Digital computer with a program-trace facility

Claim 1

In a multi-program data processing system having a memory for storing a plurality of program procedures, the last of which is an exiting operator to effect a return of system control to a prior procedure, said operators within each procedure being transferred out of memory and executed in their stored sequence by the processing system, said plurality of procedures comprising at least one procedure which contains an entering operator to effect a transfer of system control to a new procedure, and further comprising a trace interrupt procedure adapted to record the current status of selected system operating conditions, the method of tracing the flow of any particular program procedure comprising the steps of: storing in memory for each procedure a Program Control Word which indicates if a trace operation is to be performed on its associated procedure, executing a giVen procedure containing an entering operator, entering a new procedure in response to execution of said entering operator in said given procedure, generating and storing a Return Control Word upon entering the new procedure, reading from memory the Program Control Word associated with the procedure being entered, setting a trace control unit ''''off'''' or ''''on'''' in response to the status of the trace operation indicated by the Program Control Word associated with the procedure being entered, setting the control condition of the Return Control Word to indicate if the trace control unit was ''''on'''' or ''''off'''' prior to its setting in response to the Program Control Word of the procedure being entered, sequentially executing the program operators of the new procedure, executing a trace interrupt porcedure upon the completion of execution of each program operator within the new procedure if the trace control unit is ''''on'''', returning from the new procedure back to said given procedure in response to execution of the exiting operator at the end of the new procedure, reading out from memory the Return Control Word associated with said given procedure, and setting the trace control unit ''''off'''' or ''''on'''' in response to the status of the trace operation indicated by the control condition of the Return Control Word.