IBM 1620 1 Manual page 51

Write Disk/WLRC
WDGN or 38 -
Q
11
of 1
This instruction causes a read/write head to be
selected as specified by the sector address in the
disk control field. The selected head then scans the
sector addresses recorded on the disk track until a
matching address is found.
If
a matching address is
not found within one complete revolution of the
disks, or if a read-only flag is sensed in the matching
address, the operation is terminated and the Address
Check indicator (36) is turned on. When the sector
address in the disk control field matches a sector
address in disk storage, writing begins from the core
storage location specified and continues sequentially
through higher-numbered positions for the indicated
number of sectors.
The sector address in the 1620 register OR-1 is in-
cremented by one each time a sector is written, and
the address preceding each sector in disk storage is
compared against it to ensure the correct sequential
succession.
If
an address fails to compare or if a
read-only flag is sensed in a matching address, the
operation terminates and the Address Check indicator
( 36 ) is turned on.
If the number of sectors written goes beyond one
disk surface, a shift to the read/write head for the
next disk surface is made automatically and writing
continues, without loss of time. However, if the end
of the cylinder is reached and the sector count has
not been decremented to 000, the operation ter-
minates and the Cylinder Overflow indicator (38)
is turned on. Therefore, the greatest number of sectors
that can be written with one instruction is 200, one
full cylinder.
MBR-E
(16) and
MBR-O
(17) are turned on to in-
dicate a parity error in data from core storage. In
addition, each character transferred to disk storage
is checked for parity. Failure to meet the parity check
causes the Write Check indicator (07) to turn on.
Any parity check will terminate the operation.
During Write Disk instructions the Write Address
switch must be off.
This instruction checks, in addition to the sector
count, that the correct number of characters is trans-
ferred from core storage to disk storage. A group
mark stored in core storage in the location following
the last position of the record provides the correct
termination of the operation. It is important to use
the wrong-length record check whenever practical to
avoid the loss of disk storage data by writing beyond
the intended number of sectors.
Group marks in disk storage do not affect the
operation, and are replaced by data from core storage.
Execution Time. Average T
==
22
+
25 ms
Check Disk/WLRC
CDGN or 36 - Q
11
of 1
This instruction provides the means for checking data
written in disk storage against the same data in core
storage. This verification is in addition to the record
length check and parity check. Data written in disk
storage should be verified by a Check Disk instruction
after every Write Disk instruction, while the original
data is still in core storage.
A Check Disk instruction can also be used to ascer-
tain if core storage data, such as tables, constants, etc.,
has been changed.
Execution of this instruction causes the head to be
selected as specified by the sector address in the disk
control field. The selected head then scans the sector
addresses recorded on the disk track until a matching
address is found.
If
a matching address is not found
within one complete revolution of the disks, the opera-
tion terminates and the Address Check indicator (36)
is turned on.
When the sector address in the disk control field
matches a disk sector address, reading begins at the
sector specified and continues for the indicated num-
ber of sectors. As each character is read from disk
storage it is compared, bit-by-bit and character-by-
character, with the data in core storage beginning
with the address specified. Failure to compare ter-
minates the operation at the end of the sector being
read and turns on the Wrong-Length Record Check
indicator (37).
During Check Disk instructions, the sector address
in the 1620 register OR-1 is incremented by one
each time a sector is checked, and the disk address
preceding each sector is compared to ensure the cor-
rect sequential succession. If an address fails to com-
pare, the operation is terminated and the Address
Check indicator (36) is turned on.
If the number of sectors read goes beyond one disk
surface, a shift to the read/write head for the next
disk surface is made automatically and reading con-
tinues without loss of time. If the end of the cylinder
is reached however, and the sector count has not
been decremented to 000, the oper:ltion is terminated
and the Cylinder Overflow indicator (38) is turned on.
Therefore, the greatest number of sectors that can be
checked with one instruction is 200, one full cylinder.
Each character read out of disk storage is checked
for parity. Failure to meet the parity check causes
the operation to terminate at the end of the sector
being read and the Read Check indicator (06) to
turn on. In addition, the
MBR-E
indicator (16) or
the
MBR-O
indicator (17) turns on to indicate a parity
error in a character from 1620 core storage. Any
parity check terminates the operation.
47