two's complement question

This is a discussion on two's complement question within the C Programming forums, part of the General Programming Boards category; I'm learning about binary and two's complement. I cannot replicate the result from the following problem with my current knowledge ...

  1. #1
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    two's complement question

    I'm learning about binary and two's complement.

    I cannot replicate the result from the following problem with my current knowledge of two's complement - subtraction:

    "subtracting -5 from 15 is really adding 5 to 15, but this is hidden by the two's complement representation:"

    Code:
     11110 000   (borrow)
      0000 1111  (15)
    − 1111 1011  (−5)
    ===========
      0001 0100  (20)
    The question is: in the last 4 bits (left nibble), how can you borrow from 0 if it's 0? The example above seems to be borrowing from the next colum where the top bits are all 0's. Can someone explain how this is happening?

  2. #2
    Gawking at stupidity
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    It's the same way you borrow in base 10. If you have 48 - 55, then you'll have nothing to borrow from in the tens place. So you just add a 1 making 48 become 148.
    If you understand what you're doing, you're not learning anything.

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