RM80 skip sectoring

[utzoo!decvax!ucbvax!unix-wizards]

>From decvax!shannon at Berkeley Fri Sep 25 23:49:01 1981
In response to sdyer at bbn-unix's question:

The skip sector feature was added to the RM80 because they expected
to have disks with more bad spots on them than can be handled by
DEC STD 144.  The skip sector feature allows the first bad spot
on each track to have a replacement sector at the end of the track.

Very little of the skip sector feature is automatic.  The read or
write will abort when it runs into a sector with the skip sector bit
set.  The device driver must then set the "skip sector error inhibit"
bit and continue the transfer at the next sector.  The first bad sector
on each track, and all subsequent sectors on that track, will be marked
with the skip sector bit.

The skip sector error inhibit bit suppresses the interrupt when a skip
sector bit is encountered on a sector.  It also allows you to access the
extra sector at the end of the track, which is otherwise not used.

If a track has more than one bad sector, all but the first will be marked
with BSE's (bad sector errors) and must be handled by some other mechanism
(e.g., DEC STD 144).  The 4.1bsd bad block handling (which I wrote)
handles all of this transparently on all types of transfers.

					Bill Shannon
					decvax!shannon or
					CSVAX.shannon at Berkeley