[aengelke]

The same site hosts [1], but that's not nearly as nice as the 32-bit version. It's also a bit outdated.

[1]: https://www-user.tu-chemnitz.de/~heha/hs/chm/x86.chm/x64.htm

[fweimer]

Thanks. Looks like the original now has some clarifications, including more detail regarding the REX prefixes: https://wiki.osdev.org/X86-64_Instruction_Encoding

[non_obsolete]

sandpile.org is your friend.