To help you with the project, you’ll be making the Return Stack component.

Components (subcircuits)

A component or subcircuit is kind of like a function: you have inputs (“parameters”) and outputs (“return values”). You must use components in your project, and they will be very helpful in organizing the many parts of your CPU.

Components in Logisim have some neat features:


Read the specifications on the project details page here.

Get your inputs and outputs and tunnels all set up and ready to go. Here’s how.

Starting off

Make a new subcircuit (Project > Add Circuit…) and name it something like “Return Stack”.

Your return stack will need a number of inputs and outputs. What I like to do is place down all my inputs and outputs first, so I don’t forget any. It’s like the first line of a function.

I also recommend using a tunnel for each input/output, so you can move the inputs and outputs around without disturbing your circuitry.

For the clock input, use a regular input. Do not use a clock component. There will be 1 clock component in your CPU, on the main circuit, which you wire into this one.

Saving yourself some tedious work

Here’s a tip: let’s say you have an input + tunnel like this:

and you want to make a new input with a different name and number of bits.

So what you can do is:

  1. Draw a box around the input and tunnel to select them both
  2. Copy and paste it
  3. Draw a box around both the newly-copied input and tunnel
  4. Change the “Label” and “Data Bits” on the left, and it will change both things!

For example, I made a copy and selected it:

and then I changed the data bits and label:

Making the sp register and RAM

There is 1 register in this component, sp, and it’s 8 bits. Make it.

Then make a RAM with the following settings:

It should look like this:

The str input is the write enable. You can ignore the sel, ld, and clr inputs.

Make sure you hook the clock input up to the register and RAM. It’s an easy thing to forget.

Copy and paste tunnels when you can! It will avoid spelling mistakes and saves a lot of time. Grayed-out tunnels are not connected to anything else.

Making the sp register count up or down (or neither)

The is call and is ret inputs will control the behavior of the sp register.

If either one is 1, the register should change (be enabled). If they’re both 0, it should not.

Do not use comparators for this. Think about the logic of it. You can do this with a single gate.

When is call is 1, the data going into sp should be sp + 1.

When is call is 0, the data going into sp should be sp - 1.

Testing it

Set both is call and is ret to 0. Tick the clock input (just toggle it between 0 and 1). Nothing should happen.

Set is call to 1. Tick the clock input. The sp value should increase by 1 each tick.

Set is call to 0 and set is ret to 1. Tick the clock input. The sp value should decrease by 1 each tick. It’s ok if it wraps around to 0xFF.

Making the RAM work

The only time we write to the RAM is when is call is 1. So hook is call to its str input.

The A (address) input can be one of two things:

The D (data) input should be PC + 1.

The D output (on the right side) will be your return address output.

At this point… you’re basically done!

Testing it

Set is call and is ret to 0. Set the PC input to some non-zero value. Tick the clock. Nothing should change.

Now change is call to 1. The address input to the RAM should be 0 (the current value of sp). The data input to the RAM should be PC + 1. Tick the clock. Two things should happen:

I set PC to 5 (binary 101), and ticked the clock, and now my RAM looks like this:

Try pushing some more values by changing PC and ticking the clock.

Now let’s see if returning (popping) works.

Change is call to 0 and is ret to 1. The RAM should not be enabled (str should be 0). The address input to the RAM should be pc - 1. The output from the RAM should be the most-recently-pushed value.

I pushed 3 things, so my sp = 3, and before I tick the clock, things look like this (the highlighted RAM value is the one that is being read):

Now, when you tick the clock, the sp should decrease by 1.

If everything works, congrats! You’re done! Now you did a little bit of your project :)

(for fun) Making it look nicer

my ALU is a bear, your argument is invalid

You don’t have to do this, but you can if you want.

Go to the main circuit (double click on the left). Then place your return stack. It looks kinda dumpy by default.

If you go into your return stack circuit again, go to Project > Edit Circuit Appearance. Here you can move the pins around, draw things, put labels on things, whatever you want. Even something like… a bear.


Once you’re sure your circuit works, you can submit.

Name your circuit file username_lab9.circ, like jfb42_lab9.circ.

Submit here.

Drag your asm file into your browser to upload. If you can see your file, you uploaded it correctly!

You can also re-upload if you made a mistake and need to fix it.