Protection of software programs stored in read-only memory from unauthorized access

A controller comprising:

• a microprocessor;
• a read-only memory for storing scrambled program data words;
• a random-access memory;
• a data bus for transporting information signals between said microprocessor and said read-only memory and said random-access memory;
• an address bus for transporting address signals from said microprocessor to said read-only memory and said random-access memory;
• and
• a data converter connected in said data bus for descrambling data words when said read-only memory is accessed through said address bus to produce a replica of original program data words and applying said replica to said microprocessor for operating said controller, and applying data words to said microprocessor without descrambling the data words when said random-access memory is accessed through said address bus.

A method for protecting a software program stored in a read-only memory from unauthorized access, comprising the steps of:

• a) scrambling an original software program with a keyword;
• b) storing the scrambled software program into the read-only memory;
• c) reading the scrambled software program from the read-only memory;
• and
• d) descrambling the read software program with a replica of said keyword to produce a replica of said original software program, and supplying the replica of the original software program directly to a microprocessor.

A method for protecting a software program stored in a read-only memory from unauthorized access, comprising the steps of:

• a) dividing an original software program into a plurality of data blocks;
• b) scrambling each of the data blocks with a keyword corresponding to the data block;
• c) storing each of the scrambled data blocks into the read-only memory;
• d) reading each of the scrambled data blocks from the read-only memory;
• e) generating a keyword corresponding to the read data block;
• and
• descrambling the read data block with the generated keyword to produce a replica of the data block of said original software program, and supplying said replica of the data block directly to a microprocessor.