Using Huge Pointers with Blocks of Memory Larger Than 64K (79194)
The information in this article applies to:
- Microsoft Windows Software Development Kit (SDK) 3.0
- Microsoft Windows Software Development Kit (SDK) 3.1
This article was previously published under Q79194 SUMMARY
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.
| Modification Type: | Major | Last Reviewed: | 10/29/2003 |
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| Keywords: | kb16bitonly KB79194 |
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