ID Number: Q77473
3.00
WINDOWS
Summary:
In Windows version 3.0 standard and enhanced modes, rules for using
far pointers have been relaxed substantially. This is due to the use
of the protected mode of the 80286, 80386, and 80486 processors. In
this mode, far pointers are no longer a segment:offset value.
Protected mode far pointers are made up of a selector:offset value.
The selector is an index into a descriptor table. Each descriptor
contains information about a block of memory, such as size, location
in memory (linear memory if in enhanced mode, physical memory if in
standard mode), and access rights. For more information on the
descriptor table, refer to Chapter 3 of "The 80386/80486 Programming
Guide" by Ross P. Nelson (Microsoft Press).
The protected mode of the 80286, 80386, and 80486 processors allows
memory blocks to be moved in memory without invalidating their
selector:offset value. This is accomplished by changing the reference
to the location in memory in the descriptor rather than changing the
actual selector value.
Because of this functionality, using far pointers in standard and
enhanced modes is much easier. In the following four circumstances,
far pointers to data can be assumed to be valid:
1. Global memory that is allocated with GlobalAlloc as movable and
nondiscardable
The far pointer returned from GlobalLock can be assumed to be valid
as long as the GlobalUnlock function has not been called. Once
GlobalUnlock has been called, the far pointer can no longer be
assumed to be valid because the memory may be discarded. However,
the far pointer will remain valid in the case of memory that is
nondiscardable and is not an automatic data segment. Because of the
selector technology discussed above, movable memory can move
without invalidating the far pointer.
Note: If a block is reallocated using GlobalReAlloc and the new
size requires a different number of selectors (for example 50K
reallocated to 110K or 65K reallocated to 63K), the base selector
value may be changed. Each selector can refer to a maximum of 64K
of memory.
2. Far pointers to static or global data within an application
Static and global data within an application is stored in the
static data area of the application's DGROUP. Because the whole
DGROUP segment is moved, the far pointer will still be valid after
a move in protected mode.
3. Automatic data within a function as long as that function has not
been exited
Far pointers to automatic data will be valid as long as no return
has been executed from the function that allocated the data.
Automatic data is stored on the stack. When an application returns
from a function, the memory allocated by that function is no longer
allocated and cannot be assumed to be valid. This then makes memory
that a far pointer references subject to corruption.
4. Local memory that has been locked with LocalLock
Memory allocated with LocalAlloc comes from the local heap. Memory
on the local heap can be moved around within the DGROUP segment,
which will cause the offset value of its location to change. This
will invalidate any far pointers to the memory. The LocalLock
function fixes the memory within the local heap.
For compatibility with future versions of Windows, far pointers to
data within an application should NEVER be passed to other
applications. The current version of Windows uses one LDT (local
descriptor table) for all descriptors associated with data. In the
future, one LDT may be used for each application's data, while a GDT
(global descriptor table) may be used for shared data. If a
selector:offset combination from one application is used in another
application in such an environment, the selector will be used to
reference a descriptor in the called application's LDT. The descriptor
in the called application's LDT will most likely not contain the
correct reference for the memory block. The only supported way to
share data between applications is global memory allocated with the
GMEM_SHARE (also known as the GMEM_DDESHARE) flag.
Selectors that are aliased, using AllocDStoCSAlias or AllocSelector,
will not be updated when the memory is moved. For this reason, far
pointers that include aliased selectors cannot be assumed to be valid
unless the memory has been fixed into place with GlobalFix.
For a more detailed description of memory management under protected
mode and Windows 3.0, see Chapters 17 and 18 of "Peter Norton's
Windows 3.0 Power Programming Techniques." This book is available from
Bantam Computer Books and is coauthored by Paul Yao.
Additional reference words: 3.00 3.0