IBM 7074 General Information Manual page 12

access mechanism corresponds to the desired address.
Also, a seek order need not be repeated for consecutive
single-record operations on the same track.
Prepare to Verify, Single Record (DVSR-82)
The order for this operation conditions the file control
for a single-record type of operation and to perform
the following functions:
l. Select the desired module and access mechanism.
2. Specify the address to be verified.
3. Prepare to transmit data from or to the addressed
record in response to a subsequent read or write
command.
The order sent to the file control is used in conjunc-
tion with the command. Access and module number
specify the unit to be selected. Since the access mech-
anism is already located and held in the track position
and the desired head is selected by the previous seek
operation, the remainder of the order is used for verifi-
cation purposes only.
The file control (in single-record mode) compares
each record address as it comes under the read head
against the address furnished by the order until the
desired address is found.
Information can be read from or written into record
areas only as defined by format tracks. Reading or
writing continues until either a
STOP
signal is issued by
the computer or the file control recognizes the end of
record, depending upon whether a read or write com-
mand is being executed.
Prepare to Write, Format (DWRF-83)
This order conditions the file control to write a format
track for the cylinder specified by the address portion
of the order (the last two positions of this order are
insignificant). To address a format track, the format
two-position key-lock switch must be in the
WRITE
position; the track address must be one of the track
addresses of the cylinder. To write a format track, this
order must precede the write command.
Prepare to Verify, Track with No Addresses
(DVTN-84)
This order, followed by a read or write command, per-
mits the reading or writing of only the records on a
particular track; all addresses are skipped over. The
address portion of this order must specify the home
address.
This order instructs the file control to do the fol-
lowing:
l. Select the desired module and access mechanism.
2. Supply the home address to be verified.
3. To receive or transmit only the records in re-
sponse to a subsequent read or write command.
Reading or writing begins at the first record follow-
ing the home address, and continues through the
records skipping over addresses until the computer
signals
STOP
or the file control recognizes the index
point. Non-verification of the home address results in
an
UNUSUAL END
signal, with no transmission of data to
or from disk storage.
Prepare to Verify, Cylinder Operation (DVCY-85)
This is an optional feature. It permits reading or
writing of data (skipping over addresses), beginning
at the first record after the home address of the ad-
dressed track and continuing through successive rec-
ord locations and the tracks of the cylinder until either
the end of cylinder is reached or the computer signals
STOP.
This order is always followed by a read or write
command. The following functions are performed in
the cylinder operation mode:
l. Select desired module and access mechanism.
2. Specify desired home address to be verified.
3. Transmit data after the subsequent read or write
command has been given.
Prepare to Write Check (DWRC-86)
This order is used with a write command to check any
record, track, or cylinder of tracks of information.
This order performs a bit-for-bit comparison, com-
paring the information recorded on the disk with the
same information stored in core storage of the com-
puter. If data agree, the order terminates with an
END
signal; if data disagree, the order terminates with an
UNUSUAL END
signal. The sequence of commands and
orders to write and check recorded data is:
Seek
Prepare to Verify
(Single, Track, Cylinder)
Write
Prepare to Write Check
Write
*Must be same address
xxxxxxxx*
xxxxxxxx*
It is not necessary to follow a write command with
a write check operation.
Set Access Inoperative (DSAI-87)
This file control order causes the file control to discon-
nect the addressed access unit from the file control.
(It permits the programmed disconnection of a faulty
access unit from the system.) Any subsequent com-
mand to this access will result in an
UNUSUAL END.
Operations 11