Thread: Binary multiplication

1. The problem statement, all variables and given/known data

i try to do multiplication of two's complement number. I'm using straightforward method, which
give me wrong answer..

i know i can't use this method.. but.. i just want to know why..
why i can't use this method?



3. The attempt at a solution
1001 x 0101 = 0101101 which is 45.. it should be -35..

hey naspek

Well first, you need to convert the Negatives to a Positive to do the arithmetic.

1. NOT all of the bits to invert them : 0000 1001 becomes 0110
2. ADD 1 : 0111

so now you have positive 7. Do your multiplication against it, and get 35 0010 0011

Now you need to convert that, using 2's compliment BACK

1. NOT the bits 0010 0011 becomes 1101 1100
2. Add 1 : 1101 1101

That should be the Two's Compliment representation of -35.

Hope that helps!