Two's complement question,is this correct?

[CEStudent92]

Do the subtraction of 11000-01001 in two's complement. So I have to convert the "01001" in two's complement and then add 11000 to it...So basically is -24-(+9) and the result should be 33 but what I get isnt 33..help :/

[D H]

That first number isn't -24. It's either +24 or -8, the former if numbers are more than 5 bits long, the latter if numbers are 5 bits long.

[CEStudent92]

That first number isn't -24. It's either +24 or -8, the former if numbers are more than 5 bits long, the latter if numbers are 5 bits long.

 

You are wrong.The first number starts with 1 so it is negative.

[D H]

No, I'm not wrong. With two's complement and five bit binary numbers, how is -8 represented?

Answer: 8 is 01000. The one's complement is 10111, two's complement is 11000. -8 is 11000.

 

So with two's complement and five bit binary numbers, how is (-8)-(+9)=-17 represented?

Answer: You can't represent 17 or -17 in a five bit binary number. The largest positive number is 15, the largest negative number, -16.

 

You can perform the operation anyhow, but you'll get overflow.

Edited November 17, 2012 by D H

[CEStudent92]

No, I'm not wrong. With two's complement and five bit binary numbers, how is -8 represented?

Answer: 8 is 01000. The one's complement is 10111, two's complement is 11000. -8 is 11000.

 

So with two's complement and five bit binary numbers, how is (-8)-(+9)=-17 represented?

Answer: You can't represent 17 or -17 in a five bit binary number. The largest positive number is 15, the largest negative number, -16.

 

You can perform the operation anyhow, but you'll get overflow.

 

Right,this was the mistake I was making! Because I got the answer -17 and I wondered how..thanks!