Read - IBM 7074 General Information Manual

Write Track With No Addresses
The conditions necessary for this operation are that
a prepare to verify (track without addresses) order be
followed by a write command. The address received
with the prepare to verify (track without addresses)
order is compared with the home address transmitted
from the 1301. If the home address fails to compare,
the no-record-found bit is registered and
UNUSUAL END
is issued to the system. If the address compares true,
the file control skips the first address area into the
record area and writes the record. The file control
continues skipping addresses and writing the records
until the index point is sensed. A
NORMAL END
is is-
sued to the system, providing no error conditions oc-
cur during data transfer. In the event of a parity error,
the operation will be terminated at the end of the
record in which the error was detected and
UNUSUAL
END
issued.
If computer signals stop in the middle of a record,
data transmission stops, but the file control continues
writing blanks to end of record area, then writes the
check characters.
Read or Write Cylinder Operation
Operation in the cylinder mode is an optional feature
of the 7631. A cylinder mode read or write is set up
by having the computer send a prepare to verify
(cylinder operation) order to the file control. The ad-
dress issued with the order is the home address of the
track on which the operation is to start. The home
address is compared bit by bit in the 7631 after the
read or write is received.
On a compare equal, the data transfer operation
begins with all of the records on the addressed track
being read or written and with the record addresses
ignored in the same manner as a track-without-ad-
dresses operation. Rather than causing
END
when the
last record of the track has been processed, the 7631
sends a new head address to the 1301 and reading or
writing continues. The head address sent to the 1301
is one address higher than the previous head address.
By indexing one head address on each disk revolution,
the 7631 continues reading or writing until the entire
cylinder of information has been processed. The
END
signal is generated in the 7631 when the highest order
head of the cylinder has been operated on.
If the computer signals
STOP
in the middle of a rec-
ord, data transmission stops; but, if reading, file con-
trol continues to read to end of record, and then com-
pares check characters; if writing, the file control
writes blanks to end of record, then writes the check
characters.
No wrap-around feature is included in cylinder
operation to cause operation to begin again at the
low-order head after the high-order head has com-
pleted its reading or writing.
Write Check Operation
Each of the write operations has a companion write
check operation which is optional and under pro-
gram control. The operation requires the following
sequence of orders and commands:
Prepare to Verify . . . XXXXXXXX
Write --------
Prepare to Write Check XXXXXXXX
Write --------
The 7631 will compare the data recorded in the ad-
dressed record, bit by bit, with the write data from
the system, at the same time generating check char-
acters that will be compared with those previously
generated and recorded on the track, following the
record. The end of this operation is the same as a
true write operation. A compare error during a write
check will set the data compare check bit in the status
data.
During a write check operation on a format track
the format gap detector circuits in the 7631 are
checked to determine if they are within their specified
tolerance. A file control circuit check is noted if the
circuits do not meet specifications.
On reception of a
STOP
signal the 7631 continues
comparing data already received, then continues till
end of the present record area and issues a
NORMAL
END
signal.
Read
Read Home Address Operation
This operation requires that a prepare to verify (home
address) is sent to the 7631 and that a read command
follows. The read (home address) is analogous to the
write home address. The most useful application of
this operation is to recover tracks of information in a
file dump operation. This operation does not depend
on the format track for recognition of data and ad-
dress areas. With this operation and assuming that
a proper gap is provided between index point and
the beginning of the home address area, the com-
puter can restore a file to normal operation after a
major mechanical change. The 7631 compares the
physical address portion of the home address and
begins reading with the home address identifier. All
address and record data of the track are read. The
termination of this operation occurs when the 7631
senses the index point. At that time either
NORMAL
or
UNUSUAL END
is issued, depending on the state of the
data check. The computer may terminate the opera-
Operations 15