1. In a data processing system having two processor units arranged such that a first one of such processor units is substantially idle while the second one of the processor units is executing instructions, the improvement comprising: a. the first processor unit including means for detecting the beginning of an interval during which the second processor will be executing an instruction and in which the first processor will be idle; and b. means for selecting a diagnostic operation to be run in said first processor while the second processor is executing said instruction, the diagnostic operation being selected from among a plurality of available diagnostic operations and being selected such that the first processor will finish executing the diagnostic operation before the second processor finishes executing said instruction.