64 bits? Why, back in my day, we had to push our 8 bit computers uphill, 5 miles, in the snow... The CP/M Operating System.

Wikipedia has a great entry on CP/M, including technical info and some links. I am starting to use Wikipedia a lot.
The "Unofficial" CP/M Web Site has lots of technical information, source code and resources for the CP/M operating system.
Gaby’s Homepage for CP/M and Computer History has a wide variety of info on CP/M, and lots of excellent links!
The Computer Chronicles special on Gary Kildall is a must-see.

Vintage Computer Computing Technology Manuals

Vintage Computer Manuals