Overflow



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When two numbers of n digits each are added and the sum occupies n + 1 digits, we say that an overflow occurred. When the addition is performed with paper and pencil, an overflow is not a problem since there is no limit to the width of the page to write down the sum. An overflow is a problem in digital computers because the width of registers is finite. A result that contains n + 1 bits cannot be accommodated in a register with a standard length of n bits. For this reason, many computers detect the occurrence of an overflow, and when it occurs, a corresponding flip-flop is set which can then be checked by the user.

The detection of an overflow after the addition of two depends on whether the numbers are considered to be signed or unsigned. When two unsigned numbers are added, an overflow is detected from the end carry out of the most significant position. In the case of signed numbers, the leftmost bit always represents the sign, and negative numbers are in 2' sbinary numbers complement form. When two signed numbers are added, the sign bit is treated as part of the number and the end carry does not indicate an overflow.

An overflow cannot occur after an addition if one number is positive and the other is negative, since adding a positive number to a negative number produces a result that is smaller than the larger of the two original numbers. An overflow may occur if the two numbers added are both positive or both negative. To see how this can happen, consider the following example. Two signed binary numbers, + 70 and + 80, are stored in two 8-bit registers. The range of numbers that each register can accommodate is from binary + 127 to binary - I28. Since the sum of the two numbers is + I50, it exceeds the capacity of the 8-bit register. This is true if the numbers are both positive or both negative. The two additions in binary are shown below together with the last two carries.

carries: 0 I

+ 70 0 I000110

+ 80 0 I010000

+ I5O I 0010110

carries: I

- 70 0 I OI110IO

-80 I 01100000

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- I5O   110IOIO

Note that the 8-bit result that should have been positive has a negative sign bit and the 8-bit result that should have been negative has a positive sign bit. If, however, the carry out of the sign bit position is taken as the sign bit of the result, the 9-bit answer so obtained will be correct. Since the answer cannot be accommodated within 8 bits, we say that an overflow occurred.

 


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