On Fri, 23 Feb 2018 14:12:37 -0800, Charles Mills wrote:

>That is what I thought. How can a national grid work if SCE is zigging when 
>PG&E is zagging?
>-----Original Message-----
>From: Jesse 1 Robinson
>Sent: Friday, February 23, 2018 1:49 PM
>Long before I came to work for one, I heard that power companies are pretty 
>rigorous in maintaining a constant sine wave, whether 50 or 60 Hz. The power 
>might go out (!), but it won't wander in frequency. 
But now I'm confused.  The description of TIMER says:
    For TUINTVL, the address is a fullword containing the time interval.
    The time interval is presented as an unsigned 32-bit binary number;
    the low-order bit has a value of one timer unit (approximately 26.04166

That has to be right, or else programmers would have noticed.

And a less official source (but agreeing with Tony) says:

    IBM System/360 architecture
    If the interval timer feature is installed, the processor decrements the 
word at
    location 80 ('50'X) at regular intervals; the architecture does not specify 
the interval
    but does require that value subtracted make it appear as though 1 were 
    from bit 23 300 times per second.

But bit 23 must have 256 times the value of the low-order bit, and
    26.04167 * 256 = 6666.66752
... which is 1/150 second, not 1/300 sec.

Was the interval timer the only source of time-of-day on those early models?
If so, the External interrupt handler must reload its register before another
tick is lost -- easy enough at power frequencies, challenging for a higner
resolution interval timer.

I suspect CDC 6600 had such a problem.  I spotted in the source code a fudge
factor, uncommented, which could most plausibly be an accommodation for
missed ticks.  (Or for k != ki.)

-- gil

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