CMOS: Difference between revisions
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Century determination when no century register is present
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This meant that software didn't know if there was a century register, and (if there is) which register it might be. To fix that problem the ACPI specification included a "RTC century register" field at offset 108 in its "Fixed ACPI Description Table". If this field contains zero then the RTC doesn't have a century register, and if the field is non-zero then it contains the number of the RTC register to use for century.
If there is no century register then software can guess. For example, a piece of software written in 1990 can use the (2 digit) year register to determine the most likely century - if the RTC year register is higher than or equal to 90 then the year is probably be "19YY" and if the RTC year register is less than 90 than the year must be "20YY". In this way, software can correctly determine the century for up to 99 years after the software is written.
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