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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: ancient disks, Architectural implications of locate mode I/O
Date: Sat, 6 Jul 2024 21:28:25 -0000 (UTC)
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According to Stephen Fuld <SFuld@alumni.cmu.edu.invalid>:
>> I believe that's what they did with VSAM.
>
>Agreed in the sense that VSAM replaced ISAM, but, and I am getting
>beyond my depth here, I wasn't aware that PDSs used ISAM.  I had
>thought they were a thing unto themselves.  Please correct me if I am
>wrong.  In any event, PDSs in their original form lasted beyond the
>introduction of VSAM, or the PDS search assist functionality wouldn't
>have been needed.

A PDS had a directory at the front followed by the members. The
directory had an entry per member with the name, the starting
location, and optional other stuff. The entries were in order by
member name, and packed into 256 byte records each of which had a
hardware key with the name of the last entry in the block. It searched
the PDS directory with the same kind of channel key search it did for
ISAM, leading to the performance issues Lynn described.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly