System critical registers are protected from accidental modification, and Flash self-programming is protected from accidental execution. This is handled globally by the Configuration Change Protection (CPU.CCP) register. Execution of either of these actions is only possible after a signature has been written to the CCP register. After writing the correct signature to the CCP register, the desired action must be executed within four instructions. Interrupts are kept pending during these four cycles.
Refer to the CPU section in the Relevant Devices data sheet for details on CCP.