FAQ35

Affected products:  All microcontrollers
Question: Why can the T89C51RD2 not work in ROMless mode?

Answer  : By default, the T89C51RD2 security locks are set
on the maximum security level (level 3) to prevent piracy.
This security level doesn’t allow the MOVC instruction
in external execution, verify using parallel programmer,
external execution.
The Romless mode can be allowed if the lock bits
are programmed either 0, 1 or 2 by a parallel programmer.