Hi Ian,

On 01-Dec-99, Ian Moote wrote to JONATHAN DE BOYNE POLLARD:

 JP>> The year 2079 limitation is due to the "windowing fix" employed
 JP>> to solve the problem that the PC's RTC chip in most PCs does not
 JP>> update the century byte in NVRAM when the year byte rolls over
 JP>> from 99 to 00.

 IM> Because of my job I would be very, very interested in hearing any
 IM> factual information concerning an RTC used in a PC which _does_
 IM> update the century byte.

As most (I believe all) PCs made these days have the RTC integrated
into the chipset with the BIOS also stored in the chipset it becomes
difficult to separate chipset solutions from BIOS solutions :-(.
Although I can see no reason for not making it a hardware solution.

 JP>> The original RTC chip that was used didn't *have* a century byte.

 IM> Again, if you have any factual information regarding RTC's which
 IM> _do_ directly employ a century byte, I would be interested in
 IM> hearing about it.

I can dig out from data sheets RTCs that do support a century byte if
you want. It is doubtful if many will be compatible with the PC RTC.

 JP>> On most PCs the area where the century is stored is not one of
 JP>> the RTC registers, but is just an ordinary NVRAM area.

 IM> Yes, I have quite a bit of experience with that, thank you.

 JP>> The windowing fix works on the assumptions that years 80 to 99
 JP>> are in the 20th century and years 00 to 79 are in the 21st
 JP>> century, and patches the century byte accordingly.

 IM> Very, very sad. DOS is good until 2099. The RTC/CMOS itself is
 IM> good up to the year 9999. Too bad that IBM felt that they had to
 IM> break that.

The RTC is only good to 28th February 2100, thereafter it will be
wrong every century year _except_ the years divisible by 400.

 JP>> Of course, this is a textbook case of a software bodge to work
 JP>> around a bad (or at least exceedingly shortsighted) piece of
 JP>> hardware design.

 IM> Yes, I agree. And I've heard that criticism of the AT's RTC
 IM> before. Could you give me the part number for an RTC which was
 IM> readily available in 1983 that tracked real years -- "1983"
 IM> instead of just "83"?

Sorry, I can't help there :-(. I don't have stock lists from then
around any more, I needed the space :-).


George

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