Memory Use Limitations in Microsoft Windows (84778)

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In enhanced mode, Windows versions 3.0 and 3.1 can use features of the Intel 386 (and compatible) microprocessors to supplement the installed physical RAM memory with virtual memory in a disk-based swap file. As physical memory fills, Windows sends the least-recently used pages of memory to disk. When this data is later accessed, Windows pages it back in from disk. This activity is transparent to applications in the system.

Windows 3.0 cannot address any memory with an address higher than 100000h (16 MB). Windows 3.1 can address at most 256 MB of physical RAM memory, and the largest swap file Windows 3.1 will ever create is 256 MB. To be used by Windows 3.1, all physical RAM memory must have an address lower than 7FC00000h (2,044 MB).

While the combination of 256 MB of physical RAM memory coupled with a 256 MB swap file provides a theoretical limit of 512 MB, the actual maximum is somewhat less than 512 MB.

The architecture of the Intel 386 processor defines a Local Descriptor Table (LDT), which is used in the memory protection scheme. Windows 3.1 uses one LDT that provides 8192 descriptors. Each descriptor can describe a block of memory containing at most 64 KB. While multiplying the number of descriptors by the maximum amount of memory yields the 512 MB theoretical limit again, Windows will not access this much memory for the following two reasons:

• Windows reserves some descriptors for its own use.
• Most memory allocations that use a descriptor do not allocate the 64K maximum allowed.

Both of these factors reduce the number of available descriptors and the amount of memory that Windows can use.

While the About dialog box in the Program Manager reports an estimate of the amount of memory available, Windows does not maintain sufficient information to accurately report the amount of free memory at any given time. Maintaining more accurate information regarding the amount of free memory would seriously degrade system performance because memory is allocated and freed very frequently.

The values reported by the GetFreeSpace and GlobalCompact functions are less than or equal to the amount of free memory available in the system. Because of their imprecise nature, an application should use the value reported by one of these functions only for an informational display. The application must not assume that it is possible to allocate that much memory.

The largest block of memory that an application can allocate is affected by the degree of memory fragmentation. Memory available in fixed portions of memory may be available for allocation only as a fixed block. For example, consider the case where the GetFreeSpace function returns 100K available, and 64K of the available space is located in the fixed portion of the heap with the remainder in the movable and discardable portion of the heap. An attempt to allocate a 100K block of movable and discardable memory will most likely fail.