PURPOSE: To accurately trace a normal program by providing this system with a write control circuit for controlling writing in the 2nd register and a decoding circuit for deciding whether a program being executed at present is an application program or a monitor program.

CONSTITUTION: When a register A READ request signal 7 is inputted, data stored in a register A1 are read out to a data bus 3 by opening a switch A4. On the other hand, a write control circuit 6 operates based upon the signal 7 so that a certain specific bit in a register B2 is changed. When the monitor program is executed by the decoding circuit 8, a write inhibiting circuit in a logic circuit 9 is driven and the opening of a switch B5 is inhibited, so that even if the contents of the register A1 are read out in the monitor program, the data of the register B2 can not be rewritten. Consequently accurate tracing for debugging can be attained.