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).

For the '52 there are 5
1. The direct addressable "intenal" RAM;
2. The indirect addressable "intenal" RAM;
3. The SFRs;
4. The "XDATA" Space;
5. The "Code" Space (which can contain both executable code and constant data).

For many modern derivatives there are 6
1. The direct addressable "intenal" RAM;
2. The indirect addressable "intenal" RAM;
3. The SFRs;
4. The internal "XDATA" Space;
5. The external "XDATA" Space;
6. The "Code" Space (which can contain both executable code and constant data).

Erik