From mboxrd@z Thu Jan  1 00:00:00 1970
From: norman@oclsc.org (Norman Wilson)
Date: Sat, 12 Dec 2015 20:41:48 -0500 (EST)
Subject: [TUHS] why does tar have the tape device hard coded into it and
 why is it mt1 instead of mt0
Message-ID: <20151213014148.5BBF5440AE@lignose.oclsc.org>

/dev/makefile on the V7 distribution tape (or at least the
unpacked image I have that I believe to be same) says:

ht:
	/etc/mknod mt0 b 7 64
	/etc/mknod mt1 b 7 0
	/etc/mknod rmt0 c 15 64
	/etc/mknod rmt1 c 15 0
	/etc/mknod nrmt0 c 15 192
	/etc/mknod nrmt1 c 15 128
	chmod go+w mt0 mt1 rmt0 rmt1 nrmt0 nrmt1

According to /usr/sys/dev/ht.c, the minor device
number was used as follows:

minor(dev) & 07		slave unit number
minor(dev) & 070	controller unit number
minor(dev) & 0100	tape density: set == 800 bpi, clear 1600
minor(dev) & 0200	no-rewind flag

It takes some digging in the source code (and the PDP-11
Peripherals Handbook) to understand all this numerology.
In most of the code, minor(dev) & 077 is just treated as
a unit number (fair enough).  The use of 0200 appears only
as a magic number in htopen; that of 0100 only as a magic
number in htstart, and that only implied: the test is
not minor(dev) & 0100, but
	unit = minor(bp->b_dev) & 0177;
	if(unit > 077)

Not so bad when the whole driver is only 376 lines of code,
but it wouldn't have hurt to make it 400 lines if that
meant fewer magic numbers.

Anyway, what all this means is that /dev/*mt0 and /dev/*mt1
both actually meant slave 0 on TU16 controller 0, but mt0
was 800 bpi and mt1 1600 bpi.  Hence, I would guess, tar's
default to mt1.

My first exposure to the insides of UNIX was in the High
Energy Physics group at Caltech.  Some of our systems had
multiple tape drives and every drive supported multiple
densities, so we invented for ourselves a system like that
many other sites invented, with names like /dev/rmt3h to
mean the third tape drive at high density.  (Hence the
USG naming scheme of /dev/rmt/3h and the like--not that
we taught it to them, just that many places had the same
idea.)

Our world wasn't nearly as exciting as that of our neighbors,
across the building and three floors down, in the Space
Radiation Laboratory.  They had a huge room full of racks
of magtapes full of data from satellites, and many locally-
written tools for extracting the data so researchers could
work on it.  The hardware was an 11/70 with eight tape drives,
and at any given time at least half the the drives would be
spinning.  One of the drives was seven-track rather than
nine-track, because some of the satellite data had been
written in that format.

Fair disclosure: I had a vague memory that the `drive number'
in the device name had been recycled for other purposes,
but couldn't remember whether it was density or something
else.  (I'm a little surprised none of the other old-timers
here remembered that, but maybe I worked with tapes more than
them.)  But I had to dig into the source code for the details;
I didn't remember all that.  And I did have to climb up to the
high shelf in my home office for a Peripherals Handbook to
understand the magic numbers being stuffed into registers!

Norman Wilson
Toronto ON