Addition and Subtraction of Binary Numbers using sign bit:
Sometimes an underscore () is used to distinguish the sign bit from the magnitude bit.
Thus, if a computer is capable of handling numbers of 5 bits (i.e., sign bit and 4 magnitude bits) then the number  1011 is represented by 11011 where 1 denotes the negative sign and 1011 is the magnitude of the numbers.
Similarly 01101 indicate the binary number + 1101.
A negative numbers may also be represented by using 1’s complement of the magnitude of the number.
Thus the binary number – 1011 may be represented as 10100 where the MSB 1 means that the number is negative and 0100 is the 1’s complement of the magnitude of the given number.
Similarly, 10011 indicate the number – 1100 and so on.
In the same way 2’s complement may be used to represent a negative binary number.
For example,  1011 may be represented as 10101 where 1 means that the number is negative and 0101 is the 2’s complement of the number 1011.
The addition and subtraction of binary numbers using sign bit methods of representing negative numbers may be used conveniently in the design of computers to evaluate sums and differences of binary numbers by addition only.
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