Very early intel "EM64T" chips (aka amd64 compatible) had too short virtual address size of 36bit instead of 40, which is why Windows 64bit didn't run on them,

Very early intel "EM64T" chips (aka amd64 compatible) had too short virtual address size of 36bit instead of 40, which is why Windows 64bit didn't run on them, but some linux versions did. Rest is well explained by sibling posts :)