Checking - IBM 7074 General Information Manual

tion earlier by issuing a
STOP.
Upon receipt of
STOP,
the 7631 will terminate data transmission and inter-
nally complete reading of the record on which it was
operating at the time of the
STOP.
At the end of the
particular record, the 7631 will send either
NORMAL
or
UNUSUAL END,
depending on the state of data check.
Read Track with Addresses Operation
Conditions necessary for this operation are that a pre-
pare to verify (track) order be issued to the 7631, fol-
lowed by a read command. The file control begins the
operation by comparing the home address which ac-
companied the prepare to verify (track) order against
the recorded home address on the selected track. A
failure to compare will cause "no record found" to be
set in the status data and causes
UNUSUAL END
to be
issued to the system. A successful compare permits
the 7631 to begin reading at the first record address
area. The record address is read out in its entirety and
sent to the system, followed by the record and then
the next address and record, etc. The operation ter-
minates when the 7631 senses that it has completed
comparing the check characters of the last record of
a track. At that time a
NORMAL END
or
UNUSUAL END
is issued, depending upon the state of data check. A
STOP
signal prior to the logical end of this operation
will cause the 7631 to stop data transmission and
signal a
NORMAL END
at the end of the record on which
it is operating at the time of the
STOP
signal.
Read Single Record Operation
This operation requires the completion of a prepare
to verify (single record) order and read command. As
with write (single record) operation, the read (single
record) operation has no predetermined starting point
on the disk, that is, when the instruction is received
by the 7631, it immediately begins searching for a
record address. Upon finding one, the 7631 does a
bit-by-bit comparison. A failure to compare causes the
control to continue searching on the next record ad-
dress. "No record found" is registered if the 7631
passes the index point twice in its search for a partic-
ular record. Upon obtaining an address compare true
the 7631 causes the record immediately following that
address to be read and the data to be sent to the system.
The 7631 verifies the legality of the check characters
following the end of the record and sends the
NORMAL
or
UNUSUAL END
at that time. A
STOP
signal prior to
the logical end of this operation will cause the 7631 to
stop data transmission and to signal a
NORMAL END
at
the end of the record.
Read Track with No Addresses
This operation requires that a prepare to verify (track)
and a read command, in that sequence, be executed.
16
The 7631 waits for the home address area before be-
ginning to compare the address supplied with the
prepare to verify order against the recorded home
address. A compare failure causes the 7631 to register
"no record found" in the status word. If the address
compares successfully, the 7631 carries out the read-
track operation by skipping over the first record ad-
dress and reading the first record. The operation con-
tinues with addresses ignored by the 7631 and only
record data being sent to the system. The operation
ends when the 7631 finishes the last record of the track.
At that time the appropriate
END
signal is issued. An
EARLY END
may occur because of a system
STOP
signal.
In that case, the 7631 finishes the record on which it
is operating and issues a
NORMAL
or
UNUSUAL END
as
the case may be.
Checking
The 7631 File Control checks for twelve individual
error conditions possible in the operation of disk stor-
age. The sensing of anyone of these conditions causes
an
UNUSUAL END
signal to be sent to the computer.
Parity or Character Check
The 7631 is an odd-parity machine. As characters are
transferred from the computer, the bit count of each
is checked. If an even number is detected, this check
bit as well as the summary data check bit, will be set
into the status data.
The character code check is made by generating
three check characters for each address and record,
as bits are read from the disk surface during a read
operation. These newly generated characters are then
compared bit by bit with those previously generated
and recorded, in the gap following each address and
each record during the write operation. Failure to
compare will indicate the occurrence of an error.
Invalid Address Check
If a seek operation is attempted using an invalid
address, the access mechanism may be moved to a
customer engineer track (a data track for customer
engineering use). If this happens, and a read or write
command is given, an invalid address check occurs
and an immediate
UNUSUAL END
signal results.
Response Check
The response check indicates that a character was not
received by the 7631 within the allotted time. Re-
sponse check can only occur as a result of a read or
write operation.