
Seek Head
Description:
The Seek Head command transfers the track address to the control
unit. The format of the seek address parameter is (00CCHH), where (00) is 2
bytes of binary zeros and (CCHH) is a 4-byte track address. The (HH) part of the
track address replaces the (HH) value in the current track address.
The control unit saves the seek address to do a cache directory search or to select
the head at a later time.
Chaining and Restrictions:
Unit check status with the sense data containing
command reject with format 0, message 2 occurs if this command is:
Not preceded in the command chain by a command that establishes track
orientation.
In a Locate Record or Locate Record Extended domain.
If the file mask inhibits Seek Head commands (bits 3 and 4 are ‘11’), the command
is terminated with unit check status (file protected).
The control unit requests a 6-byte set of parameters from the channel. If the
channel sends fewer than 6 bytes, the command is rejected with unit check status.
The sense data contains command reject with format 0, message 3.
The track address is checked for validity for the current access authorization and
the logical volume type. (See Appendix A, "Device Characteristics" on page A-1.)
If the seek address parameter is not valid, the command is rejected with unit check
status. The sense data contains command reject with format 0, message 4.
If the Seek Head command is preceded in the channel program by a Define Extent
command, and the seek address is outside the defined extent, the command is
terminated with unit check status. The sense data includes File Protected.
Ending Status:
After the control unit receives a valid parameter, it checks the
cache directory to determine if the track is in cache; it saves the track address and
presents channel end and device end status.
The track accessing operation is deferred as described under “Seek Operations” on
page 4-46.
Hex Code
Data Address
Count
1B
The main storage location of the seek address.
6
Chapter 4. Command Descriptions
4-49
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