PURPOSE: To shorten an accessing time, by switching an addition value to a specific value when a virtual computer accesses to the specific area of a main memory in a real computer.

CONSTITUTION: When a flip-flop 4 is set at '0', namely, when an address set at a main memory address register 1 is the one that does not access to a hardware, a selector 5 selects a virtual main memory base register 3. In other words, the value of the virtual main memory base register 3 is added on the output of the main memory address register 1 at this time. Meanwhile, at the time of being accessed to the hardware area, it follows that the selector 5 selects a value '0' of a fixed value generation circuit 6 by setting the value of the flip-flop 4 at '1', and the address to be used for the accessing to a main memory device on the real computer always goes to the value set at the main memory address register 1 regardless the value of the virtual main memory base register 3.