PDP-4

The PDP-4 is an 18-bit computer system that was introducted in 1962 as a successor to the Digital Equipment Corporation's PDP-1. However, this was a completely new system, incompatible with the PDP-1. For example, the PDP-1 used ones' complement arithmetic whereas the PDP-4 used two's complement, a much faster way to implement multiple precision and floating-point operations.