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Multiplication with binary unsigned numbers


Active member
Sep 13, 2013
1111010 x 1001 is a negative * a negative so I should get a positive right?

which I got 0001001010.

BUT, if it's -122 x -9 = 1098. How come I get a positive number that doesn't equal 1098. Aren't you supposed to disregard the most left bit if it's more numbers than your original like wouldn't i disregard the 1 carry in 10001001010, because if I included it in the answer wouldn't that make it -122 * -9 which = -1098. And how is that possible.


Well-known member
MHB Math Scholar
Jan 30, 2012
1111010 x 1001 is a negative * a negative
Why are you saying that? First, are you talking about 2's complement? Second, what number of bits do you have? Only when you know the number of bits, you can say which numbers are negative in 2's complement: the leftmost bit must be 1. But the leftmost bit here means that you have to use all bits, even if this means leading zeros. For example, if you have 8 bits, then 1001 is 00001001, the leading bit is 0 and the number is positive.