1. A microprocessor system comprising: a read-only memory (ROM) storing microcode; a random access memory (RAM) storing microcode and one or more data bits for the selection of corresponding ROM or RAM microcode for execution; a multiplexer coupled to receive as input signals the ROM microcode and RAM microcode, and having its control port coupled to receive said one or more RAM data bits, and providing an output signal selected from the RAM or ROM input signals as determined by said one or more RAM data bits.