What is Lockstep CPU?

Answer: B. Two CPU cores execute the same instructions simultaneously, and their outputs are compared every clock cycle; a mismatch is reported as a fault

Lockstep is a scheme in which two identically configured CPU cores run the same program and a comparator checks their outputs against each other. If one result changes due to a bit flip from cosmic rays or a circuit fault, it is detected as a mismatch, giving high diagnostic coverage for CPU faults. Because the two cores do the same work, performance equals that of one core. Comparing two cores alone cannot tell which one is correct, so after detection the system performs a reset or a transition to a safe state.

Answer: A. If the same cause, such as supply noise or radiation, produces the same error in both cores at the same time, the comparison cannot detect it, so this makes identical errors less likely

Lockstep detects faults through differences between the two cores' results, so it cannot detect identical errors produced in both by the same cause. Such failures are called common cause failures. Momentary supply voltage drops or clock disturbances affect both cores simultaneously, so delaying one core's execution by, for example, 2 clock cycles and realigning the results for comparison makes it less likely that the same error occurs at the same timing. Placing the cores apart likewise keeps localized causes from affecting both.