Abstract: A computer system has a CPU, a main memory and an additional memory storage which is difficult from the main memory and an external storage. The additional storage may transmit data with the main memory and/or the CPU.

Abstract: A data processing apparatus incorporating a function referred to as the scan-out diagnosis function for reading out and checking sequentially internal states of the apparatus. In addition to the address designating means for the scan-out diagnosis function, an address register is provided which is validated in place of the address designating means only when the latter is inoperative, to thereby designate a particular flip-flop. An observing means is provided for observing continuously the state of the particular flip-flop as read out. Thus, the state of the flip-flop designated by the address register can be continually visually observed independently of the address designating means for the scan-out diagnosis function.