8-bit CPU control logic: Part 3

Introduction

In this tutorial, we will explore how to utilize two EEPROMs to translate the instruction cycle ring counter and the contents of the instruction register into the appropriate control signals for each microinstruction in an 8-bit CPU. This process is essential for understanding how control logic works in CPU design. By the end of this guide, you will also see how to run your first full program on the CPU.

Step 1: Understand the Instruction Cycle Ring Counter

• The instruction cycle ring counter plays a crucial role in tracking the current instruction being executed.
• Familiarize yourself with how the ring counter operates and its relationship to the microinstructions.
• The ring counter outputs a series of signals that indicate which step of the instruction cycle is currently active.

Practical Tip

• Draw a diagram of the instruction cycle to visualize how the ring counter progresses through each state.

Step 2: Set Up the EEPROMs

• Use two EEPROMs to store the control signals for each microinstruction.
• Each EEPROM will correspond to different aspects of the control logic:
  • One for the instruction cycle state.
  • Another for the contents of the instruction register.

Implementation Steps

1. Connect the two EEPROMs to your CPU setup.
2. Write the necessary data into the EEPROMs that correspond to the control signals needed for each microinstruction.
3. Ensure that the data addresses in the EEPROM match the outputs of the instruction cycle ring counter.

Common Pitfall

• Double-check the connections and data addresses to avoid mismatches that can lead to incorrect control signals.

Step 3: Program the Control Signals

• Program the EEPROMs with control signals based on the outputs from the instruction cycle and the instruction register.
• Use the following structure to program the EEPROMs:
  • Identify the control signals needed for each state.
  • Map these signals to the corresponding data addresses in the EEPROM.

Example Control Signal Mapping

Address  | Control Signal
---------|----------------
0        | Load A
1        | Load B
2        | Add
3        | Store Result

Step 4: Running the First Full Program

• With the EEPROMs programmed, you can now run your first complete program on the CPU.
• Prepare a simple program that utilizes the control signals you’ve set up to perform basic operations (e.g., addition).

Steps to Execute the Program

1. Load the program into the instruction memory.
2. Start the CPU and let it execute the instructions.
3. Monitor the output to ensure the operations are performed correctly.

Practical Tip

• Use a logic analyzer to observe the signals and ensure everything is functioning as expected.

Conclusion

In this tutorial, you learned how to set up control logic using EEPROMs in an 8-bit CPU. By understanding the instruction cycle ring counter and programming the EEPROMs with the correct control signals, you can successfully run your first program. Moving forward, experiment with more complex programs and refine your understanding of CPU architecture.