BCD Subtraction Woes - Part II =)

Discussion in 'Homework Help' started by lotusdeity87, Feb 24, 2008.

  1. lotusdeity87

    Thread Starter New Member

    Feb 24, 2008
    3
    0
    Hi,

    I am having some trouble with BCD subtraction as well.

    I understand how to take the 10s complement and add it to the other number. But, I dont really understand what to do with the extra digit -- do you ignore it? Here are some examples I worked out which might illustrate my problem:

    1) 13 -5

    0001 0011
    +0000 0101
    0001 1000

    We get "1" and "8" but what to do with the 1?

    2) 8-13 = 8+(-13) where 10's comp of -13 is 100-13 = 87

    0000 1000
    +1000 0111
    1000 1111 <-- invalid so add 6
    +0000 0110
    1001 0101

    which is "9" and "5" but it should be -5 ... so I really dont know what to do in this case.


    3) 26-84 = 26+(-84) =-58 and the 10s comp of -84 is 100-84 = 16

    0010 0110
    + 0001 0110
    0011 1100 <-- invalid so add 6
    + 0000 0110
    0100 0010

    which is "4" and "2" and is clearly no -58.

    Thank you very much.

    Sorry ... I posted it in the old thread accidentally but this is a different question altogether.
     
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