16 bit registers
  AC - accumulator - work register.
  IR - instruction register - holds instruction being decoded.
  MBR - holds data being transfered between cpu and system, bi-directional.

12 bit registers
  PC - points to next instruction to be decoded.
  MAR - holds current address to be accessed.
      Uni-directional, address always placed on address bus via MAR by CPU.

8 bit registers
  InREG - input register accessed with the "input" instruction.
  OutREG - output register accessed with the "output" instruction.
    Registers themselves not shown. However a field "input" is provided when
    an input is required and a scrollable "OUTPUT" window is provided for
    displaying output.