It's actually the DASD controller that detects EOF on a Read Data CCW.

As for PDSE, the dataset is formatted into control intervals and there are data 
in the dataset used by the access method to delineate logical records and 
members.


--
Shmuel (Seymour J.) Metz
http://mason.gmu.edu/~smetz3

________________________________________
From: IBM Mainframe Discussion List <IBM-MAIN@listserv.ua.edu> on behalf of 
Christopher Y. Blaicher <cblaic...@syncsort.com>
Sent: Friday, February 23, 2018 9:49 AM
To: IBM-MAIN@listserv.ua.edu
Subject: Re: Count with no Data and no Key ? (CKD internals again)

Radoslaw,
The access methods detect the key=0, count=0 records and process them as EOF, 
which is how they signify the end of a PDS member.  Every member has an EOF 
record with the rare exception of a member that ends at the end of the track on 
the last track of a data set.  I don't know how that is done for a PDSE as I 
have never dug into that.  I had to deal with EOF when writing EXCP and STARTIO 
code, but I used BSAM when dealing with PDSE's, there weren't many back when I 
was doing that.  Maybe others can comment on PDSE structure.

Chris Blaicher
Technical Architect
Mainframe Development
P: 201-930-8234  |  M: 512-627-3803
E: cblaic...@syncsort.com

Syncsort Incorporated
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-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf 
Of R.S.
Sent: Friday, February 23, 2018 8:35 AM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: Count with no Data and no Key ? (CKD internals again)

I don't know internals, but definitely one can use short blocks on DASD, 18B is 
not a limit. You can use 18 or 5 if you want. Of course the tracks utilization 
will be extremely poor, but that's different story.

BTW: I'mt not sure, but it seems the null record inside a member is when the 
member span track boundary.
Obviously there are multiple EOFs (null records) inside a PDS.

--
Radoslaw Skorupka
Lodz, Poland






W dniu 2018-02-23 o 13:32, Elardus Engelbrecht pisze:
> Paul Gilmartin wrote:
>
>> Also, just curious, I understand that on tape a block <18 (? still?) is 
>> bypassed as a noise record.
> Wow! Interesting! Where is that documented?
>
>
>> But it's possible that with RECFM=VB the last block might, by happenstance, 
>> consist of as few as 8 bytes (BDW=8; RDW=4).  How do tape drivers deal with 
>> this?  Poorly?  With RECFM=VBS, a null segment could be used for padding.
> Now you made me curious. [1] Damn! ;-)
>
> I really hope someone can answer you...
>
> Groete / Greetings
> Elardus Engelbrecht
>
> [1] - Currently outside my scope of work, but hey, I worked with that things 
> as a Storage Admin in the prehistoric days. 8-)
>
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> For IBM-MAIN subscribe / signoff / archive access instructions,
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