cartridge tape designators

[Mike Jipping]

> I, too, would like to know the difference between rst0 and rst8 on 1/4
> inch tape.  The only references I have found are on page 84 of the
> "Network and System Administration" manual, part 800-1733-10, Rev A, May
> 1988.

About a year ago, John Gilmore submitted to Sun-Spots a well-written
description of cartridge tape formats.  I've included it below.  Save it
and spread it around.

      Mike Jipping                        Internet: jipping at cs.hope.edu
      Hope College                          BITNET: JIPPING at HOPE
      Department of Computer Science         Voice: Hey!

=========================================================================
>Date:    Thu, 18 Aug 88 02:56:03 PDT
>From:    hoptoad.UUCP!gnu at cgl.ucsf.edu (John Gilmore)
>Subject: Cartridge tape formats and sizes (/dev/rst0 versus /dev/rst8)

There seems to be massive confusion about cartridge tapes.  It's really
simple combinations of three different parameters.

There are two variants of the mechanical tape drive -- 4-track and
9-track.  The tracks are used like on an 8-track audio tape (run all the
way down the tape on one track, then mechanically slide the head up or
down and do another pass of the tape.)  The 9-track version can store
9/4ths as much data as the 4-track version.  The 9-track version will read
4-track tapes but the tapes it writes can be marginal for reading on
4-track machines, because the tracks are thinner.  Mostly they work.

There are two variants of the tapes themselves.  One is 450 feet long
(DC300XL style) and the other is 600 feet long (DC600A style).  They use
different magnetic coatings because the tape has to be thinner to fit 600
feet into a cartridge.  Some older tape drives can only read/write the 450
foot tapes because their heads can't cope with the new magnetic coatings.
There is a sense hole on the cartridge (up near the write protect tab)
that lets the newer drives figure out how to set up the head for this
particular tape.  The actual end-of-tape sensing is done with small holes
punched in the tape itself, detected with an LED, a mirror in the
cartridge, and a photocell, so that works fine for either tape length.

There are two variants of the bit format that controllers record on the
tape.  One is called QIC-11, the other is QIC-24.  QIC-11 is the original
Archive format (Archive Corp. started the whole 1/4" streaming cartridge
business).  When a standards committee got a hold of it, they changed it
(of course) to QIC-24.  In both cases, the tape contains 512-byte blocks
of data with small headers on them.  For QIC-11, the block number in the
header is 8 bits; for QIC-24, the number is 24 bits.  That is essentially
the only difference between the two.  It was changed because in unusual
error recovery situations it's possible for the tape to move more than 256
blocks (at 90 inches per second and 8000 bits per inch, things go by
quickly -- think about it) and the controller could lose track of where it
was on the tape.  Both formats hold the same amount of data on a given
tape.

If you make up a table of this stuff, you start seeing some familiar
numbers:

	Tape	450'	600'
Drive
4-track		20MB	[no such drives used in Suns]
9-track		45MB	60MB

The hardware takes care of 4-track/9-track and tape size issues, so all
you have to specify in software is whether you want QIC-11 or QIC-24
formatting.  /dev/rst0 is QIC-11 and /dev/rst8 is QIC-24.  I linked them
to /dev/rst.qic11 and /dev/rst.qic24 so I could just do it without looking
it up.

I believe Sun should have made the tape driver software attempt to read
tapes in both QIC-11 and QIC-24 format, like reel-to-reel tape drives
which will read whatever density you throw at them.  They didn't,
unfortunately, so if you try to read a tape that's in the other format, it
looks like a totally empty tape (you get a "no data" error).  Just rewind
and try again with the other format.  If you get "no data" in both
formats, you really have a blank tape (or one recorded in yet some other
random format).

There are a few other manufacturers who use QIC-24 tape drives; the IBM
PC/RT is one.  Apollos may be another, I'm not sure.  Very few of the IBM
PC 1/4" tape drives use QIC-24; they all went off in different directions.
I don't know of any current production machines that use QIC-11 only; it's
obsolete.

Sun used to make all their distribution cartridges in 4-track, QIC-11
format on 450 foot tapes [20MB], since they can be read by all Suns.
Starting with SunOS 4.0, they are now making Sun-4 tapes in QIC-24 on
9-track, 600 ft tapes [60MB] which reduces the number of tapes by a factor
of 3.  I am not sure whether Sun-3 tapes have been switched, though I
think all Sun-3's can read 60MB QIC-24 tapes unless they were upgraded
from Sun-2's.  However, older boot PROMs can't boot from a QIC-24 tape
(they never ask the tape controller to try QIC-24 mode, and it's too dumb
to do it itself), which is why you may need a boot PROM upgrade from Sun
Tech Support to boot SunOS 4.0 from 1/4" tape.

[I wrote the 'ar' driver for 1/4" tape on Sun-1's -- my first Unix driver,
and it was really bad -- and maintained the boot code for tape drives
through the first Sun-3's.]

	John Gilmore