Abstract: A computer system provides a RESET-signal for resetting registers upon start-up of the system, and includes a central processor unit (CPU) of an optional type and a detect circuit for determining which optional type. On start-up, the CPU sends out an initial memory reference at the highest possible address. The first type of CPU has a memory address register of N bits. The memory address register of the second type of CPU has a memory address register of N+M bits. The upper M bits input to the memory address register of the first type of CPU are grounded. A detect circuit is activated by the CPU addressing memory and receives at least one of the M upper bits. If that bit is not grounded, then the CPU is of the second type and, if grounded, it is of the first type. The RESET-signal upon start-up assures that the detect circuit will be activated on start-up.