official gnu as reference for x86 instructions - specifically cmpxchg

[anand akhare]

Hello gcc team,
    my question is quite basic. I am looking for exhaustive complete reference for x86 instructions in gnu assembly. I searched a bit but most of my searches are pointing outside gnu.org. I am specifically looking for cmpxchg and atomic instructions related reference. I could not find anything definitive there or I am might be missing obvious.
    I do not see separate email alias for gnu assembler (gas) so, I think gcc might be handling questions on gnu assembler.
Pls provide any link reference which is official reference gnu assembler instruction for x86,

Regards
Anand