The information in this article applies to:
- Microsoft Windows Device Driver Kit (DDK) for Windows versions 3.0
and 3.1
SUMMARY
In the future, Microsoft Windows and applications developed for Windows are
likely to run in read-only memory (ROM). The text below lists some things
to consider during the process writing software that may be burned into
ROM. Some things are obvious, some are not, and some are specific to system
implementation.
MORE INFORMATION
- Code Segment (CS) Variables
Do not use CS variables. Although the discipline of writing code
for protected mode has eliminated many code segment variables,
numerous modules use the PrestoChangoSelector function, which
causes problems. Never allocate a CS alias unless you have a very
good reason. Read-only tables in CS are acceptable. If each table
is referenced only within the segment in which it is defined, the
code can be patched to correct errors.
If, for very serious performance reasons, a CS variable is
required, the program must copy the entire code segment into RAM.
Try to make the code segment as small as possible.
- Code Patching
Do not expect any dynamic code patching to work properly. (If an
application replaces a device driver, this is acceptable as long as
the device driver is built into the ROM image, or the ROM image
device drivers are "replaceable," which may or may not be the
case).
- Accessing the Executable File
Do not use the AccessResource function or otherwise open the
program's EXE file. When a module is placed in ROM, the application
cannot access it with MS-DOS read or write functions. Always use
the FindResource, LoadResource and LockResource functions to access
resources directly. There is no method to work around this
limitation; if an application uses AccessResource to access a small
part of a large resource, break the resources into smaller
segments.
- Module Paths
Do not load a module with an explicit pathname unless you know it
is not in ROM. The LoadModule function searches the ROM for a
module only if the function call does not contain an explicit path.
- WinFlags
Import the __WinFlags variable only to check the WF_PMODE bit. In a
ROM-based Windows implementation, this address must be part of the
ROM image, therefore, other bits may be incorrect. Use the
GetWinFlags function to retrieve the run time value of WinFlags.
In general, do not import a value from a module using an absolute
address if the value can change at run time. __WinFlags is the only
such value in the Windows core.
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