# Binary adding/subtracting, am i doing this right?

1. Nov 27, 2005

### mr_coffee

Hello everyone, i'm alittle confused, we have an exam tomarrow and i want to make sure i'm doing this right. I'm suppose to do the following:
Perform the indicated subtraction with the following unsigned binary numbers by taking the 2's compliment of the subtrahend.
so for one of them:
10110 - 1111
I take the 2's compliment of 1111 which is:
1's compliment: 0000
2's compliment: 0000 + 1 = 0001
now i add 0001 to 10110
10110
+0001
=
10111

now since i'm only adding a 4 bit number to a 5 bit number my final answer will only still be in 4 bits right? so it would be
0111 as the final answer right?
I used a binary calculator and it said the answer is 111, but i think it just wasn't programmed to included the 0 infront.

2. Nov 27, 2005

### mr_coffee

Also what would
101 - 101000 equal? the calculaotr came out with a huge number:
1111111111111111111111111111111111111111111111111111111111011101

when i take the 2's compliment of 101000 and add it to 101 i get: 011101

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