# 2's Complement Binary Numbers

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**Offset Binary**

One other coding scheme is occasionally used. Suppose the actual numbers (base 10 equivalent) run from -128 to +127. If we simply added 128 to every number, then the range becomes 0 to 255, the same as straight binary. This is called offset binary. The coding for -128 is 0000 0000, the coding for 0 is 1000 0000, and the coding for +127 is 1111 1111. If one is aware in advance that this coding is used, it is common to subtract 2N-1 from each number as it is read into a computer so that it can be stored as a signed integer, either sign-magnitude or two's complement. We deal with this convention no farther here, but one will occasionally spot a data converter that uses this convention. Usually, a converter using one of the other conventions is available and is more commonly used.