When I said "addressed as disk drives (DASD)" I was referring to IBM's
DASD's (Direct Access Storage Device's) CKD (Count Key Data) format
introduced with the System/360 which supported variable length records, or
as Paul calls them "variable length sectors" on disk drives, data cells and
drums amongst other devices. The Count field of each Record defined the
length of both the optional Key field the Data field so that on a single
track you could have records of different length - not likely but possible.
It was also possible to write a record longer than a track using record
overflow.
Variable length CKD records are still supported today in IBM's mainframe
systems by emulation on arrays of fixed sector HDDs.
Tom
-----Original Message-----
From: Paul Koning <paulkon...@comcast.net>
Sent: Friday, April 12, 2024 12:13 PM
To: t.gard...@computer.org; cctalk@classiccmp.org
Cc: Tom Gardner <tom94...@comcast.net>
Subject: Re: [cctalk] IBM 360
> On Apr 12, 2024, at 2:10 PM, Tom Gardner via cctalk
<cctalk@classiccmp.org> wrote:
>
> Data Cell - Tape, Card or Disk?
>
> I'm pretty sure the developers thought of the media of the IBM 2321
> as tape rather than cards, although the strips (of tape) were
> addressed as disk drives (DASD) not tape.
Actually, they look like a disk. See the 2841 manual on Bitsavers (that's
the controller which drives 2311 disks as well as 2321 data cell devices).
It says that each strip has 100 tracks, read/written by a movable heads unit
that has 20 heads on it, so 5 positions. And it shows the layout of each
track, which is the conventions count/key/data layout of 360 disk drives
like the 2311. Yes, variable length "sectors", you'd specify in the JCL
what you wanted for blocksize of that particular file. If I remember right,
the block length could vary from one block to the next, which is pretty
wild. (Contrast with the EL-X8 disk drive, which also has variable length
sectors, but more limited: a choice of one of 5 possible sizes, chosen on a
per-track basis when you first write or format that track.) Apart from
those, I only ever remember seeing fixed size sectors, though the actual
lengths might be strange -- like the CDC mainframe disks with sectors of 322
12-bit words.
paul
