🚀 go-pugleaf

You can send this to the list if you think it's useful.  This info has
been gleaned from various sources.  I wish I could give credit, but I have
no idea of who said what...

There are two parameters to consider when dealing with quarter inch
cartridge (QIC) tapes.  One is number of tracks (4 or 9), the other is
encoding format (QIC-11 or QIC-24).  Older Sun-2's can only write 4-track
QIC-11 tapes.  Newer Sun-2's can only write 9-track QIC-11.  And all
Sun-3/4's can only write 9-track QIC-11 or 9-track QIC-24.  As far as I
know no-one writes 4-track QIC-24 tapes.  The number of tracks is
determined by the tape drive head.  There is no way to write 4-track tapes
with a 9-track drive (they're in the right position, but not wide enough
to necessarily be readable by a 4-track drive).  The QIC-11 versus QIC-24
format is determined by the controller and is software selectable if both
formats are supported.

QIC-11 has only 1 byte defined for block number, QIC-24 has 3 bytes.  This
means that QIC-11 has less overhead per block and is therefore higher
density than QIC-24 (proven by experiment).  I don't think that sun uses
that block number, or that it ever will get used.  I think it is supposed
to be used for high-speed random positioning.

Sun made a major boo-boo when they switched to QIC-24 for 4.0.  Not only
do they get less data per tape (maybe .5M less per 450 foot tape), they
also had to update all those PROMS.  All they needed to do was to switch
to 9-track.  If you look at some of the unbundled software for 4.0, you'll
see that they now use QIC-11 with either 4-tracks or 9-tracks.