My understanding (from Wikipedia) is that the x64 instruction set supports executing 16-bit protected mode code from long mode, but cannot execute real mode code without being switched out of long mode because long mode lacks virtual 8086 mode. Therefore, it stands to reason that real mode DOS apps can't be run in Win64 w/o software emulation or dynamic translation. However, why was support for Win16 protected-mode apps excluded when support for them seems (at least at first glance) to be reasonably implementable and is included in newer versions of Win32? Was it just a matter of demand not being high enough to justify implementation costs (and the win32 version was already implemented), or is there a good technical reason?