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

[Jeff Mogul]

In the mtio(4) manual page for 4.2BSD, it is stated that
	"The files mt0, ..., mt7 are 800bpi, mt8, ..., mt15
	are 1600bpi, and mt16, ..., mt23 are 6250bpi."
This is a change from the corresponding 4.1BSD manual page,
mtop(4), in which it is stated that
	"The files mt0, ..., mt7 are low density, and mt8, ...,
	mt15 are high density."

This change seems to be a good idea, since it means that the
mapping from minor device number to density is now independent
of the particular hardware.  Unfortunately, the change did not
make it into the TU78 driver, which still believes that /dev/rmt0
is a 1600bpi device, and /dev/rmt8 is 6250bpi.

There are two ways of resolving this conflict (if you don't
count "change the manual page" as an option): 
	(1) Fix the device driver
	(2) Modify /dev/MAKEDEV so that
		/dev/rmt8	is on minor device 0
		/dev/rmt16	is on minor device 8
My preference is to fix the device driver (which involves a one-word
change to the source code), but I wondered if anybody had a good
reason for taking the other approach.

Another question arises: what minor devices should mt0, ..., mt7 map
on to for the TU78?  The only other 6250bpi tape supported in the
4.2BSD distribution is the "ut" tri-density drive, which has an
800bpi setting and thus a natural choice for mt0.  The TU78, however,
does not work at 800bpi.  The choices I see for mt0 are
	(1) Leave it as it is (1600 bpi)
	(2) Remove it from /dev
The latter avoids conflict with the language of mtio(4); the former
avoids confusing the poor users.  Any comments?

-Jeff Mogul
decvax!decwrl!Shasta!mogul
mogul at Shasta.ARPA