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Institute for Computing Systems Architecture

Computer Systems Colloquium

"Onwards to the Year 2000"

Gordon Brebner, Division of Informatics, University of Edinburgh

3.30 pm, Thursday 16 December

Room 2511, James Clerk Maxwell Building

Abstract

The "Y2K" problem has attracted much attention in recent years. The concern is that, where the year in a date is stored using only two decimal digits, moving from 1999 (99) to 2000 (00) might cause confusion, for example, software believing that it has been plunged back to the year 1900. In this pre-Christmas talk, a variant of the problem is introduced and briefly examined: the Y2K Nibble Carry Problem. Suppose that BCD arithmetic results in the value 99 being advanced to 100, i.e., an extra BCD digit is generated. We identify two variants: (a) the American-date big-endian problem, where the carry nibble interferes with the last nibble of data stored before the date; and (b) the European-date little-endian problem, where the carry nibble interferes with the first nibble of data stored after the date. The implications for some example unluckily-trampled data types will be outlined. The audience will be invited to contribute their own examples, in a Christmas party game-style scenario. Mince pies will be served afterwards.


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