Disk formats

posted Jun 8, 2014, 4:52 AM by Jeff Ogden   [ updated Jun 8, 2014, 1:19 PM ]
Original comments and questions from Jeff are in black. Replies from Mike in blue.

On Jun 8, 2014, at 1:08 AM, Mike Alexander wrote:

I do know the answers to some of these questions (as I'm sure Gary does too).  Comments appear below.  They were somewhat hastily entered so may have mistakes.


--On June 7, 2014 at 9:28:49 PM -0400 Jeff Ogden wrote:

Another item of historical interest is the evolution of the format of
the MTS file system on disk (or data cell).

I came across this as part of the article "New Disk System to be
Installed" in the 9 May 1973 issue of the CC Newsletter:

Although any physical record format for the new disks would be
incompatible with that of the old ones, we have chosen to
"page-format" the new disks in order to eliminate, hopefully, even
this minor incompatibility (the need to regenerate saved note/point
information of sequential files) when acquiring new hardware in the

So, it looks like MTS disks at U-M became page formatted over the
weekend of June 30th.

This of course begs the question of what they were before they were
"page-formatted". It seems to be "track" oriented, but I'm unclear on
the relationship between a page and a track. Was there a time when
the block size for files on disk wasn't a page (4096 bytes)?

The block size was never 4096 bytes until we went to page formatted disks.  Before that the basic disk format was derived from OS/360, not TSS.  Each track was formatted into a fixed number of records that filled the track up.  On early disks I think it was one record per track, but later on there were more.  Tracks were formatted the first time they were allocated (or maybe each they were allocated).  before that they only contained a record 0 and no other records.

And on a related, but slightly different topic: I know that the basic
data structures on the MTS Volumes were based on formats from TSS. I
know too that at some point we moved from something called VAM2 to
VAMX. But in general I'm pretty vague on the details behind those
names. I suspect that there is more information in some of the
distribution notes. I suspect that some of the MTS Newsletters have
more information as well.

Was VAM2 the name for the original TSS format? Was there a VAM or
VAM1? VAMX was our own elaboration of the VAM2 format, wasn't it? I
think we continued to use the TSS DASDI and DUMP/RESTORE stand-alone
utilities until the move to VAMX.

There may have been a VAM or VAM1, but it never got outside IBM.  VAM2 was the format used by TSS and adopted by MTS when it went to page sized records on the disk.  At that time we also preformatted the disks so MTS could assume the page sized records were already there.

VAMX was essentially the same as VAM2 except for one small, but important change.  Page addresses on the disk were 32 bits.  In VAM2 this consisted of 4 bits unused (except in DSCB pages), 12 bits volume number, and 16 bits page number.  Disks got big enough that 16 bits wasn't enough for a page number so in VAMX the division was 4, 8, and 20.  Other than that I think VAM2 and VAMX were the same.

Did the original TSS formats include the catalog or just the volume
label, PAT and relocation entries, and DSCBs?

The catalog was entirely MTS's (mostly Gary's) invention.  The label format is from OS/360 (mostly) while the DSCBs, PAT, and relocation entries are taken from TSS.

On Jun 8, 2014, at 1:23 PM, Mike Alexander wrote in response to several comments and questions from Jeff:

On Jun 8, 2014, at 10:34 AM, Jeff Ogden wrote:

Thanks Mike.

So, before page formatted disks, MTS file buffers were larger or smaller than 4096. Weird. I have 4096 baked into my brain.

And before page formatted disks, file space was allocated in terms of tracks rather than pages or records?

Yes, that’s why old FS tapes show file sizes in tracks, not pages.

I know the file routines supported FBA [fixed block architecture] devices. Did U-M ever have any FBA disks?

Yes, we had support for several FBA devices.  It wasn’t hard since the file system used a fixed block size by then.

That same issue of Computerworld has an article about the Superdisk on page 1.  There were two models, the largest of which had a capacity of 800 MB, which was quite large for the time.