RM80 and its skip sector feature

[utzoo!decvax!ucbvax!unix-wizards]

>From ARPAVAX.wnj at Berkeley Fri Oct  2 00:36:56 1981
On an RM80 disk each sector either is or isn't marked with a special
bit.  Sectors which are preceded on their track by a bad sector have
this bit set; other sectors do not.

There is a bit in the controller interface, which must have the same
value as this disk bit for data to be read.

Skip sector works because there are really 32 sectors per track, while
the controller pretends there are 31.  You normally don't set the bit
in the interface and sector 32 is ignored.  If a sector marked on the
disk is encountered, an error occurs.  You set the bit in the interface
and read the sectors after the skip sector.  You have to do all the
arithmetic, the controller does nothing, except to complain.

This is all necessary because of the limited number of bad blocks which
can be represented under standard 144.

	Bill Joy

P.S.  This is all rather confusing, but the answer is you probably
didn't really want to know anyways.  All this was explained to me by
Bill Shannon at DEC.  I can hardly believe anyone would have done this,
but if I had about 10 operating systems to support, I probably wouldn't
change the bad block format either.