Designing a 4 bit ALU Shift LR

[sickkid]

Homework Statement

HI

This 4 bit ALU requires 8 functions in a bit slice, and satisfies following criteria

Also, it can only be constructed via basic logic gate..and,or,not,xor...

No filp-flop...etc.

S1 S2 S3 OUTPUT
0 0 0 Shift A Through (X = A)
0 0 1 Shift input A Left (X = A Shifted right by 1 bit)
0 1 0 Shift Input A Right (X = A Shifted left by 1 bit)
0 1 1 Logical AND (X = A AND B)
1 0 0 Logical XOR (X = A XOR B)
1 0 1 Logical NOR (X = A NOR B)
1 1 0 Full adder (X = A + B)
1 1 1 Subtraction (using the Full Adder) (X = A - B)

Homework Equations

Designing a 4 bit ALU

https://www.physicsforums.com/showthread.php?t=265676

The Attempt at a Solution

I've complete the other six tasks(but subtractors doesn't work properly?!)

However,shifting right&left really beat me up...this is what i got in one bit ALU...and then got stuck

http://tinyurl.com/ybhqex2unable to build a shifter to fill in the blank

0 0 1 Shift input A Left (X = A Shifted right by 1 bit) =D1
0 1 0 Shift Input A Right (X = A Shifted left by 1 bit) =D2 somehow i hav rough ideas to build a shifter but seems can't satisfy the requirement or not this case

2 bit shifter
http://tinyurl.com/yjvdx344 bit shifter
http://tinyurl.com/ye9enhd

 

[KataKoniK]

Hello,

Thank you for sharing your progress so far. Building an ALU using only basic logic gates can be challenging, but it is a great learning experience. I have a few suggestions for your design:

1. For the shift operations, you can use a multiplexer to select between the original input and the shifted input. This can be achieved by using the S3 bit as the selector for the multiplexer. For example, when S3 is 0, the output of the multiplexer will be the original input, and when S3 is 1, the output will be the shifted input. You can use this idea for both left and right shift operations.

2. For the subtraction operation, you can use a two's complement approach. This involves inverting all the bits of the second input (B) and adding 1 to it. This will give you the negative of B, which can then be added to A using the full adder to get the subtraction result.

I hope these suggestions help you in completing your design. Keep up the good work!