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Designing 8 bit adder?

  1. Nov 25, 2012 #1
    Hi I already created a 4 bit adder but for my assignment I need an 8 bit one. Is it correct if I design to separate 4bit adders and connect their outputs (Cout) with an AND gate?

    4 bit adder:
    https://www.dropbox.com/s/ao60s6dgtrnhal3/4bit adder.PNG
  2. jcsd
  3. Nov 30, 2012 #2
    your picture is too small to read, so its hard to tell for sure; but i think you want to xor the values. for example, lets say your inputs are 11001010 and 00101111. breaking it up to two 4 bits additions, we get...
    values 1110 = A 1001 = B
    Bcarrybit = 1
    (A << 4) xor B xor (Bcarrybit << 5)
    hope that helps.
  4. Dec 1, 2012 #3


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    Homework Helper

    To make an 8 bit adder from two 4 bit adders, the carry output from the low order 4 bit adders goes into the carry input of the high order 4 bit adder. The outputs are not "connected", you just have an 8 bit output (4 from each adder) plus a carry outpur from the high order adder.
    Last edited: Dec 1, 2012
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