PURPOSE: To change the processing contents of a main memory by switching a part of address data outputted from a data processor to a code for control with the designation of a microprogram.

CONSTITUTION: A data processor (MPU)1, when a writing address is outputted from a memory address register (MAR) 4 to a main memory control device (MCU)2, inputs a part of the address outputted from the MAR 4 into a selector 5 controlled by a switching signal (S) by a microprogram. The selector 5 outputs a code X for control by the value of the signal S and sets it to a control code register (CR)7. Next, the code X is decoded by a decoder 8, and through a stage control part 11, a series of processings such as the deletion and addition of the stage is executed. Thus, the processing contents are changed corresponding to the memory address with a different access speed.