On 23 February 2018 at 19:16, Paul Gilmartin <
0000000433f07816-dmarc-requ...@listserv.ua.edu> wrote:

> 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
>     microseconds).
> 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 subtracted
>     from bit 23 300 times per second.

I remembered it as bit 23 when I posted, and I first assumed I had just got
it wrong. But the last S/370 POO (and the earliest for S/360 - both on
Bitsavers) do say, with slightly different wording, that it is bit 23 that
is effectively counted down at 300 Hz.

That first S/360 POO also has this table:

  23       300  cps    3.33  ms
  24       600  cps    1.67  ms
  25        1.2 kc      833  µS
  26        2.4 kc      417  µS
  27        4.8 kc      208  µS
  28        9.6 kc      104  µS
  29       19.2 kc       52  µS
  30       38.4 kc       26  µS
  31       76.8 kc       13  µS

> 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.

I'm wondering if the problem relates to the timer itself being a signed
integer (an external interrupt becomes pending when the timer goes from
positive to negative, but it keeps on counting), but the TU arguments to
things like STIMER are said to be unsigned. But that's perhaps more of a
clue than an explanation. So I dunno.

> Was the interval timer the only source of time-of-day on those early

Yes. The TOD clock was new with S/370. (Well, one never knows about the
360/85, 91, and 195 unless one has had hands-on experience.)

> If so, the External interrupt handler must reload its register before
> tick is lost -- easy enough at power frequencies, challenging for a higner
> resolution interval timer.

Heh... They thought of that early on. I can't find it in the POO at the
moment, but it is no accident that the fullwords at locations 4C and 54 are
"reserved" by both S/360 hardware and software. Then an MVC for length 8
from address 50 to address 4C both saves the current value and sets a new
one. Since the timer is not (visibly) updated during instruction execution,
there is no loss of information.

Tony H.

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