# Basic Circuits ALU design

Discussion in 'Homework Help' started by MikeAlvarez, Mar 10, 2013.

1. ### MikeAlvarez Thread Starter New Member

Mar 10, 2013
2
0
I am to create a ALU cell that can perform these operations:

mode
m1 m0
----------
0 0 y = 2^(floor(log(2a))*i.e. Set all bits to 0 except most significant 1
------------------------
0 1 y = A + B/2
1 0 y = A
1 1 y = A+B

I have been working on this for a while, trying to figure it out. I am allowed to use an infinite amount of 2-to-1 Muxes with one select input, an infinite amount of 4-to-1 Muxes, and ONLY 1 full adder.

I am having extreme difficulty figuring out how to do modes 00 and 01. I don't know how I should compare the bits to know which one is the most significant. Please help point me in the correct direction. Keep in mind I am using 1 ALU cell symbol in Quartus to create a 4-bit ALU with inputs A3-A0, B3-B0, m1, m0, and outputs Y3-Y0. Thanks!

2. ### WBahn Moderator

Mar 31, 2012
23,089
6,941
Given a binary value X, how can you get X/2?

For the log part, worst case you have four 4-input combinatorial logic problems.

For a more elegant approach, what information do you need to determine if a given bit in the value should be the single 1-bit in the output? Think in terms of information that can come from the left and then be passed to the right.

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3. ### MikeAlvarez Thread Starter New Member

Mar 10, 2013
2
0
Given a binary number... to get x/2 I think I should right-shift the most significant bit equal to 1. ex: 1010 = 10 so I shift the most significant 1, 0110 = 5. Am I right? I just don't really know how to implement this. I get the concept just now beginning to learn about alu cells and how it flows. I don't know how to write the logic to perform this. Can I get a hint?

4. ### WBahn Moderator

Mar 31, 2012
23,089
6,941
Is 0110 really equal to 5?

Write out the bit patterns for 0 through 15. Now write the bit patterns for each of these divided by 2. What relationship do you see?