On 23 February 2018 at 12:03, Steve Smith <sasd...@gmail.com> wrote:
> Timer units just happen to be what the high-order word of the traditional
> TOD clock counts.  Whether that figured as a design point back in the day I
> have no idea.  I presume the span covered (1900-2047) and the precision
> (specified as microseconds in bit 51, implying much room for more in the
> last 12 bits) were more important considerations.  Just my guess, though.

Just a note on terminology that we seem to need every ten years or so:

Timer Units are not TOD clock units. Timer units are approximately
26.04167 microseconds. They come from the long-gone S/360 Interval
Timer, which was the fullword at location 80 (x'50'). This was defined
so that bit position 23 is decremented every 1/300 second, which
conveniently allowed an implementation that decremented either bits 21
and 22 every 1/50 second, or bits 21 and 23 every 1/60 second, thus
being able to run on a 50 or 60 Hz power line. Of course only the
smallest 360 models actually used the power frequency for timing, but
the definition lives on.

Timer Units are still referenced in various places in z/OS, for
example the TU operand on the STIMER[M] macro.

Tony H.

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