[ntp:questions] Linux 11-minute mode (RTC update)

Maciej W. Rozycki macro at linux-mips.org
Thu Apr 10 19:04:33 UTC 2008

On Thu, 10 Apr 2008, Unruh wrote:

> OK, if you say so. I am used to the CPU timer being very different from the
> rtc, but I am only used to PCs. It sounds really weird to have the rtc and
> the cpu timer being coupled like that. I would expect the timer to just
> keep going, and not to be dependent on the actual value that one stuck into
> the rtc. 

 Well, I cannot see a specific reason not to use an RTC as the source of 
the timer interrupt if such a chip has been designed into a system since
the beginning -- which is the case with these DEC machines.  With the IBM 
PC architecture the reason of using the 8253 and later on the 8254 for the 
timer interrupt is certainly historical as an RTC was not included in the 
design before the PC/AT.  Therefore some other chip had to be used.

 Another matter is they used the two other channels of the 8253/8254 to 
drive memory refresh and the speaker respectively so the PIT was meant to 
be there already, but if the RTC was available from the beginning, it 
would have probably been the obvious choice for the timer interrupt 
regardless and the remaining channel of the 8253/8254 could have been used 
for something else, such as a watchdog.

 I am told there is a piece of operating system software that uses the RTC 
as the source of the timer interrupt on the PC anyway.


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