IBM System/370 145 Manual page 48

sequential position on the track. Therefore, sector location can be
calculated for fixed-length records.
Two new disk commands are provided for use with rotational position
sensing: SET SECTOR and READ SECTOR. If the sector address of a record
is known or can be calculated, a SET SECTOR command can be included in
the disk channel program to cause the control unit to look for the
designated sector. Once the control unit accepts the sector number
provided by a SET SECTOR command, both the block multiplexer channel and
the disk control unit disconnect and are available for another I/O
operation. When SET SECTOR is used for positioning, the time the
channel is busy during the search for a record is reduced from an
average of 8.3 ms to an average of 260 microseconds for 3330-series
drives. (Allowing for the worst case of speed variation and for disk
pack interchange, the search time for a record, from sector found to
beginning of desired record, can vary from 120 microseconds to 380
microseconds on a 3330-series drive.)
The READ SECTOR command is useful for sequential disk processing and
for write verification. When chained from a READ, WRITE, or SEARCH
command, READ SECTOR provides the sector number required to access the
record processed by the previous CCW. This sector number can be used to
reposition the track in order to verify the record just written or in
order to read or write the next sequential record. These two new sector
commands, used in conjunction with the block multiplexer channel, permit
a single command-chained channel program, which frees the channel and
disk control unit during seek and rotational positioning operations, to
be initiated for each disk operation.
When record 10 is known, the two channel programs shown below
illustrate direct retrieval of a record from an OS BDAM data set on a
direct access device without RPS, such as the 2314 (key not written).
The seek operation can be overlapped with other seeks and one data
transfer operation on the same selector channel. (Commands shown in the
two channel program examples are only those that illustrate the
advantage of RPS. Thus, commands such as SEEK HEAD and SET FILE MASK,
which are used by data management to ensure correct operation, are not
shown. )
Channel program 1. Initiate the stand-alone seek to position the disk arm.
Command
Chaining
Flag
Command
SEEK (Seek address)
Selector Channel
and Disk Control
Unit Status
Free as soon as
control unit accepts
seek address
Channel program 2. Initiate the data transfer operation after the seek
is complete.
Command
Chaining
Flag
CC
CC
38
Command
SEARCH ID EQ (ID - sequential position
on the track)
TIC (Back to search if ID not equal)
READ DATA (Storage address of input area)
Selector Channel
and Disk Coiltrol
Unit Status
Busy (12.5 ms on
average for 2314)
Busy
A Guide to the IBM System/370 Model 145