Binary Subtraction of Two Unsigned Integers

Question

For unsigned integers $ X = 00110101 $ and $ Y = 10110101 $, determine the following values:

$ X + Y = (\text{My answer is}) ~ 11101010 $.

$ X - Y = ~ ??? $

$ Y - X = ~ ??? $

What have you tried? Perhaps try to define 01-10 and 10-01 in binary first, and then try to tackle your problem. — gt6989b, Feb 20 '13 at 23:57
Yes I understand that this would come out to -10000000. However, "unsigned" integers can't be negative...so how do I do this then — Silent Man, Feb 21 '13 at 00:04
If your answer is supposed to be unsigned, $X-Y$ has no answer — Ross Millikan, Feb 21 '13 at 00:14
@RossMillikan he is likely asking how this is actually done on a computer, say in C++ programming -- using 2's complement... See Amzoti's answer below. — gt6989b, Feb 21 '13 at 00:16
Amzoti and I have spent some effort to answer your question. Why did you delete it? — Ross Millikan, Feb 21 '13 at 06:14

score 7 · Answer 1 · answered Feb 21 '13 at 00:09

7

Your answer is correct for $X+Y = 11101010$

Hint (Algorithm): $X-Y$

Determine $Y’s$ $2’s$ complement $X+$ (2’s complement of $Y$)
If $X \ge Y$, an end carry will result. Discard the end carry.
If $X \lt Y$, no end carry will result. To obtain the answer in a familiar form, take the 2’s complement of the sum and place a negative sign in front.

You may want to display in a different form, but you did not specify (hint: convert the first answer to 2's complement form).

$$X - Y = -10000000 ~~\text{and}~~ Y - X = 10000000$$

answered Feb 21 '13 at 00:09

Amzoti

56,093

Good work, Amzoti! +1 – amWhy May 01 '13 at 02:15
1

@amWhy: I have to admit that this has never been a good way to have defined it in general. I suppose this makes it easy for logic circuits, but hard for humans! :-) – Amzoti May 01 '13 at 02:17
1

I agree wholeheartedly! But I enjoy doing "two's complement" arithmetic...I don't know why...I guess it's an exercise in logic, to some extent... – amWhy May 01 '13 at 02:17

score 1 · Answer 2 · answered Jan 10 '15 at 12:04

1

For X=00110101 and Y=10110101,

Binary Addition X + Y = 11101010

Binary Subtraction X - Y = -10000000

Binary Multiplication X * Y = 0010010101111001

Binary Division X / Y = 0000

Check results @ Binary Arithmetic Calculator

answered Jan 10 '15 at 12:04

Adam Biju

11

score 1 · Answer 3 · answered Feb 21 '13 at 00:17

1

For $Y-X$ you do it just like base $10$. In this case there are no borrows. For unsigned, there is no answer to $X-Y$ because negative numbers cannot be represented.

$$\ \ \ 10110101\\ \underline{-00110101}\\\ \ \ 10000000$$

answered Feb 21 '13 at 00:17

Ross Millikan

374,822

@SilentMan: yes, the high order bit is not taken as a sign, it is taken as a bit of the number. So in $8$ bits you can represent the numbers $0$ through $2^8-1=255$, while with signed numbers you can represent $-128$ through $127$ in two's complement – Ross Millikan Feb 21 '13 at 00:22
"Unsigned" computer arithmetic usually means that the operations are done $\bmod 2^n$, where $n$ is the word size. It does not mean that $X-Y$ is undefined when $X<Y$, but rather that the result is $X-Y+2^n$. – MJD Feb 21 '13 at 00:25
@SilentMan: I would say you do two's complement to represent negative numbers. It is one representation, sign/magnitude is another. – Ross Millikan Feb 21 '13 at 00:28
@SilentMan:$-128_{10}=10000000$ in two's complement as shown here If you have more bits, you put more $1$'s on the left. You can't represent $+128_{10}$ in a single byte of two's complement. – Ross Millikan Feb 21 '13 at 00:46

score 0 · Answer 4 · answered Mar 22 '15 at 06:30

Step: 1. Take 2's compliment of -ve number.

Step: 2. Add it to +ve number

Step: 3. If most significant bit is 0 then write answer with avoiding end carry

Step: 4. If most significant bit is 1 then take 2's compliment of answer again and place negative sign with answer

X-Y => 00110101 -10110101 Take 2's compliment of 10110101 =>01001010 =>00111011

Then add it to first number =>10000000

We know that MSB is 1. So the answer should be in negative. To make it negative take 2's compliment of answer again => 01111111 => 10000000 X-Y =>-10000000

For Y-X Y => 10110101 X =>-10110101 Take 2's compliment to X => 01001010 -X => 01001011

Add it to Y 110000000 There is an end carry so answer will be in positive automatically. Ignore and carry and write answer 10000000

Binary Subtraction of Two Unsigned Integers

4 Answers4