The fallowing is a list of computer system that might be affected by Y2K.

Central or mainframe computers. See Mainframe for more details.

Telephone systems with programmable switches. Phone systems are almost all controlled by software.

Personal Computers. The device inside the PC that retains the date and time when the computer is shut off often cannot see a four-digit year. It will give the operating system an invalid date after January 31, 1999. See Personal Computer for more details.

Automated security systems with computers that automatically lock and unlock doors and elevators. See Microchip for more details.

Dispatching systems that use the Global Positioning System (GPS) to locate vehicles. GPS devices determine distance by measuring how long it takes to get a signal from several orbiting satellites. Unless corrected, many will fail on August 22, 1999.

Automatic data collection systems like those on electric meters.

I. Your personal computers

Every personal computer contains a set of permanent instructions that control the most basic functions of the system. They are known as the "basic input/output system", or BIOS of a computer. The BIOS accesses an internal clock in the computer. The date in this clock generally has one digit for the century (9 for 19) and two digits for the year. When the clock rolls over to '0' the century information will be lost and the operating system will be functioning with incorrect values. The system will either roll over to January 1, 1900, or will revert to an incorrect system date.

This problem may not be immediately apparent, since the date on the screen will show 01-01-00, but the internal date is incorrect and the problem will eventually manifest in the operating system or in the applications that run on it.

What personal computers are affected? Most of the older versions of Microsoft's MS-DOS and Windows 3.1 will have a run-in with the millennium bug. Newer Pentium-based computers and Microsoft Windows 95, and Windows NT operating systems are more likely to calculate the date correctly, but not all of them. It's still a good idea to test them. Older PC's can be upgraded, but it may be difficult to find parts and service. The Macintosh Operating System is ready for the millennium bug, but even if your system software is OK, you'll need to check some of the programs you use.

II. Mainframe Computers

The earliest computers were mainframes, and it was on these computers that the first applications for business, industry, government and science were run. Writing programs for the early mainframes required eliminating every possible character to save storage space and increase program speed. This is where the convention of using two digits for the date began.

As a result, the applications that run on these computers will be vulnerable to Year 2000 problems. Many of the original applications are still in use today, they are poorly documented, and the languages used to program them are no longer in use. Many of them will also fail to calculate the leap year correctly.

Microchips

There are over 30 billion microchips in use today in electronic components. These microchips control security elevators and doors, telephone switches, traffic lights, electric substations, and hundreds of small household and office appliances.

Experts predict that only about 2% of the embedded systems in use will fail. But 2% of 30 billion is still a sizeable number, and there is no way of knowing exactly which 2% will fail. In most cases the device that uses these chips will just stop working. Elevators will descend to the basement or go into security mode. VCR's will still work, but you probably won't be able to program them.

How can you tell if a device may have year 2000 problems? If it has an LCD display of any kind, it probably uses a microchip. If the display tells you the date, it will obviously be date-sensitive. Look for ways that a device responds automatically to different conditions, particularly related to the time or date. If it would be inconvenient or dangerous for the device not to function, it's a good idea to check it out well ahead of time.