On 23 February 2018 at 12:03, Steve Smith <[email protected]> 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. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
