Program Counter

The program counter stores an eight bit number used to track the current position/instruction in program memory.

It’s similar to a regular register but has some extra functionality:

  • It can add one to it’s current value.
  • It’s value can be asynchronously reset to 0.

Interface and Operation

This is the interface to the program counter:


This is how it operates:

Name Bit width Description
data 8 Reads bits from, or asserts bits onto this connection.
contents 8 Always outputs the current value held in the program counter.
clock 1 Clock signal from the clock module.
input_enable 1 While high, the program counter stores the value on data on a rising clock edge.
count 1 While high, the program counter increments the stored value by one on a rising clock edge.
output_enable 1 While high, the program counter asserts it’s content onto data.


  • Two 74HCT161s are used to store, increment and clear the 8 bit value.
  • A 74HCT245 is used to provide a tri-state output to allow asserting values onto the bus, or not.
  • A 74HCT04 is used to invert the incoming input_enable, output_enable and count signals to drive the active low inputs on the 74HCT161s and 74HCT245.
  • LEDs with current limiting resistors are used to display the current contents.

The electronics are laid out on the breadboard like so: