Erik,

I was responding to the phrase:
"is not much different from a regular '51".

The '251 is very different from a std '51 architecture but implementation can start from a '51 departure point which has it's implication for risk analysis. When you need to move to 16 memory access you can migrate to the PAGE Mode Architecture.

The 251 is not different in that it can execute std. '51 binaries (much faster) and can be placed into a std '51 socket. These features are very significant in that it allow incrementalism in migration to 16 bit code implementation. 16 bit instructions can be implemented as needed and with critical specific analysis.

If you are starting a new design the processor most suited should be determined. If this is a 16 bit micro-controller/processor model such as the XA,186, ... then so be it.

regards,
p