A relatively fast system control processor, such as an Intel 8051, is substituted for an Intel 8042 microprocessor in a PC/AT type compatible personal computer. In one embodiment of the invention, a System Control Processor Interface (SCPI) is provided between the central processing unit (CPU) and the system control processor (SCP) to maintain compatibility with the PC/AT bus. The combination of the faster SCP and the SCPI interface improves the overall system performance. Control circuitry is also provided for setting the A20 signal relatively quickly to allow memory access above one megabyte. In an alternate embodiment of the invention, a Mouse Keyboard Interface (MKI) is provided. The MKI provides even quicker switching of the Gate A20 signal by eliminating the need to interrupt the SCP. The MKI also provides support for a type PS/2 mouse.