I've always had the Intel manual but I was wondering if anyone knew if it made any difference which one you read for the first time? AMD has 3DNow and their timing is different as far as instruction execution goes. So if you read the AMD manual could it mess you up while programming a P4? Or vice versa?
Yes, there are differences in optimization.. some small, others important.
IMHO you should read expecially the manual of the CPU you want to optimize for.
And also remember that the P6 (Pentium Pro, II and III) and Pentium 4 are very different each other.