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>