Microsoft KB Archive/75497

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Writing Embedded Applications for Microsoft Windows

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Q75497

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The information in this article applies to:


 * Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.0, 3.1

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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

 * 1) 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.
 * 1) 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 &quot;replaceable,&quot; which may or may not be the case).
 * 1) 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.
 * 1) 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.
 * 1) 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.

Additional query words: 3.00 3.10 DDKMISC

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