Well, the S/360-20 had TPS (Tape Processing System, not to be
confused with Tape Operating System) which had a RESVOL that was
a tape. And there were utilities to update the tape in place (not
copy to another tape and then back, actual update blocks in
place). From time to time one would have to make a new copy of
the REsVOL to, kinda defrag it as I remember.
By the time I got to TPS (Tape Processing System), IBM was no
longer putting out maint. Or we just weren't getting any maint
for it from IBM (1977-78 when I was working on that system).
The Noodle snatcher had "data cells" which were segments that had
x strips of tape hanging in them (where x is a number that I've
forgotten). It was tape that could be randomly accessed, so it
was a kind of DASD.
It had a very interesting instruction. RFWBS, Read Forward, Write
Backward with Shredding. Yep, catch that mylar strip (aka,
noodle) on the edge of the slot and it would peal/slice it in
two. That could wreck you whole day.
And if you had just had maintenance done, and one of the segments
was not correctly fastened in place, when that sucker would
rotate, it would fling that segment right through the "glass" and
well, things just got exciting for a bit.
Makes one very glad to have things like thumb drives that we have
today. Now if I could just get one big enough to IPL z/OS...
Regards,
Steve Thompson
On 05/18/2016 09:07 PM, Paul Gilmartin wrote:
On Wed, 18 May 2016 17:50:53 -0700, Charles Mills wrote:
... It was indeed a direct access storage device. Not a disk, but DASD
nonetheless. Certainly not magnetic tape (though it had a family resemblance!)
and certainly not unit record.
What's the criterion? Max/Min latency ratio? Statistical distribution of
latency
between randomly selected records? For tape, it's sort of triangular; for DASD
it has a sort of plateau.
Addressable by block and writable randomly without corrupting other blocks?
DECtape had preformatted addressable blocks; it held a filesystem with a
directory. It moved from reel to reel past a stationary R/W head. It met some
criteria for DASD and some for tape; it was both.
-- gil
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