protected mode and long mode protected mode was first introduced in intel providing mechanism protecting resource and assisting multi task os such as unix os and so forth and enhanced in intel processor by extending computing capability to bits from bits eventually becomes part of ia architecture most x 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 x also transit itself from real mode and protected mode thorugh and ack too windows x 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 switches itself out of ia dominated legacy mode to a completely self contained mode which means even though a x 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 x codes without needing additional physical execution units for itself like it found in early itanium processor so ia architecture might or might not have relationship with x x amd or intel ia e and amd amd is a switched out architecture from ia providing compatibility mode to support most ia software and representing itself in the long mode within amd processors it is a new architecture even though most lying on ia architecture ia e is not a mode but an extention of ia or newer revision of ia introducing bit computing capability for its fundemental architecture intel em t is the new brought feature to this kind of processors enabling it for bit computing of course em t is not a mode too but a feature like pae could be enabled to extend the bit computing capability without em t enabled yamhill processor would act like a tradtional pentium after em t enabled bit registers would be visible to applications bit traditional os might need minor fixes to support applications referring bit registers to do the 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 bit registers and stacks and so forth but without needing to compile the os from the scratch like it in amd virtual address might be extended up to bits while physical address up to bits k segments of gib could be spreaded to tib physical space with or without paging because the difference between bit and bit computing is bit width of register rather than the whole system so bit and bit codes is also differentiated by the referenced resgistered without needing a submode such as compatibility mode to support tradtional bit software after em t disabled yamhill processor would act like a northwood pentium