# Creating an ALU slice with 8 operations

Hey, I'm trying to implement a 4bit ALU based on this 8bit alu design. The ALU has 8 different operations, given by an input of X Y Z

The 8 operations are (edit: the last one is just A not, made a typo)

I'm pretty lost as to how to start on the abext. I've determined I need two muxes, one for A and one for B, encoded as either " 0 = Pass A/B, or 1 = Preform a logical operation". However, I'm totally lost as to how to implement each logical operation according to the input of XYZ. I tried to make a 2 input decoder that takes Y, Z and enables "and" gates that are paired with the output of the decoder and the output of each mux, all connected to an or, however that only gives me one output, and I need two.

I'm really just not sure how to create an ALU that demands these requirements, I've never made a normal 4 operation ALU, let alone one with 8 operations. What I'd really like is a a nudge in the right direction in the form of some components I might need to achieve the ia and ib output. Or an example of a similar ALU, as I haven't been able to find one on the internet, and my textbook does not explain the implementation of the abext.

I think for the A mux the selector is X, and for the B mux the selector is X or'd with inv Y. That's all I've tried. Really stuck, would appreciate some help.

(Progress Edit 1) So with the help given below, I arrived at this solution to the abext schematic. Its a little messy, as I'm not the best at organizing these things, but it gets the job done!

https://i.sstatic.net/TJEfd.png

What I've done is for input a, created a 4 to 1 mux paired with a 2 to 1 mux, the 4 to 1 mux has its selectors as Y and Z, and the 2 to 1 has it's selector as X. If x is low, we just take the raw input of a, otherwise we take a logical operation.

For b, we I used two 2 to 1 mux, the first mux takes the input b and b not, this is for addition and subtraction. It's selector is Z. The other mux takes the input of the first mux, and 0. If X' and Y' are both low, it takes the input of the first mux, otherwise it's 0.

Off to hopefully finish the rest of this thing!

(Progress edit 2)

And I'm finished! The rest of the design follows pretty much exactly to the diagram, but here's my implementation of the logic for CIN incase anyone is interested in that. https://i.sstatic.net/fgJw5.png

• Are you building it from gates and muxes, or are you using some level of abstraction? This may seem like a strange question, but have you tried prototyping parts in a logical gate simulator or in Minecraft? Commented Nov 6, 2013 at 18:40
• I'm using xilinx ISE design suite, so simulating is quite easy. I'm building it from the symbols included with that. So decoders, muxes, gates, it's all there. Commented Nov 6, 2013 at 19:26
• ah, in that case I misunderstood the question as low-level homework. Nevermind my comment. Commented Nov 6, 2013 at 19:30

The basic idea is that the adder after the abext cells can do the heavy lifting needed for the first few operations, with some simple transformations of the inputs. Some options (not sure if these are all optimal):

• A + B → ia = A, ib = B, cin = 0
• A - B → hint: do you know how to convert B to 2's complement?
• A + 1 → ia = A, ib = 0, cin = 1
• A → ia = A, ib = 0, cin = 0
• A and B → ia = A and B, ib = 0, cin = 0 (just include the logic gate inside abext)
• The other 3 logic operations could be implemented similarly to "and", using ib = 0, cin = 0, or to balance things out you could have 2 of them have the logic gate in the ib path and set ia = 0.

A simple way to do this would then be to have 2 8-input multiplexers in each abext cell, and apply the inputs as specified in the list above: e.g., for the ia multiplexer, the inputs would be (A, A, A, A, A and B, A or B,...) when XYZ = (0,1,2,...). There are more optimized ways to do it, like by noticing that ia = A when X = 0, instead of duplicating A to the first 4 mux inputs.

• Thanks! This helps a lot. I'll try it again and see how far I get. And I actually miswrote that last one, it's supposed to just be a not. Commented Nov 6, 2013 at 19:22
• Hey, thanks a ton! Thanks to your help, I was able to finish the abext design. Edited a screenshot of it into my main post. Commented Nov 7, 2013 at 11:32