MEMORY PROTECTION SYSTEM

PURPOSE:To ensure protection of a memory not only in a mapping mode but also in a direct mode by latching an effective ring level RLE and comparing it with a reference ring level RLR. CONSTITUTION:A selection circuit 10 selects the RLR of a latch circuit 7 and the RLE of a latch circuit 6 when a mode flag 12 in a program state PSW1 designates a mapping mode and a direct mode respectively to use it as the comparison input at one side of a comparator 9. The other comparison input of the comparator 9 is always equal to the RLE of the circuit 6. Therefore, access is given to a main memory with no limit in case RLE=0 is satisfied in the mapping mode. The access of the main memory is complied with access control information AR in case of RLERLR. While in the direct mode the conditions are always secured for RLE=RLR and the memory protection is limited in accordance with the access information AR.