Where to find gcc abi specs?

If someone points me to where I can find the gcc abi specs - I don't
know if that is the correct name - then I can finish the support for bit
fields in structures and unions.

I'm looking for a document where the implementation specific behaviour
of gcc is specified - depending on the target platform.  How are
bitfields packed, and so on.  x86 architecture is most iunteresting for
me.  A bit difficult to find out by experimantation, I would like to
avoid that.

Thanks,

Thomas



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