HP 9895A Disc Memory
Command
Set
A-3
Parallel Poll
Response
Parallel poll
is
used
as
an
additional
means
of communication
between
the
9895A
and
the
bus
controller.
If
the
9895A
is
ready
to
accept
the next
part
of a
command
sequence,
it
will
respond
to the parallel poll
conducted
by
the
bus
controller.
After
accepting
most
secondary
command
bytes, the
9895A
disables the parallel poll re-
sponse.
This indicates
that
the
device is
busy
processing
the
current
part
of
the
command
sequence.
The actual disabling of parallel poll
response
may
occur
up
to
100
microseconds
after the
secondary
is
accepted
by
the
9895A. Thus, if
the
9895A
has
parallel poll
enabled,
and
the
bus
controller
is
fast
enough
to
send
a
command
sequence
and
then
conduct
a
parallel poll before the
9895A
has
disabled
the
poll,
the
bus
controller would
see
the
wrong
parallel poll response. To solve this
problem,
an
intentional delay
can
be
introduced,
or
a
DSJ
command
(this disables parallel poll)
can
be
issued
before
other
commands.
The
exception
to
the parallel poll
response
interlock
concerns
the
Clear
commands.
The
DSJ
command,
unlike
the
Clear
commands,
may
not
be
a valid
or
recognized
command
in
all
states
of
the
controller. Thus, the controller
may
reject
or
not
even
see
the
DSJ
command
and
not disable the parallel poll
response.
If
the
bus controller
had
expected
the
DSJ
to
lower
the
parallel poll in
the
case where
the
controller
can
not
accept
the DSJ,
the
bus
controller would
see
the wrong parallel poll after
the
Clear
command.
Cylinder and Track Numbering
Starting from
the
outer
cylinder, cylinders are
numbered
sequentially from a to 76.
These
numbers
are also
the
physical track
addresses.
A track
is
the
intersection of a cylinder
and
a
head.
There
is also a logical track
address
associated with
each
good
track.
If
a disc
has
no
bad
tracks,
the
logical
address
of a track
is
the
same
as
the
physical address.
A disc with
bad
tracks
can
be
made
to
look like a slightly smaller disc with
no
bad
tracks.
To
do
this, the
bad
tracks
are
specially
marked
to indicate
that
they
have
no
logical
address.
A
track
marked
in this way
is
referred to as
an
invisible track.
The
remaining
good
tracks
are
sequentially assigned logical track
numbers.
Logical track a is
the
outer-most
good
track (it
mayor
may
not
be
physical track 0).
During normal operations,
the
user
need
be
concerned
only with logical
addresses.
The
9895A
controller
will
take
care
of finding
the
proper
physical address.
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