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Oh, and another almost-mandatory compiler extension to C and derivatives that's not properly described by the relevant standards : inline assembly. As awful as GCC's syntax for it can be, it's often much better than keeping separate .s files and writing headers for them, because...
-Doing so is overkill for those assembly snippets of less than 10 lines that form most of CPU-specific code.
-When you're writing ASM, you don't write portable code anyway.
-It's better for code clarity.
-Except for that class of tiny CPU-specific code chunks, ASM is often used for high-performance code, that kind of code where even a CALL's penalty can be too much... Edited 2010-10-08 17:00 UTC
Assembly place is on .s files.
I for one vote for not having inline assembly.
I don't see any problem with having to write a few external functions.