This issue came up in the discussion about RTOS, but since it is a more generalized issue, I choose to post it separately.

Many, many moons ago Philips and Intel went into collaboration about coming up with an "advanced 8051" and this led to a basic design of a "16 bit 8051". When Intel insisted on bit compatibility, Philips opted out of the collaboration and went their own way.
Thus we have today the Intel 251 which is bit compatible in 'native mode' and is not much different from a regular '51 but for the 16 bit width.
The Philips XA, however, is a vastly improved architecture and to satisfy the compatibility issue it is "source compatible" with the '51.

I am still using the '51 for a lot of jobs, it is a great little bugger for many things. I do, however, cringe when someone try to cram more into the '51 than what it makes sense to ask it to handle.

One of the designs I have implemented succesfully in some cases is an XA as the master and '51 as slaves in RS-485 systems, this works very well.

Have fun,

Erik