Record Accessing
An associative addressing (search) operation can find a specific record, which
causes the control unit to read some area from the device and compare (associate)
it with a value from the channel. The Locate Record command or one of the
search commands is used to do the search operation.
Data Transfer Commands
Data transfer commands are commands that transfer data that is, or will be,
recorded on the track. All data transfer commands except Read IPL and Read
Multiple CKD can be in a Locate Record or Locate Record Extended domain.
A Define Extent or Read IPL command must precede the Locate Record command
in the same channel program. Only one Define Extent command can be in a
channel program. However, a channel program can have more than one Locate
Record or Locate Record Extended command.
Brief descriptions of the Define Extent, Locate Record, and Locate Record
Extended commands follow:
Define Extent Command:
Defines the extent that the channel program will operate in. An
extent
is a set
of consecutively addressed tracks that a channel program can access. The
limits of an extent are defined by specifying the addresses of the first and last
tracks in the extent.
Defines attributes of, and limitations on, the commands that follow in the
channel program.
Defines usage and control attributes for cache for the remaining channel
program.
See “Define Extent” on page 4-8 for a complete description.
Locate Record and Locate Record Extended Commands:
The Locate Record
command:
Specifies the operations and the number of consecutive records (or tracks).
Specifies the address of the first track and the orientation state to establish
before starting data transfer.
Defines the bounds and attributes of an operational
domain
. This domain
begins with the transfer of the Locate Record parameters and extends through
the last operation specified in the Locate Record parameters. Some data
transfer commands run differently in the operational domain of a Locate
Record.
The domain does not necessarily extend to the end of the command chain.
For example, the domain of a Locate Record command with the Format Write
operation code extends through the number of formatting write commands
specified by the Locate Record count parameter.
The Locate Record command cannot be used to transfer
nonconsecutive
records
in the domain of a single Locate Record command. However, a Locate
Record Extended command can read
nonconsecutive tracks
in some
operations.
Chapter 3. Elements of Data Access
3-3
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