Microsoft KB Archive/68644

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DOCERR: Segment and Handle Limits and Protected Mode Windows

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Q68644

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1

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SUMMARY
Page 16-25 of the &quot;Microsoft Windows Software Development Kit Guide to Programming&quot; for Windows 3.0 and page 350 of the &quot;Windows Software Development Kit Guide to Programming&quot; for Windows 3.1 state:

"Under the Windows standard mode and 386 enhanced mode memory configurations, there is a system-wide limit of 8192 global memory handles, only some of which are available to any given application." With regard to standard mode, this statement is incorrect. The limit is 4096 global memory handles when Microsoft Windows runs in standard mode and 8192 when Windows runs in enhanced mode. Standard mode uses two LDT entries for each memory handle while enhanced mode uses one LDT entry for each memory handle.

MORE INFORMATION
Each memory block has an associated handle that Windows and applications running in the Windows environment use to access the block. The Intel 80286 or higher microprocessor maintains records of each segment using a descriptor, which describes the attributes of each memory block such as its base address, size, attributes (read, write, execute), and privilege level. In enhanced mode, each memory block has one handle and at least one descriptor. In standard mode, each memory block has at least two descriptors. Because Windows and its applications run in 16-bit protected mode, global memory blocks larger than 64K (the maximum segment size in 16- bit protected mode) use several descriptors.

The architecture of the Intel CPUs requires that each descriptor table fit into its own 64K segment. Because each descriptor is 8 bytes long, each descriptor table can hold a maximum of 8192 descriptors (64K divided by 8 bytes equals 8K). Therefore, each descriptor table can manage up to 8192 segments. For more information, see the &quot;386 DX Microprocessor Programmer's Reference Manual,&quot; (Intel Corporation).

Windows creates a global descriptor table (GDT) for the system, and one local descriptor table (LDT) for itself and all applications running under Windows. Because all applications use the same LDT, a maximum of 8192 descriptors are available to Windows and applications. Enhanced mode can manage a maximum of 8192 segments because each segment requires one descriptor. Standard mode can manage a maximum of 4096 segments because each segment requires two descriptors. Because descriptors are used for code, data, and resource segments, the maximum number of descriptors available for applications to use for global memory blocks is considerably less than 8192 and 4096.

Windows does not provide a way to determine how many selectors are available for a given application to use. All Windows API functions fail when there is not enough free memory to satisfy a given request, or when there are not enough descriptors available. Either way, Windows is unable to satisfy the application's memory request. It is important that selectors be used with deliberation because each Windows-based application must cooperate with other Windows-based applications. Instead of allocating many small blocks from the global heap, an application should allocate fewer blocks that are larger in size. The application can then divide each larger memory block into pieces for its own use.

If small memory blocks are required by the application, use the local memory-management routines provided by Windows. A local handle does not impact the selector limit at all because a local memory block is allocated inside a global memory block. For example, the LocalAlloc function allocates a memory block from an application's or a dynamic-link library's (DLL's) local heap.

In some circumstances, it may be advantageous to employ a more sophisticated memory-management scheme called multiple FAR heaps. This technique is useful if the application requires a number of blocks of memory, say 100, and the total number of bytes in each block is not above 64K. Chapter 18 (pages 602-617) of &quot;Windows 3.1 Power Programming Techniques&quot;, 2nd edition by Peter Norton and Paul Yao, contains more details on this technique. In this chapter, Norton and Yao state the steps necessary to perform local heap allocation in a dynamically allocated segment. This chapter effectively describes the technique of using multiple FAR heaps and also provides some sample code.

This technique involves four steps:


 * 1) Globally allocate a block of memory with GlobalAlloc.
 * 2) Lock the block of memory with GlobalLock.
 * 3) Initialize the block of memory by calling LocalInit.
 * 4) Modify the local memory-management routines so each updates the DS register to point to the new heap just before a call to a local memory- management routine. When access to the new heap is complete, immediately restore the DS register.

Overall, this technique uses fewer global handles and less memory overhead with each call.

Additional query words: 3.00 no32bit 3.10 docerr

Keywords : kb16bitonly

Issue type :

Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310