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Homework Help: Shift LR in 4 bit ALU?

  1. Apr 2, 2010 #1
    1. The problem statement, all variables and given/known data


    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)


    2. Relevant equations

    Designing a 4 bit ALU

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

    3. 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/ybhqex2


    unable 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 cant satisfy the requirement or not this case

    2 bit shifter
    http://tinyurl.com/yjvdx34


    4 bit shifter
    http://tinyurl.com/ye9enhd
     
  2. jcsd
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