overflow

Computer Science An Overview _J. Glenn Brookshear _11th Edition

The point is that computers can make mistakes. So, the person using the

machine must be aware of the dangers involved. One problem is that computer

programmers and users become complacent and ignore the fact that small values

can accumulate to produce large numbers. For example, in the past it was com-

mon to use patterns of 16 bits for representing values in two’s complement nota-

tion, which meant that overflow would occur when values of 21532,768 or

larger were reached. On September 19, 1989, a hospital computer system mal-

functioned after years of reliable service. Close inspection revealed that this date

was 32,768 days after January 1, 1900, and the machine was programmed to com-

pute dates based on that starting date. Thus, because of overflow, September 19,

1989, produced a negative value—a phenomenon for which the computer’s pro-

gram was not designed to handle.