Early PDP-1's had the MUS (MUltiply Step) operation, which had to be applied repeatedly in order to perform multiplication. Singe-step multipy (MUL) was first available as an extension, later it became the default.
More on the multiply and divide subroutines at https://www.masswerk.at/spacewar/inside/insidespacewar-pt6-g... – automating these subroutines in hardware meant adding a special clock, which would take over control in order to perform the according steps.
