INFO: Win32s Translated Pointers Guaranteed for 32K

• Microsoft Win32s versions 1.0, 1.1, 1.2, 1.30, 1.30a, 1.30c

SUMMARY

Translated pointers are guaranteed to be valid only for 32K, rather than 64K, which selectors are usually limited to. This limitation is for performance reasons.

Selectors are tiled every 32K. A 0:32 pointer can be quickly translated into a 16:16 pointer, which will be valid for a minimum of 32K. In other words, the offset portion of the 16:16 pointer is not guaranteed to be 0 (zero) when translated. As a result, even though the translated selectors have a limit of 64K, the offset passed to the 16-bit side may be as large as 32K-1.

Selectors are created on a 32K alignment so that if you pass several pointers to the same range, the Universal Thunk (UT) uses the same selector. Selectors are freed when application terminates.

The alternative is to create a selector for each and every translation, which is very slow.

MORE INFORMATION

For any given address, there are two selectors that point to it, but only one has a limit less than 32K:

           +-------+-------+-------+-------+-------+-------+
           |Selector 2(64K)|Selector 4(64K)|Selector 6(64K)|
   +-------+-------+-------+-------+-------+-------+-------+
   |Selector 1(64K)|Selector 3(64K)|Selector 5(64K)|       |
   +-------+-------+-------+-------+-------+-------+-------+
   |  32K  |  32K  |  32K  |  32K  |  32K  |  32K  |  32K  |
   +-------+-------+-------+-------+-------+-------+-------+

1. Do a GlobalAlloc() on the 32-bit side. Be sure to use GMEM_MOVEABLE.

   If the buffer in more than 64K in size, make sure to type cast the
   return value from GlobalLock() to a huge pointer.

For more information on how to share large amounts of data between the 16-bit and 32-bit side of an Universal Thunk under Win32s, please refer to the following Knowledge Base article: