The Pentium patents have expired too, and if we go by the usual 20-year expiry, then that would mean everything up to the Pentium III (1999) would be in the public domain. IANAL.
Isn't the point of patents that instead of ideas being kept secret (and often vanishing when their inventors die) they are published in exchange for a limited time monopoly on their use? In that case when a patent expires it does indeed become public domain.
Sure, Intel didn't have any serious patents on the 8080 but they did copyright the assembly language mnemonics. So the Z80 was 100% compatible at the binary level but had to use other names for the instructions. Same thing with trademarks.
What I was saying is that what is described in the patent text can be freely used after the patent expires.