The long mode 64b instruction set is similar enough to normal 16/32b i386 ISA that droping the 16/32b part will make no significant difference both in terms of

The long mode 64b instruction set is similar enough to normal 16/32b i386 ISA that droping the 16/32b part will make no significant difference both in terms of IP and hardware resources. On similar note, first AMD's 64b chips had some intentional limitations in backward compatibility with i386 ISA (that's why Microsoft says that it is not possible to run 16b programs on 64b windows due to hardware limitations), but newer chips do not have these limitations, probably because any cost savings are insignificant (which given the fact that most of the die area today are caches and not logic makes perfect sense).