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2

For PC XT types, there were multifunction cards using a MC58167 from National Semiconductors. Other RTC chips were also used.


4

If the latter, was it common to lose clock accuracy if the timer rate was changed by a running program? It was certainly possible, yes; it's hard to say how common this kind of distortion was, but it was fairly easily avoidable. A well-behaved program that hooks the timer interrupt needs to call the original handler at its original rate. If it changes the ...


13

Or was the clock maintained in software, and based off of something like the 18.2 Hz system timer interrupt? Exactly that. It is a 32 bit counter incremented by one every time INT 8 is triggered by the 8253 counter #3 (via INT 0). If the latter, was it common to lose clock accuracy if the timer rate was changed by a running program? That depends much on ...


27

Or was the clock maintained in software, and based off of something like the 18.2 Hz system timer interrupt? This is exactly how time was tracked; you can see the implementation of the timer tick handler in the IBM PC Technical Reference, page A-77. It updates a counter, stored in memory as a double word at 0x0040:0x006C, and checks for elapsing days, ...


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