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Extended Asm in the GCC manual describes how to use inline assembly in gcc. Unfortunately, it is not very precise or thorough. When describing the '+' constraint modifier, it says you should not use '+m', but Linus claims this is not true (see Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints).
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).
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http://yarchive.net/comp/linux/gcc_asm.html
A 2007 mailing list post Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints by Linus which describes that "+m" as an asm output constraint is acceptable. This is in conflict with Extended Asm in the GCC manual, which Linus claims needs to be updated.