The 4100 series machines were based on bit-parallel CPUs employing silicon transistors. For each of the 4120 and 4130 computers there was originally a choice of ferrite core store: either 2 microsecond or 6 microsecond cycle time. The 4130 had a faster CPU and implemented floating-point operations in hardware. Floating-point was performed by software extracodes in the 4120. The Autonomous Transfer Unit (ATU) was an add-on extra for the 4120 but was built into the main cabinet of a 4130.