A lockstep controller operates a lockstep system of three or more CPU-GPU pairs, comparing the outputs from the CPU-GPU pairs and, by way of a majority vote, provides the output for the lockstep system. Based on comparing the outputs, if one of the CPU-GPU pairs provides outputs that disagree with the majority outputs, it can be switched out of the lockstep system. The removed CPU is replaced by a backup CPU. So that the backup CPU can be part of a CPU-GPU pair, a portion of the address space from the GPU of one of the other CPU-GPU pairs is assigned to the backup CPU to operate as a replacement CPU-GPU pair, while the CPU already associated with this GPU retains another portion of the GPU's address space to continue operating as a CPU-GPU pair.