I remember around 2000 it was 3 manuals, last time I checked it was 5 or 6
New instructions, new functionality (virtualization) etc
But the old manuals still have a lot of relevant stuff, especially if you want to work at the low-level (like switching to protected mode, page tables, etc)
http://i.imgur.com/odqvETR.jpg
I believe I got all 3 volumes/5 books around 2008, each being hundreds of pages thick, thousands in total. The one I'm missing is the optimization reference, but that's what they look like (not my photo).
And it was free for me, including shipping to Canada. It might be worth a shot to see if they've started it up again.
http://www.intel.co.uk/content/www/uk/en/processors/architec...
And yes, virtually everything remains compatible. So if you want to write a 32 bit code generator and don't need access to newer features (synchronization primitives are the most important bits that were added later, also stuff like vector instructions, etc...) that 30-year-old document is all you need.
And if you give up binary portability and restrict yourself to the source portability which helped sell people on the 8086 and the 8088 to begin with, you can use a manual for an 8080 processor and convert the code to the earliest 16-bit subset, prior to the expansions the 80286 brought.
And if that's too recent for you, you can move to the IBM mainframe series derived from the System/360 architecture, and use layer upon layer of emulation to run software written in the Johnson administration.
Screw diamonds: Legacy systems are forever.
No, thank you