Abstract: A computer system includes a number of storage elements encoded with space selection instructions and at least one current-space storage element that together allow a computer to address a larger number of devices than allowed by a limited number of address terminals on the computer. Specifically, the current-space storage element is encoded with a space-selection signal that indicates a "current" address space, i.e. which one of a number of mutually exclusive address spaces is currently accessed. The space-selection signal can be changed by any device using a data bus to store signals indicative of a new address space as the "current" address space in the current-space storage element. The current-space storage element is included in a first device that can be accessed by the computer at any time by driving a signal active on a high address line of an address bus.