8051 architecture code memory is a read-only memory space in normal circumstances. There is the MOVC instruction to read it and the instruction fetch process to read it.

Some designs put the code memory external and then through hardware gimic they OR the PSEN/ and RD/ lines together to make the OE/ for the external memory. The WR/ goes to the WE- pin of the RAM. Under these special conditions the "new" overlapped code/data space can be read with MOVC or MOVX and written with MOVX.

Also many detivates now have on board programmable FLASH memory. On some of these there are special SFR bits that can be set to permit the device to program its own FLASH. In many of these the MOVX instructions take on a new meaning to support reading and writing the FLASH memory for programming while the SFR bits are set.

Michael Karas