The security lock ... prevents a programmer from reading the memory. In addition (8051-family), the security lock prevents the microcontroller from executing code on the expanded bus of Ports 0 and 2. Thus an attacker cannot add a memory and use MOVC instructions to would force the microcontroller to read out the contents of protected memory ...........
If a knowledgeable user were to physically erase the security bit in an EPROM-based microcontroller, the memory contents would remain to be read.