Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!
This page lists publically available datasheets and programming guides from various vendors. It includes CPU, chipset, Super I/O, and many other datasheets.
AMD provides their AMD64 Architecture and Software manuals, which are a very useful reference for AMD systems, of course (but also explain lots of non-AMD-specific x86 stuff too):
Other AMD documents:
Intel provides their Intel® 64 and IA-32 Architectures Software Developer's Manuals which are a very useful reference to the x86 architecture in general:
Turion64:
Athlon64:
Opteron:
Phenom:
Athlon64/Opteron:
Opteron:
Family 0Fh:
You can use this page to search for specification datasheets. If it doesn't give you result, then use Google to search them.
Mobile:
Embedded:
Xeon:
Not public (yet?). However, some of the information may be contained in the public docs in the RS880 section.
Apparently there are other docs as well, not sure if they provide any additional information or if they are part of the public documents above:
VIA has a portal with some source code and datasheets.
The recent AMD southbridges starting from SB710 have an embedded controller called AMD IMC
(bought by Winbond, now Nuvoton)
There are many datasheet archives on the web which may have other (usually older) datasheets which already disappeared from vendor websites.