A-32
HP
9895A Disc Memory Command
Set
Buffered Read
Type:
Purpose:
Description:
Disc Read Commands
Disc read.
Data is transferred through
an
internal buffer in 9895A
before being sent to the
HP-IB. This allows HP-IB data
transfers to
be
asynchronous with the disc,
and
to vary
from
an
arbitrarily low rate to
about
190K bytes
per
sec-
ond. The maximum
number
of bytes to
be
transferred in a
buffered read is
256
(1
sector); the read request must
be
repeated
for each additional sector transferred.
Following reception of the Read command, parallel poll
response is disabled
and
the status of the specified unit is
checked.
If
the unit can
be
accessed, then the current cy-
linder number
is
read
and
compared with the target cylin-
der address.
If
they differ, a seek to the target cylinder is
performed. This may occur
if
the actuator has slipped
or
if
an
auto-increment to the next cylinder is required.
If
an
HP format disc is being used, then the target sector's
256
bytes are read into the controller's buffer.
If
the read
completes
successfully,
then
the
target
address
is in-
cremented by one sector.
If
the sector
is
not found, a
eRe
error
is
indicated,
or
a D bit
is
encountered, the target
address
is
not incremented.
If
an
IBM
disc is present, then the target sector's
128
bytes
are read into the controller's buffer.
If
the read completes
successfully, then
the
target address is incremented by
one.
If the target sector
is
not found, a
eRe
error
is
indi-
cated, or the D bit is encountered, then the target address
is not incremented.
After the data has
been
buffered into the controller, the
parallel poll response is re-enabled, indicating that the de-
vice has data ready to transmit. The bus controller should
request the data by issuing the
"Send
Data" secondary.
Upon receiving the secondary, the 9895A again disables
parallel poll.
If
the read was terminated before data was loaded into the
buffer (i.e., any error except
eRe
or
D bit on), the device
will
respond by sending
an
EOI
tagged byte
and
enabling
parallel poll response.
If
there was no error, the sector's worth of
data
is made
available. The bus controller can take any
number
of bytes
up to a sector's length.
If
more than
one
sector is re-
quested, the 9895A
will
send a byte tagged with
an
EOI.
The number of bytes taken has no effect
on
the updating of
the target address.