HELP! (possibly the hardest question i've ever seen)

Discussion in 'Homework Help' started by fakeaccount, Sep 16, 2008.

  1. fakeaccount

    Thread Starter New Member

    Sep 16, 2008
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    My homework is to build a 16 bit adder with "4 bit carry look-ahead". I know how to build a simple 16 bit adder but how do i implement the "4 bit carry look-ahead". In addition i also have to make it expandable so that i can make a ALU which has a subtractor , multiplier and divider (which i'll worry about later). So for the mean time can somebody please tell me how to implement a "4-bit carry look ahead" on a 16 bit adder? (has to be done on a software called tanner).

    Thanks in advance!
     
  2. smarra427

    New Member

    Sep 16, 2008
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  3. scubasteve_911

    Senior Member

    Dec 27, 2007
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    I don't know what a 4-bit carry lookahead is. I would look into the 'simple processor', which accepts commands in their registers to perform various functions in the ALU.

    Is this a parallel input? Or, is it a machine? (sync , or async)

    Is the 16-bit adder one with 16-bit inputs, or outputs?

    Can you use the logic outputs of the adder (the sum) to create the carry look-aheads with some logic?

    Steve
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Get the data sheet for a 74LS182. It's a lookahead carry generator.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    Here's a hint: Get the datasheet for the CD4008 4 bit full adder with look-ahead carry and duplicate the functionality.
     
  6. veritas

    Active Member

    Feb 7, 2008
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    A [*edit* simple] lookahead carry is just ANDing the two previous bits of the operands together.
     
    Last edited: Sep 23, 2008
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