hi,

If I have a register in a peripheral IC mapped into my XDATA space I'm not going to kid myself that it's external RAM

Well, you are right - from point of schematic view it is separate IC, really: it is different chip, it is not physical RAM but latch, etc... But from program model it is XRAM. Even with programming you will use:

unsigned char xdata that_register _at_ 0x1234;

So your example is not good one. As I already said: this question may not be answered without additional conditions. If we talk about hardware then that peripheral register is different memory; if we talk about program model then it is XDATA, i.e. indirect accessed memory.

About registers, you said:
No we don't, because this area is plain old RAM. Some of it just happens to be accessible in special ways.

Oh, well, so SFRs are just a direct accessed RAM but happens to be accessible in special ways. Comments?

Finally, I try again: it is useless task to calculate the number of memory types as long as we do not define the conditions by which we determine these types.

Regards,
Oleg