32 128 128
64 256 256
128 512 256
256 1024 256
The actual virtual memory size stays at 256 MB once you reach 64 MB of total
physical memory, and never gets any bigger.
Note that in the preceding paragraph, physical memory is referred to as "total
physical memory." This is because this 256 MB limit is purely a limit on the
total AMOUNT of physical memory. This limit has nothing to do with the location
on the machine bus of the memory (the memory's address). This distinction is
important because on many "large memory" machines, the physical memory is not
all in one contiguous block.
This means that for Windows 3.1 running in standard mode, the limit for the
maximum usable physical memory address is 4096 MB; the limit for 386 enhanced
mode is 2044 MB.
Additional query words: 3.10 3.11 HIMEM 64Mb 512Mb 256Mb AST power premium eisa ram
======================================================================
Keywords : win31
Technology : kbWin3xSearch kbZNotKeyword3 kbWin310 kbWin311
Version : WINDOWS:3.1,3.11
=============================================================================
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.
Copyright Microsoft Corporation 1986-2002.