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Sandeep.S's GCC-Inline-Assembly-HOWTO is a nice introduction to writing inline assembly in gcc. Unfortunately, it does not mention the + (plus) constraint modifier, and it is wrong about the semantics of asm volatile (see Warning box above).
http://kernel.org/doc/Documentation/volatile-considered-harmful.txtVolatile Considered Harmful in the Linux Documentation describes why data should not be marked volatile.
A collection of posts by Linus on inline assembly in gcc that Norman Yarvin found interesting. The ones from 2007 are quite relevant to this document (some are also cited individually here).
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