Protected Mode and Long Mode Protected Mode was first introduced in Intel 80286 providing mechanism protecting resource and assisting multi task OS such as UNIX OS 2 and so forth and enhanced in Intel 80386 processor by extending computing capability to 32 bits from 16 bits eventually becomes part of IA 32 architecture Most x86 OS only running within protectected mode such as Windows NT Linux and so many BSD distros but MS DOS working under real mode some extender might switch through and back from protected mode for extending physical memory addressing likewise Windows 3 x also transit itself from real mode and protected mode thorugh and ack too Windows 9x not only depends the real mode as bootstrap but also sits itself from and to that mode Long Mode is a kind of protected mode not a real mode but does not belong to the Protected Mode in IA 32 switches itself out of IA 32 dominated Legacy Mode to a completely self contained mode which means even though a x64 processor eliminates its legacy part it would be fulfunctioned From the aspect of implementation or Microarchitecture level the legacy mode codes could share the same physical execution resources designed for the x64 codes without needing additional physical execution units for itself like it found in early Itanium processor So IA 32 architecture might or might not have relationship with x86 64 x64 AMD64 or Intel64 IA32e and AMD64 AMD64 is a switched out architecture from IA 32 providing compatibility mode to support most IA 32 software and representing itself in the Long Mode within AMD64 processors It is a new architecture even though most lying on IA 32 architecture IA32e is not a mode but an extention of IA 32 or newer revision of IA 32 introducing 64 bit computing capability for its fundemental architecture Intel EM64 T is the new brought feature to this kind of processors enabling it for 64 bit computing Of course EM64 T is not a mode too but a feature like PAE could be enabled to extend the 64 bit computing capability Without EM64 T enabled Yamhill processor would act like a tradtional Pentium 4 After EM64 T enabled 64 bit registers would be visible to applications 32 bit traditional OS might need minor fixes to support applications referring 64 bit registers to do the 64 bit computings but programmers must take care of the computing results without intervented by other processes But most likely as it in PAE OS might need a patch to support this feature totally at least to manage the extended descriptors and extended 64 bit registers and stacks and so forth but without needing to compile the OS from the scratch like it in AMD64 Virtual Address might be extended up to 52 bits while physical address up to 46 bits 16 K segments of 4GiB could be spreaded to 64TiB physical space with or without paging Because the difference between 32bit and 64bit computing is bit width of register rather than the whole system so 32 bit and 64 bit codes is also differentiated by the referenced resgistered without needing a submode such as compatibility mode to support tradtional 32 bit software After EM64 T disabled Yamhill processor would act like a Northwood Pentium 4