Abstract: An extended memory system includes a processor, a plurality of input/output devices, a real memory having first and second portions thereof, the first portion storing a system operation program and the second portion storing a plurality of user programs. The processor produces logical addresses when executing user programs or input/output routines, which logical addresses are translated to real memory addresses by one of two first translation memories. System operation instructions common to all user programs are stored in a low order address portion of the real memory. Different user programs are mapped into mutually exclusive portions of the real memory by means of the two translation memories.