Peter,

Erik gave you some useful reference which you should read.

Additional Source material can be found at:

http://developer.intel.com/design/mc..._mcs51.htm

Carefully examine the 8051 Memory Mapping especially the "Internal RAM" "Internal SFR's" and "External Data Memory" maps. Once you have this down you will find the difference between opcodes that specify "External Data Memory" and "Internal Resources" are pretty easy to keep track of.

regards,
p