On Thu, 14 Dec 2023 at 18:39, Lennie Dymoke-Bradshaw
<[email protected]> wrote:
> I would have said use EXCP until recently. In addition you would need to be
> APF authorised in order to open VSAM datasets using EXCP.
> However, I understand however that there are some undocumented issues running
> EXCP against data sets that are extended format. The DFP Advanced Services
> manual states the following,
> "The EXCP and EXCPVR macro instructions allow you to control the data
> organization based on
> device characteristics. The exceptions to this capability are partitioned
> data sets extended (PDSEs),
> extended format data sets, spooled and dummy data sets, TSO terminals, and
> z/OS UNIX files and
> file systems. They are not supported for user-written applications using
> EXCP."
In the pre-MVS days, EXCP was the lowest level I/O API. (A very few
programs stole the I/O new PSW and issued their own TIO/SIO
instructions, but that's hardly an API).
In MVS ("OS/VS2 Release 2") a new interface macro STARTIO (not to be
confused with the SIO machine instruction) which called the new "I/O
Supervisor" (IOS) was slid in between EXCP and the hardware I/O
instructions, and a new "EXCP processor" became just one of several
"IOS driver"s. IIRC the others in the early MVS days were ASM, JES3,
and perhaps VSAM and VTAM. Looking at z/OS 2.5 I see there are 36 such
drivers, of which "DFSMS" is one.
The STARTIO interface was never really documented externally by IBM,
though a number of individuals and companies figured out how to use
it. I would imagine that an IOS driver today can still direct CCWs
(and probably now TCWs) at an arbitrary device, and e.g. do I/O
against things such as a PDSE, an extended format dataset, or a ZFS
VSAM dataset that contains UNIX files. But of course the format and
metadata for these things is mostly undocumented, so having access to
the raw data this way is pretty much good for a backup/restore or
remote disk access product and not much else.
I have no access to the [in]famous $$$$ FAMS documentation, and I'm
not sure how FAMS relates to "Media Manager", but what little is
visible from APARs and MACLIB and discussion on this list suggests
that it is not just another layer above CCWs, but rather a much higher
level interface that understands all sorts of stuff about the
internals and metadata of various objects on DASD (and probably other
devices). It speculate that it might be that "DFSMS" IOS driver. What
level of interface handles non-CCW/TCW devices such as OSA, I have no
idea.
All idle speculation for a Friday...
Tony H.
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