It was based on Goodyear's earlier STARAN array processor, a 4x256 1-bit processing element (PE) computer. The MPP was a 128x128 2-dimensional array of 1-bit wide PEs. In actuality 132x128 PEs were configured with a 4x128 configuration added for fault tolerance to substitute for up to 4 rows (or columns) of processors in the presence of problems. The PEs operated in a single instruction, multiple data (SIMD) fashioneach PE performed the same operation simultaneously, on different data elements, under the control of a microprogrammed control unit.