# 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.