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A simple Subtract circuit

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• Easiest way to implement subtraction
  • Fact:

    Subtraction = addition of the negated value

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  • Negating a binary number (from CS255):

    Flip each bit (= apply not on each bit) Add 1 to the result.

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  • Here is a 4-bit subtract circuit constructed using a 4-bit adder circuit:

  • The logic-sim circuit is here: click here

    (Note: final "carry bit" in the adder circuit must be inverted on obtain the borrow indication)