Hi,
You can use external memory along with internal memory. External memory may be used for holding programs or data. 8051 has two banks of 64K each of external address space for program and data memory.
For reading of external Prg.Mem, you use PSEN signal. For Rd/Wr of external data memory you use RD/WR signal lines. Address lines are commonly used.
When you have only internal prog.memory or,both internal and external program memory, you make EA- pin high. In this case the chip will only access the external memory by outputting PSEN, only when the program branches to an address higher than that of internal memory. For a system with only external memory, you make EA- line always low.
As far as fixed data is concerned, you can keep it in the code memory, internally or externally and access it using MOVC instruction. Remember, externally, MOVC only generates PSEN signal and MOVX, the RD/WR signals.
If you use an ORed signal of PSEN+RD and connect to the OE- signal of an external Read/Writeable memory, you can have a system with on-the-fly program loading facility.
Regards,
Laktronics