A microcontroller provides protection to memory blocks in an embedded memory. A set of rules such as security levels mapped to memory blocks are stored in a nonvolatile supervisory memory. An algorithm for application of the rules is stored in a supervisory ROM. When a read or write operation is to be carried out, the rules are applied according to the algorithm in order to authorize or reject the read or write operation. Security levels can be modified, but only according to defined rules. In one embodiment, the security levels can only be increased.