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Microsoft Year 2000 Readiness Disclosure & Resource Center |
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Windows Operating System Interactions with BIOS and Real Time Clock |
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4. Introduction
Microsoft Windows® operating systems insulate applications from most, although not all, Year 2000 rollover issues.
This paper addresses the interaction between the computer’s Real Time Clock (RTC) and basic input/output system (BIOS) and the following Microsoft Windows operating systems:
- Windows NT® 3.51 and Windows NT® 4.0 – referred to here as Windows NT.
- Windows® 98 and Windows® 95 – referred to here as Windows 9.x.
- Windows® 3.x and MS-DOS® version 5.x or later.
A key part of preparing your computing environment for the turn of the century is knowing what your PC’s operating system will do, what the RTC/BIOS will do, and what the RTC/BIOS will do as it performs Power On Self Tests (POST) after the rollover. This paper includes easy-to-use tests to help customers determine whether or not their machine’s RTC/BIOS combination and operating system are Year 2000 ready. Microsoft recommends that customers install the free updates for operating systems and applications that Microsoft makes available from time to time—before conducting Year 2000 rollover tests.
Before IBM made the Real Time Clock (RTC) chip standard equipment on its PC AT in 1984, users were prompted to enter the date manually every time they turned on their computers. PCs since then have essentially duplicated the RTC chip, which may or may not have Year 2000 functionality.
The RTC chip is battery powered to ensure it can keep time even when the PC is turned off. The chip itself updates time, day, month, and 2-digit year. It typically contains seven registers that store time and date values. Six of the registers are updated automatically. Each one of them stores a different value: seconds, minutes, hours, days, months, and years. The year register stores the last two digits – "99" in 1999 or "00" in 2000. A seventh one—called the century register—stores the first two digits of the 4-digit year. The century register reads either "19" in 1999 or "20" in 2000 and is not updated automatically. It will change only if updated by either the BIOS or the operating system.
Many hardware manufacturers do not support direct access to the RTC. This is because there is no standard location for the century register in RTCs. For example, the RTC chip in most PS/2 computers stores the century value in a different location than the RTC chip found in other types of computers. Some Year 2000 testing applications do access the RTC directly and have detected anomalies. However, Microsoft is not aware of any non-test applications that directly access the RTC. In general, applications should always use defined application program interfaces (APIs) or BIOS interfaces to obtain date information.
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