A three-dimensional (3-D) processor system includes a first processor chip and a second processor chip in a stacked configuration. The first processor chip includes a first processor having a first set of state registers. The second processor chip includes a second processor having a second set of state registers that corresponds to the first set of state registers. The first and second processors are connected through vertical connections between the first and second processor chips. A mode control circuit operates the processor system in one of a plurality of operating modes. In one mode of operation, the first processor is active and the second processor is inactive, and the first processor operates at a speed greater than a maximum safe speed of the first processor, and the first processor uses the second set of state registers of the second processor to checkpoint a state of the first processor.