
Valid parameters include the following:
The flag byte and bytes 2–5 must be zero.
Parameter byte 6 must contain a valid suborder. The only valid suborders are
X
'
00
'
and X
'
01
'
and X
'
03
'
.
Byte 7 must be zero.
Bytes 8 through 11 must be zero unless the suborder in byte 6 is X
'
03
'
, in
which case they contain a 4-byte message identifier.
If these conditions are not met, the command is rejected with unit check status.
The sense data contains command reject with format 0, message 4.
Ending Status:
The control unit presents channel end when it validates the
parameters. Device end occurs when the data is available. In most cases,
channel end and device end occur together.
Only a Read Subsystem Data command can be chained from the Perform
Subsystem Function command with this order. If any other command is chained
from the command with this order, it is rejected with unit check status. The sense
data contains command reject with format 0, message 2 (the contents of the buffer
can be lost). The buffer’s content can also be lost if the Read Subsystem Data
command is not chained, but the control unit does not report an error. See “Read
Subsystem Data” on page 4-139 for the content of the buffer.
Table
4-35. Parameter Bytes for Prepare for Read Subsystem Data X
'
18
'
Bytes
Definition
1
Flag byte. Must be set to zero
2–5
Reserved
6
Suborder defining the data to place in the buffer:
Value
Description of the suborder
00
The status of the subsystem.
01
Cache performance statistics.
02 Reserved
03
Read Attention Messages
04–FF Reserved
7
Reserved, set to zero.
8–11
Depends on the value in byte 6:
If the suborder in byte 6 is X
'
03
'
(Read Attention Messages) then bytes
8–11 contain a Message Identifier.
Otherwise, these bytes are set to zero.
Set Special Intercept Condition - PSF Order Code x'1B'
The intent of this order is to set a condition in the subsystem that causes it to reject
specific commands for the addressed logical volume. The VM operating system in
particular, issue the order to oversee and control the use of physical resources by
guest operating systems. The guest operating system can be granted ownership of
a specific logical volume, however, in the course of using the logical volume, a
command may be issued that changes the behavior of a different logical volume.
Commands and orders that have a scope that goes beyond a single logical volume
4-132
Internal Disk Subsystem Reference Guide
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