4.3 Maximize CPU Compatibility

UEFI Drivers should be designed to maximize source code portability since it is possible to write a single UEFI Driver that compiles on all CPU architectures supported by the UEFI Specification. The list of supported CPU architectures may grow over time, so it is important to follow these portability guidelines.

The guidelines presented here apply to all CPU architectures. Chapter 28 covers portability issues specific to IPF platforms, and Chapter 29 covers portability issues that are specific to EBC.

When porting between CPU architectures, most developers take as much existing code as possible and reuse it. Unfortunately, some developers porting code do not rigorously follow the UEFI conventions, such as using only the data types defined in the Calling Conventions section of the UEFI Specification. Others may not follow best coding practices.

  • Use data types defined by the Calling Conventions section of the UEFI Specification.

  • Use compiler flag settings to guarantee that the UEFI calling conventions for the CPU architecture are followed. See the Calling Conventions section of the UEFI Specification for details.

  • If a UEFI driver contains assembly language sources, then either the source needs to be ported or it needs to be converted to C language source. Conversion to C language source is recommended. The EDK II library BaseLib, and other EDK II libraries, provide functions that may reduce, or even eliminate, the need to assembly code in UEFI Drivers.

TIP: Implement UEFI Drivers in C to maximize portability,

  • Avoid use of C++. It is not supported by EBC.

  • Avoid unaligned data accesses. Compilers, by default, generate code and data that perform aligned accesses. Unaligned data accessed are generated when features such as byte-packed structures, type casting pointers, or assembly language are used. Aligned data accesses typically execute faster than unaligned data accesses. Parsing UEFI Device Paths is a common generator of unaligned data accesses. These generate alignment faults on IPF platforms.

  • The best approach to debugging a UEFI Driver ported to a differing CPU architecture is to keep a good code base with every revision. This allows comparison with earlier revisions to see the source code before and after the problem became visible.

  • If source code is not available, the CPU register state may not be sufficient to debug a specific issue. Keep in mind that a "new" problem might have nothing to do with a recent change to the code. A pre-existing problem might not have shown up before for a variety of reasons. For example, the current developer might have included error checking or exercised the error handling registers after making an addition to the code-error checking that might not have been done before. Or a new addition might make the pre-existing problem worse, so the problem finally becomes visible in the new revision.

  • Perform a minimal port first to test simple parts of the UEFI driver. This is simply good porting practices, but even experienced developers can forget to port and test the simple things first. Start with a known-good sample driver that is extremely simple. For example, a driver that prints "Hello World". Then divide the code into sections. Begin inserting and testing the less complicated sections into the known-good driver, one section at a time. Another technique is to replace more complex code with "neutered" code that returns but doesn't actually do anything. Make sure the simple sections work and do not cause alignment faults or other errors. Only then should the more complicated sections be added and adapted to the new architecture rules. This approach can significantly cut down on debug time.