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Q79194: Using Huge Pointers with Blocks of Memory Larger Than 64K

Article: Q79194
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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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
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The GlobalAlloc() function can be used to allocate memory blocks larger than 64
kilobytes (64K). This memory can then be accessed using "huge" pointers. For
more information on huge pointers, see your Microsoft C compiler documentation.

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

GlobalAlloc() always returns a single contiguous block of memory, if it is
possible to satisfy the request. This block can be accessed using huge pointers.
Huge pointers are composed of a segment:offset address (real mode), or a
selector:offset address (standard mode or 386 enhanced mode), just like a far
pointer. Huge pointers allow access to more than 64K, by adjusting the segment
or selector part of the address, as well as the offset.

In real mode, the segment address increases or decreases by 0x1000 every time the
offset crosses the 64K boundary. In standard mode or 386 enhanced mode, memory
is addressed using selectors instead of segments. When GlobalAlloc() allocates a
block of memory, it also creates selectors to access that block. If the block of
memory is larger than 64K, GlobalAlloc() will create a set of tiled selectors.
The first selector in the set provides access to the first 64K of the block.
Subsequent selectors provide access to additional 64K portions of the memory
block.

Tiled selectors are allocated in a predefined pattern; the value of the second
selector equals the value of the first selector plus the value of __AHINCR, and
so forth. Therefore, when the offset of a huge pointer crosses a 64K boundary in
standard mode or 386 enhanced mode, the value of the selector is increased or
decreased by the value of __AHINCR.

Additional query words: 3.00 no32bit 3.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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