The 'Architecture' page says:
"the 8051 has three memory spaces, 256 bytes of internal RAM, and two 64k byte external spaces, one for data and the other for executable code."
http://www.csee.umbc.edu/courses/u...index.html

The 8051 has only 128 bytes "internal" RAM - the 8052 expanded it up to 256
http://www.csee.umbc.edu/courses/u...dware.html

There are actually four distinct memory spaces:
1. The "intenal" RAM;
2. The SFRs;
3. The "XDATA" Space;
4. The "Code" Space (which can contain both executable code and constant data).