MGMT 1030 Lecture Notes - Lecture 36: Overflow Flag
6 May 2018
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MGMT 1030 Lecture 36 Notes – Overflow and Carry Conditions
Introduction
• It is useful to be able to predict approximate sizes of integers that are represented in
complementary form without going through the conversion.
• A few hints will help positive numbers are always represented by themselves.
• Since they always start with 0, they are easily identified.
• Small negative numbers, that is, negative numbers close to 0, have representations that
start with large numbers of 1s.
• The uer − i 8-it ’s opleet, for eaple, is represeted
hereas −8
• The largest egatie ’s opleet uer, is represeted
• This is evident from the scale
• Since there is only a difference in value of 1 between 1’s ad ’s opleet
representations of negative numbers (positive numbers are, of course, identical in both
representations)
• You can get a quick idea of the value in either representation simply by inverting all the
1s and 0s and approximating the value from the result.
• We noted earlier in this discussion that overflows occur when the result of a calculation
does not fit into the fixed number of bits available for the result.
• I ’s opleet, a additio or sutratio oerflo ours he the result
overflows into the sign bit.
• Thus, overflows can occur only when both operands have the same sign and can be
detected by the fact that the sign of the result is opposite that of the operands.
• Computers provide a flag that allows a programmer to test for an overflow condition.
• The overflow flag is set or reset each time a calculation is performed by the computer.
• In addition, the computer provides a carry flag that is used to correct for carries and
borrows that occur when large numbers must be separated into parts to perform
additions and subtractions.
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