What Shmuel was pointing out that if you have the data set open
normally, the DEB will have extent entries for each extent on each
volume. Then the M will point to the m+1 th extent of the data set; if
the DS has one extent per volume, then M=2 will address the 3rd volume,
etc.
If you've been doing the OPENJ with a single volser, then JFCBVLSQ
should not be used, because you're making it appear to be a single,
uncontinued data set?
I'm trying to read a dataset that will potentially have many volumes
each of which has up to 16 extents. I'm trying to open the dataset in a
way that I get one volume at a time, with the DEB reflecting the extents
of that volume.
What you're talking about sounds like BDAM where you get a single
logical view of a multivolume dataset?
I'm wanting to do EXCP access and my understanding is that when you do
this you get a "simple" DCB which is "connected" to one volume at a time
and for which the DEB just reflects however many extents are on that
volume. Certainly this is what I see, and this is how I'm performing my I/O.
The question is why, when I declare my dataset DISP=OLD, do I get this
behaviour, where when I open volume sequence number 2 I look at the DEB
and the associated UCBs and they are indeed on the volume that I know to
be the second one, whereas when I do the same with DISP=NEW the extents
in the DEB are all on the first volume.
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