4.2BSD TU78 driver does not agree with mtio(4)

[Dave Martindale]

One way of "fixing" the discrepancy is to get rid of the un-mnemonic
names for tape drives entirely.

Several years ago, the tape naming convention was changed at Waterloo.
The name for a /dev entry for a tape contain the always-present core
"mt".  To that is appended "800", "1600", or "6250" depending on the
density, followed by a letter indicating which of possibly many drives
on the machine is intended.  When desired, a "r" is prepended to get
the name of the raw tape, and prepending an "n" to the whole thing
gives the name for the no-rewind-on-close device.

Thus "/dev/rmt1600a" is the "A" (only or best) tape drive at 1600BPI
using the raw interface.  "/dev/nrmt6250b" is the second drive at 6250,
no rewind.  There is no hint of what controller each drive is on -
the user doesn't care.  And drives that don't support certain densities
simply don't have entries for them.  Programs that have default tape
names compiled in (tar, etc) now default to "/dev/rmt1600a".

With any scheme like this, the attributes that the user wants to control
are specified by the name in a way that the user has a chance of remembering.
And the mapping between these attributes and the actual minor device numbers
used is an arbitrary convention that only the driver in question and
/dev/MAKEDEV need agree on.