The PDP-2 Program Counter
PDP-2 CPU Program Counter discription The Program Counter is a 16 bit loadable up counter. The program counter is realized using 74x193 up/down counters. The PC control signals: PC-ADV advances the PC by one. PC-SEL Selects PC load source, either the 16 bit IR address field or the data on the lower 16 bits of the M-BUS PC-LOD loads the PC with an address from the source select mux PC-OMB enables the PC counter tri-state drivers feeding the M-BUS PC-OAB enables the PC counter tri-state drivers feeding the A-BUS A-BUS A-ADR = <0:15> A-SRC = <idle,PC,IR> selecting PC or IR implies dest. of MAR M-BUS M-DAT = <0:23> M-SRC = <idle,MDR,MDR-EXT,PC,AC,IO,SW> M-DES = <idle,MDR,IR,PC,AC,IO>