Those ones are x86 Compatible 64 bit Processors. I follow a bit Wikipedia
AMD64 was designed as Extension to x86. 64Bit Processors are able to run 32
and even 16 bit. For that the Processor changes his Run Mode to
Legacy Mode: In that he can run all "older" Runmodes (Real-Mode,
Proceted-Mode, System Management Mode).
Long Mode:
64 Bit Mode: Used for 64 Bit applications on a 64 Bit OS
Compatibility Mode: Used for emulating 32 Bit Applications in 64 Bit Systems
(the Applications sees a Protected-Mode).
http://en.wikipedia.org/wiki/X86-64
