Windows 3.1 Memory Limits

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Article ID: 84388
This article was previously published under Q84388
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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.

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Article ID: 84388 - Last Review: September 24, 2011 - Revision: 3.0
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