
Chaining and Restrictions:
The Perform Subsystem Function command cannot
occur in the domain of a Locate Record or Locate Record Extended. If the
command is in a Locate Record or Locate Record Extended domain, it is rejected
with unit check status. The sense data contains command reject with format 0,
message 2.
See “Effect of Subsystem and Device State on Status Presented to Command” on
page 2-5 for the operation of this command when other subsystem and device
status conditions occur.
The valid orders of this command are accepted when the logical volume is not
available. If the command is received with this condition in effect and the order
byte is not X
'
18
'
or X
'
B0
'
, the command is terminated with status that includes
unit check together with the appropriate sense data.
The orders have different parameter lengths. If the channel sends more bytes than
the order requires, the control unit accepts only the number of bytes required by the
order. If the channel sends fewer bytes than the order requires, the command is
rejected with unit check status. The sense data contains command reject with
format 0, message 3.
See the description of the specific order for any additional requirements.
Ending Status:
See the specific order.
Table
4-34. Perform Subsystem Function Orders and Flags
Byte
Description
0
Order Byte
Hex
Name of Order
Value
00–17 Reserved
18
Prepare for Read Subsystem Data
19–1A Reserved
1B
Set Special Intercept Condition
1C Reserved
1D
Set System Characteristics
1E–AF Reserved
B0
Set Interface Identifier
B1–FF Reserved
1
Flag Byte
Bits Description
0
Must be set to zero.
1–7
See the specific order.
Prepare for Read Subsystem Data X
'
18
'
This order stores data in the control unit (in an internal buffer) so that a Read
Subsystem Data command can obtain it.
This order requires 12 parameter bytes, including the order, byte 0, to be
transferred from the channel. Table 4-35 on page 4-132 shows the definitions of
the parameter bytes.
Chapter 4. Command Descriptions
4-131
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