![]() ![]() I assume you've had enough illustrations of inverting and adding one. The two's complement representation of 69 is the following. As before, we'll add the two numbers together. The last is the binary representation for -12. To get the negative of 12 we take its binary representation, invert, and add one. Now suppose we want to subtract 12 from 69. But let's use binary instead, since that's what the computer uses. If we're to use decimal, we see the sum is 81. Suppose we want to add two numbers 69 and 12 together. In the examples in this section, I do addition and subtraction in two's complement, but you'll notice that every time I do actual operations with binary numbers I am always adding. With a system like two's complement, the circuitry for addition and subtraction can be unified, whereas otherwise they would have to be treated as separate operations. One of the nice properties of two's complement is that addition and subtraction is made very simple. If you have -30, and want to represent it in 2's complement, you take the binary representation of 30:Ĭonverted back into hex, this is 0xFFFFFFE2. ![]() So the negative of 0xFFFFFFFF is 0x00000001, more commonly known as 1. The inversion of that binary number is, obviously: But how to do that? The class notes say (on 3.17) that to reverse the sign you simply invert the bits (0 goes to 1, and 1 to 0) and add one to the resulting number. To see what this number is a negative of, we reverse the sign of this number. That's just the way that things are in two's complement: a leading 1 means the number is negative, a leading 0 means the number is 0 or positive. What can we say about this number? It's first (leftmost) bit is 1, which means that this represents a number that is negative. That is how one would write -28 in 8 bit binary. Suppose we're working with 8 bit quantities (for simplicity's sake) and suppose we want to find how -28 would be expressed in two's complement notation. You then invert the digits, and add one to the result. To get the two's complement negative notation of an integer, you write out the number in binary. ![]() Two's complement is the way every computer I know of chooses to represent integers. Therefore, after this introduction, which explains what two's complement is and how to use it, there are mostly examples. Two's complement is not a complicated scheme and is not well served by anything lengthly. Thomas Finley, April 2000 Contents and Introduction ![]()
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