Mechanisms are provided in which firmware verifies he entire system's memory scrub coverage through some additional memory controller (MC) registers/attentions and builds up a processor runtime diagnostic (PRD) scrub coverage table during every scrub cycle. Firmware may go through the scrub coverage table rank-by-rank on a periodic basis to determine whether any ranks had not been covered by hardware scrubbing. Firmware may initiate a targeted scrub and diagnostic for all of the ranks that did not have adequate scrub coverage. If for some reason the system still has some memory ranks that have not been covered by the initial hardware scrub and the targeted scrub, then the firmware may perform some course of action for fault isolation.