Memory system and method for protecting the contents of a ROM type memory

A memory system for protecting the contents of a Read-Only Memory ("ROM"), the memory system comprising:

• a ROM having an address input and a data output;
• address modifier means adapted for coupling to an external address and coupled to the address input of the ROM for applying an internal address to the ROM;
• and
• improper access detector means adapted for coupling to the external address and coupled to the address modifier means for selectively generating an alarm signal and applying the alarm signal to the address modifier means so that the address modifier means modifies the external address and applies an internal address to the address input of the ROM so that erroneous data appears at the data output of the ROM, said improper access detector means operable to generate an alarm signal in response to each one of the following possible conditions:
• (A) A program loop is not executed a predetermined known number of times;
• (B) A block of data is accessed, which block of data is of a size different from a predetermined known size;
• and
• (C) A predetermined known external address is applied to the memory system and the sequence in which the predetermined known address is applied is improper.

In a memory system that includes a Read-Only Memory ("ROM") that has an address input and a data output, a method of protecting the contents of the ROM comprising the steps of:

• (1) generating an alarm signal in response to each one of the following possible conditions:
• (A) A program loop is not executed a predetermined known number of times;
• (B) A block of data is accessed, which block of data is of a size different from a predetermined known size, and
• (C) A predetermined known external address is applied to the memory system and the sequence in which the predetermined known address is applied is improper;
• (2) modifying an external address in response to the alarm signal;
• and
• (3) applying the modified external address to the address input of the ROM so as to cause erroneous data to appear at the data output of the ROM.

A memory system for protecting the contents of a Read-only type Memory (ROM), the memory system comprising:

• a ROM having an address input and a data output;
• address modifier means coupled to an external address and coupled to said address input of said ROM for applying an internal address to said ROM that is correspondingly modified for each external address depending upon whether each address including at least one address is in a proper addressing sequence or in an improper addressing sequence;
• and
• detector means coupled to the external address and coupled to said address modifier means for detecting an improper address including said at least one address that is not in a proper sequence by monitoring each address including said at least one address to determine if each address including said at least one address is in a proper sequence and for selectively generating an alarm signal, when each address including said at least one address is not in a proper sequence, to the address modifier means which modifies the external address and applies a modified address to the address input of said ROM so that erroneous data appears at the data output of said ROM.