In the data sheet for the P89V52X2 they describe two code security (CSEC) bits. Bit 1 of the CSEC register is used for protection on internal execution. When programmed, if the internal address space of 8K bytes is exceeded, the address will rollover into internal space (upper address bits are ignored) for MOVC and instruction fetches. MOVC will still access the data EEPROM when the address >= FF00H. Bit 1 of the CSEC register is the general protect bit. When programmed, it prohibits further erasing or programming of code memory. MOVC instructions executed from external code memory are disabled from fetching code bytes from internal code memory.

Michael Karas