Read Record Zero
Description:
The Read Record Zero command transfers the entire record zero
(count and data bytes) to the channel.
When in a Locate Record or Locate Record Extended domain, the control unit is
oriented to home address when it receives this command. Data transfer begins
with the following record zero count area.
When not in a Locate Record or Locate Record Extended domain, if the control unit
is oriented to home address, the operation is the same as when in a Locate Record
or Locate Record Extended domain. If not at home address, the control unit is
oriented to index, and the operation continues according to the multitrack bit (bit 0)
in the CCW:
If the multitrack bit is ‘0’, the control unit is oriented to the record zero count
area on the same track and sends the record to the channel.
If the multitrack bit is ‘1’, the control unit advances to the next track (see
“Operations Outside the Domain of a Locate Record or Locate Record
Extended” on page 4-3), is oriented to the record zero count area, and sends
the record to the channel.
Note:
When in a Locate Record or Locate Record Extended domain, if the control
unit is not oriented to home address when starting a multitrack Read R0,
the command always advances to the next track before sending record
zero.
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 Seek, Seek Cylinder, Locate Record,
Locate Record Extended, or Read IPL command
In a Locate Record or Locate Record Extended domain that specifies other
than a Read Data (06) or a Read (16) operation and index or home address
orientation (byte 0, bits 0 and 1 are ‘11’ or ‘01’).
If the track does not have a record zero, the command is terminated with unit check
status. The sense data includes No Record Found.
Ending Status:
The control unit presents channel end and device end status
when data transfer completes. When the command completes, the control unit is
oriented to the record zero data area.
Hex Code
Data Address
Count
16
Multitrack
96
The main storage location for the
first byte of the record zero (R0)
count data.
The number of count and
data bytes to be read.
Chapter 4. Command Descriptions
4-63
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