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16-bit hexadeciaml stored in sign-magnitude

  1. Jan 26, 2012 #1
    1. The problem statement, all variables and given/known data

    What is the sum of A and B if they represent signed 16-bit hexadecimal numbers stored in sign-magnitude format?

    A = 0D34
    B = DD17

    2. Relevant equations

    A + B = C :)

    3. The attempt at a solution

    so far the hexadecimal can be converted to binary numbers

    A = 0000 1101 0011 0100
    B = 1101 1101 0001 0111

    since the hexadecimal is stored in sign-magnitude, then the most significant bit, controls the sign either plus or minus (in this case its -)

    Therefor i write the following

    0000 1101 0011 0100
    -0101 1101 0001 0111
    --------------------------------
    ???? 0000 0001 1101

    I'm stuck afterwards, because where do i borrow or get the last bits?

    Because when i use a binary calculator i get the following result
    1100 1111 1110 0011 => CFE3

    I would like to understand how to get this result, not just use the calculator...
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 26, 2012 #2

    jedishrfu

    Staff: Mentor

    you do realize that B is negative right because its sign bit is on, so you adding a positive number and a negative number.

    Your mistake is in flipping the sign bit and subtracting the B.

    Remember in binary:
    - a plus ONE is 0000 0000 0000 0001 and
    - a minus ONE is 1111 1111 1111 1111

    (((ie a minus ONE is not the sequence 1000 0000 0000 0001)))

    and when you add them together what do ya get? and where did the carry bit go?
     
    Last edited: Jan 26, 2012
  4. Jan 26, 2012 #3
    Scooooty: You should check out two's compliment, which is what jedishrfu is discussing.
     
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