This is entirely wrong. Microsoft calls it "amd64" for the same reason that everyone calls amd64: AMD developed it at a time (~1999) that Intel had convinced itself that its 64-bit future was with Itanium, not x86. It was not until Intel's Yamhill project (~2003) that Intel ceded to AMD's ISA lead (and Itanium's relative failure in the market) and... implemented amd64. But by then, every operating system had support for AMD's architecture -- and the ISA's name was not up for renegotiation.

All of this is right, but I was more referring to Microsoft sticking with the name "amd64" when most other platforms have moved to the name x86_64. The reason, according to my former colleagues at Microsoft who worked with davec, was to be petty against Intel. In most of Intel's documentation at the time, they referred to this architecture as "EM64T" rather than admit it came from a competitor

Perhaps someone ex-Intel can comment, but there was a story within Microsoft that Intel proposed a different x86 derived 64-bit architecture after AMD64, but Microsoft didn't want to support two x86-64 ISAs and that put pay to Intel's proposal. There was also talk at Microsoft about having input into the AMD64 design, but no idea what the details were there exactly.

well, it's AMD's architecture, and Intel licenses it.