Date: 07/04/2003 at 16:10:06 From: Andrew Subject: Binary Subtraction I have looked at the examples you provided and they have been very helpful, but I need help understanding the rules of subtracting binary numbers when the subtrahend is larger then the minuend. For example, I see from the answer provided in a textbook the that 101101 -111101 is -010000 but I can't figure out how the leading -0 was obtained in the answer. Getting the first four zeros in the answer from right to left is pretty straightforward. Then borrowing from the 32's position gives a two in the 16's position, so 2 minus 1 is 1 for the answer in the 16 position. That leaves a 0 in the minuend for the thirty two's position and a 1 in the subtrahend. I can't see how subtracting 1 from zero yields a -0 for this position as shown in the answer. Andrew
Date: 07/04/2003 at 23:23:09 From: Doctor Peterson Subject: Re: Binary Subtraction Hi, Andrew. Let's start by thinking about the more familiar base 10. You'll find that the same problem exists there, but you probably don't stop to think about it because the solution is more familiar there. Let's subtract 61 from 45: 45 - 61 ---- -24 I just followed the usual rules for subtraction: 5-1 is 4; 4-6 is -2. So is the answer -24? No, because if we add 61 to -24 we get 61 - 24 ---- 37 not 45. What went wrong? The problem is that when we got that negative sign, it meant only that the 2 in the tens place was negative; the 4 is still positive! So what we realy found was that (40 + 5) - (60 + 1) = (40 - 60) + (5 - 1) = -20 + 4 That is not -24, but -16, which is the right answer to the problem. So we can't subtract a larger number from a smaller one columnwise, because the sign gets mixed up. The usual way to do this is to apply the fact that a - b = -(b - a) to say that 45 - 61 = -(61 - 45) = -16 You would do the same thing in binary (and these are in fact the same numbers): 101101 111101 - 111101 - 101101 -------- -------- = -(010000) That is, you reverse the order of the numbers, subtract, and take the negative. Now, there are a couple of alternative ways to do this, one of which I don't think I've ever seen (because it doesn't save any work), while the other is essentially the way a computer handles the problem. First, we can use the fact I demonstrated above, that the negative sign applies only to the most significant digit. In the decimal version of our problem, the work would look like this: 45 - 61 ---- -2 <-- subtract tens digits 4 <-- subtract all other digits (in this case, just the ones) ---- -16 <-- subtract 4 from 20, and keep the negative sign In binary, this would be 101101 - 111101 -------- -1 <-- subtract 11 from 10 0000 <-- subtract the rest -------- -10000 <-- subtract 0 from -10000 That's not quite as simple as it looked in the decimal example, because just subtracting 1-1 wasn't enough to fully deal with the negative sign. I had to take two digits so that what remained could be subtracted normally. I can see why nobody uses this! But the computer method is to represent negative numbers in "two's complement" form. This allows us to directly add or subtract any pair of numbers, including greater from smaller and negative from positive, and come up with the correct result, as long as we interpret it correctly. In this case, if I think of the numbers as having eight digits, a negative number -x is represented by its twos-complement, which is 100000000-x. To do the subtraction, I can allow myself to "borrow" from an imaginary ninth digit, and get 00101101 - 00111101 ----------- 11110000 This represents a negative number whose value can be found by taking its twos-complement; it is -(100000000 - 11110000) = -10000. This uses the fact that a - b = -(100000000 - [(100000000 + a) - b]) The same can be done in decimal using tens-complements, which work exactly the same way. In our example, 45 - 61 ----- 84 which means -(100 - 84) = -16 You can read more about these ideas here: Subtracting and Borrowing in a Column http://mathforum.org/library/drmath/view/61168.html Two's Complement http://mathforum.org/library/drmath/view/54344.html Subtraction Using Nine's and Ten's Complements http://mathforum.org/library/drmath/view/55949.html If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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