AMD64 designates the 64-bit evolution of the Intel 32-bit x86 architecture. AMD completely switched to 64 bit quite before Intel. And amd64.s is the name of the x86-64bit architecture specific file in OCaml's source code.
More registers available
Inline memory allocation in assembly for better performance
Deprecates the x86-32bit architecture anyway
Probably the most widely available multicore CPU architecture, as new x86-32bit compatible CPUs are all 64bit capable