A Program Counter

• The program counter is more than a register.
  • It needs a simple adder to increment by 1.
  • It needs to be able to send data to the bus (fetch)
  • It needs to be able to load data from the bus (jump, branch)
• I see three control lines
  • PC to Bus when high allows the value of the pc to be transfered to the bus.
  • PC source selects the data to load into the pc.
    • This is completely arbitrary.

    • PC Src	RTN
      0	PC ← PC + 1
      1	PC ← BUS

• Constructing this from DIGITAL components
  • A 4 bit register
  • A 4 bit adder
    • a = PC
    • b = 1
    • C_in = 0
  • A mux
  • 
• Digital File