[cwyers]

All 16 and 32 bit x86 code is valid on x64. It's an extension, not a new ISA.

[geezerjay]

You got it all backwards. AMD64 is a complete revamp which happens to support x86 in its legacy mode.

Server people care for 64-bit address spaces, and that's a feature introduced by AMD64 which is not available in x86.