As far I've understood, DATA (128 first bytes of RAM) contains the variables, IDATA are the 128 following and are containing the bit RAM & registers, and the 768 following are XDATA for general purpose.

No,no, no you haven't go it.
Ok here it is:
It is all explained (with pretty pictures) in the 3 80C51_FAM documents found at:
http://www.semiconductors.philips.com/products/...other.html
first 4 pages of "family architecture" tell the whole story.

If reading this creates questions, come back

Erik