🔳Two paths to 64-bit CPU

Intel had put a lot of effort in a pure 64-bit architecture called Itanium released first in 2001. x86 was 32-bit architecture which was extended from 16-bit architecture which was extended from 8-bit architecture. There was a lot of backward compatibility and legacy to carry-on, Intel wanted to shed that weight off and if the new architecture catches on no one would come close for many years to come.

AMD on the other hand came up with Athlon-64 in 2003, which was extended from 32b-bit x86, this meant that it would be backward compatible. 32bit apps would run without needing to be recompiled and when OS (e.g. Windows) becomes 64bit native 32-bit apps will still keep running.

Intel has grossly misjudged the power of software, 64-bit native apps porting would take 8 to 10 years, the Itaniums were power hungry, datacenters would not like that.

In 2004 Intel pivoted and came out with EM64T, which was similar to Athlon-64 and began the era of datacenter on x86_64 processors.