<Li> Kernel mode device drivers must be 64 - bit versions; there is no way to run 32 - bit kernel mode executables within the 64 - bit operating system . User mode device drivers can be either 32 - bit or 64 - bit . </Li> <Li> 16 - bit Windows (Win16) and DOS applications will not run on x86 - 64 versions of Windows due to removal of the virtual DOS machine subsystem (NTVDM) which relied upon the ability to use virtual 8086 mode . Virtual 8086 mode cannot be entered while running in long mode . </Li> <Li> Full implementation of the NX (No Execute) page protection feature . This is also implemented on recent 32 - bit versions of Windows when they are started in PAE mode . </Li> <Li> Instead of FS segment descriptor on x86 versions of the Windows NT family, GS segment descriptor is used to point to two operating system defined structures: Thread Information Block (NT_TIB) in user mode and Processor Control Region (KPCR) in kernel mode . Thus, for example, in user mode GS: 0 is the address of the first member of the Thread Information Block . Maintaining this convention made the x86 - 64 port easier, but required AMD to retain the function of the FS and GS segments in long mode--even though segmented addressing per se is not really used by any modern operating system . </Li>

The 64-bit intel processor called x64 (or x86-64) has how many register