I've been playing around in Logisim to get some experience in designing basic electrical circuits. While I'm sure not the best, I was able to put together a functional ALU:

1-Bit: 1-Bit ALU

  • This ALU is capable of the operations AND, OR, XOR, NOR, ADD/SUB.
  • [A] & [B] are separately 8-bit inputs.
  • The [Sub] input designates whether adding (0) or subtracting (1).
  • The [Operation Setter] designates what operation is being performed.
  • [R] is the result of the designated operation, with the [Zero Flag] alerting us to a ‘0’ output, such as when a number is subtracted from itself.

Using the 1-Bit ALU I pieced together an 8-bit ALU (this image directly below performing an ADD operation): 8-Bit ALU

(SUB operation): 8-Bit ALU Subtraction Operation

enter image description here

I'm now attempting to take my ALU and combine it with some form of CPU (for now mainly focusing on adding/subtraction & outputting to a few 7-segment displays). I've got a 7-segment Display put together (also have another for alhpa characters AbcdEfG):

7-Segment Display (Non-Alpha)

I can't quite figure out a good way to take what I have for an ALU and place the output into my 7-Segment Display (ultimately I want to have the result display via 3 to display all possible 255 values from my 8-bit addition):

The following is what my circuit does for each different operation:

AND Operation - 1.1 = 1: AND

OR Operation - 1+0 = 1: OR

XOR Operation - 1 XOR 0 = 1: XOR

NOR Operation - 1 NOR 0 = 6: NOR

ADD/SUB Operationg - 1+1 = ERROR (Sub Flag is errored out for all OP's??): enter image description here

I'm not quite sure:

  • Why my Sub Flag isn't selectable.
  • Why my Zero Flag errors out on an Add/Sub Operation.
  • How to fix my result from my ALU to properly go into and display in my 7-segment Display (currently outputting 8-bit into an 8-bit splitter and then placing the first 4 as my input for the 7-segment Display).

Does anyone have any input for how I can correct the issues in my design and make this work? Thanks in advance!


Sub flag issue fixed. Somehow when I brought the circuit over for use in my main one, it had the properties of being an Output, and being 3-state (which I don't believe I ever set). In any case, that minor issue is fixed.

Seems the Zero Flag was also set as 3-state for some reason.

Any advice on fixing the Add/Sub issue?


1 Answer 1


In the last few pictures, it looks like you've configured the "sub flag" as an output, not an input. Since there's nothing driving that net, it's resulting in an error.

  • \$\begingroup\$ I'm not sure how those properties got changed, but you're right! The sub flag in my main circuit was both set as an Output & as 3-state. Thanks! Any advice on the rest? \$\endgroup\$ Commented Oct 12, 2014 at 5:41

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