Andy Wood wrote:
. . .
most 360s didn't actually decrement the low-order bit of the location
timer ... most were at the 3.3mills than 13+ microseconds. 360/67 had
high resolution timer feature that did decrement the low-order bit (this
is somewhat analogous to 370 timer defining the referenced bit ... but
allows different modesl to do actual updates at resolution comparable to
their instruction rate).
Well, that is completely consistent with what the S/370 POP said.
However, why was the S/360 POP different? It makes no sense to me for the
S/370 interval timer to really be different to the S/360 one, and the interval
timer was not included in the section of the S/370 POP which listed
differences between S/370 and S/360. My wild guess is that the intention in
S/360 may have been to decrement at line frequency, but it was realised very
soon that that was a bad idea, and it was changed to the 3.3ms at bit 23
rate, but for some reason the POP was not completely updated until the S/370
version.
Sure, a 3.3ms signal to decrement the interval timer could perhaps have been
derived from the line frequency with a PLL generating five times the line
frequency for 60Hz, and six times line frequency for 50Hz. However, even if
that is how it worked, calling that "counting down the timer word at a rate of
50 or 60 cycles per second" would be stretching the truth a bit.
And, if we are in agreement that the low order bit of the interval timer
corresponded to 13 microseconds, why did the operating system choose to
double that for its "timer units"?
The behavior of the interval timer *was* properly documented in the System/360
PoP, even at the -0 level, but it was documented under "External Interruptions".
Bob
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