A microprocessor system contains a read-only memory (ROM) for storing programs or firmware. Retrieval and execution of program code is controlled by a microprocessor address bus. Erroneous data in the ROM can be corrected by address comparison and translation. Trap, region, and patch tables are provided to store addresses, regions, and translated addresses. An address issued by the microprocessor is stored in the trap and region tables can be translated for selecting another programmable device, such as a SRAM or DRAM, other than the original ROM. Thus, erroneous code in the ROM can be corrected, inserted, or replaced.