The transition from x86 to x86-64 did not seem like a radical redesign of the architecture. The x86-64 bit chips are still capable of running code written for x

The transition from x86 to x86-64 did not seem like a radical redesign of the architecture. The x86-64 bit chips are still capable of running code written for x86 chips without modification (even the 16 bit code I believe). Furthermore, the programming model (as exposed to assembly) did not seem to change between the two architectures.