signal from the circuit 57 the operation of the device shown in fig7 will now be described it is assumed that the switch 6 is on a reference condition for trigger comparison is preliminarily set for the trigger comparison circuit 33 upon supply of the start signal to the control circuit 4 the storing operation is started however unless the stack pointer changing instruction or response to interrupt exists the storing prohibition circuit 58 delivers the storing prohibition signal s 58 and hence neither a write pulse s 4a nor a clock signal s 4b is delivered from the control circuit 4 and accordingly storing is not carried out to the trace memory 31 when either a stack pointer changing instruction is executed or a response to an interrupt is generated the stack pointer changing instruction execution or interrupt response generation detection circuit 56 delivers a stack pointer changing instruction and interrupt detection signal s 56 and hence the storing prohibition signal generation circuit 58 stops delivery of the storing prohibition signal then a writing in pulse s 4a and a clock signal s 4b are delivered from the control circuit 4 and the state of the buses during the execution of the stack pointer changing instruction or response to interrupt is stored in the trace.