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What are the differences among these clocks, real-time clock,wall clock,system clock?

I would be thankful for any hint on this question.Could anybody make it clear for me?

sunshilong369
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  • Real time clock is the RTC chip that your motherboard has that keeps track of time, even when the system is off. –  May 18 '20 at 01:44

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Actually google can help you, but I google for you and digest some essential part here, the whole website can be see here.

Most computers have one or more hardware clocks which record the current "wall clock" time. These are called "Real Time Clocks" (RTCs). One of these usually has battery backup power so that it tracks the time even while the computer is turned off. RTCs often provide alarms and other interrupts.

The system clock is a software clock maintained by the kernel and used to implement gettimeofday(2) and time(2), as well as setting timestamps on files, defined to be the POSIX Epoch: 1970-01-01 00:00:00 +0000 (UTC).

A key difference between an RTC and the system clock is that RTCs run even when the system is in a low power state (including "off"), and the system clock can't. Until it is initialized, the system clock can only report time since system boot ... not since the POSIX Epoch. So at boot time, and after resuming from a system low power state, the system clock will often be set to the current wall clock time using an RTC. Systems without an RTC need to set the system clock using another clock, maybe across the network or by entering that data manually.

tyChen
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  • Thank you for you clarification.I have saw the same webpage before.I could not get the idea. But after i have saw the digest you posted, i make it clear.Thank you. – sunshilong369 May 18 '20 at 05:30