A method, apparatus, system, and signal-bearing medium that in an embodiment detect an event that will cause idle cycles in the processor and issue diagnostic instructions to the processor during the cycles that would be idle. In another embodiment, the processor is periodically interrupted and diagnostic instructions are issued to the processor, where the diagnostic instructions are selected based on a history of activity at the processor and a log of previous errors at the processor. In this way, errors may be detected at the processor without undue cost and impact on performance.