The AMD64 psABI used to be hosted at x86-64.org.
I have a copy of pdf file and it says explicitly:
The architecture specification is available on the web at http://www.x86-64.org/documentation.
but http://www.x86-64.org is down for a long time already. Like a couple of months at least.
WHOIS points to AMD as the owner of the domain.
Has the authoritative document been moved to a better location, or we just have to wait until AMD sets up their server back again?