A semiconductor device has a processor, a first memory unit accessed by the processor, a plurality of page memory units obtained by partitioning a second memory unit which is accessible by the processor at a speed higher than the speed at which the first memory unit is accessible such that each of the page memory units has a storage capacity larger than the memory capacity of a line composing a cache memory, a tag adding, to each of the page memory units, tag information indicative of an address value in the first memory unit and priority information indicative of a replacement priority, a tag comparator for comparing, upon receipt of an access request from the processor, the address value in the first memory unit with the tag information held by the tag, and a replacement control unit for replacing the respective contents of the page memory units.