Read Commands
The read commands cause data to transfer from the cache to the channel. The
control unit is oriented to a specific record area and sends one or more areas or
records to the channel.
All read commands except Read IPL must be preceded in the command chain by a
command that specifies (or implies) (CCHH) parameters. If the read command is
not preceded by a Seek, Seek Cylinder, Locate Record, Locate record Extended, or
Read IPL command, the command is rejected with unit check status. The sense
data contains command reject with format 0, message 2.
All read commands except Read Track and Read Track Data are valid outside the
domain of a Locate Record or Locate Record Extended.
The Read Multiple CKD and Read IPL commands are not valid in the domain of a
Locate Record or Locate Record Extended.
If the read command is preceded by a Define Extent command, the control unit
determines if the track is in the extent limits described by the preceding Define
Extent command. If the track is not in the specified boundaries, the read command
is terminated with unit check status. The sense data includes File Protected.
Most read commands can be in multitrack mode. (See “Operations Outside the
Domain of a Locate Record or Locate Record Extended” on page 4-3.) Only two
of the read commands accepted by Internal Disk; Read Multiple Count Key and
Data, and Read IPL are defined as single-track mode only. Only the Read Track
command is defined as multitrack mode only. All other commands may be
executed in either mode.
If the channel truncates data transfer before all data bytes transfer, the control unit
continues reading data until the end of the record. The truncation of data transfer
by the channel does not cause an exception condition.
The control unit presents channel end and device end status after it completes
processing the record. If the control unit finds an exception condition after the
channel stops data transfer, the status includes unit check with channel end and
device end.
Chapter 4. Command Descriptions
4-61
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