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special functions by use of the sense command may
define modifier bits for the control of these
functions. The special sense operations may be
initiated by a unique combination of modifier bits,
or a group of codes may specify the same function.
Any remaining sense command codes may be
considered invalid, thus causing the unit-check
indication, or may cause the same action as the
basic sense command, depending upon the type of
device.
The sense information that pertains to the last
I/O operation or other action at a device may be
reset any time after the completion of a sense
command addressed to that device. Any command
addressed to the control unit of a device, other
than the no-operation command and the command
which results from a TEST I/O instruction, may be
allowed to reset the sense information, provided
that the busy bit is not included in the initial status.
The sense information may also be changed as a
result of asynchronous actions, such as when
attention or not-ready-to-ready device-end status is
generated.
A CCW used in a sense operation is inspected
for every flag-CD, CC, SLI, SKIP, and PCl. Bit
positions 0-3 of the CCW contain modifier bits.
Transfer in Channel
CCW Address
o
4
8
31
II
I I I I I I I I I I I I I I I
I
32
63
The next CCW is fetched from the location in
storage designated by the data-address field of the
CCW specifying transfer in channel. The
transfer-in-channel command does not initiate any
I/O operation at the channel, and the I/O device is
not signaled. The purpose of the transfer-in-
channel command is to provide chaining between
CCWs not located in adjacent doubleword
locations in an ascending order of addresses. The
command can occur in both data and command
chaining.
The first CCW designated by the CAW must not
specify transfer in channel. When this restriction is
violated, no I/O operation is initiated, and a
program check is generated. The error causes the
status portion of the CSW, with the program-check
status bit set to one, to be stored during the
execution of START I/O or START I/O FAST
RELEASE being executed as ST ART I/O. When
START I/O FAST RELEASE is executed
independent of the device, the error causes an
interruption condition to be generated.
To address a CCW on integral boundaries for
doublewords, a CCW specifying transfer in channel
must contain zeros in bit positions 29-3l.
Furthermore, a CCW specifying a transfer in
channel must not be fetched from a location
designated by an immediately preceding transfer in
channel. When either of these errors is detected, a
program check is generated.
The contents of the second half of the CCW, bit
positions 32-63, are ignored. Similarly, the
contents of bit positions 0-3 of the CCW are
ignored.
Command Retry
Some channels have the capability to perform
command retry, a channel and control-unit
procedure that causes a command to be retried
without requiring an I/O interruption. This retry is
initiated by the control unit presenting either of
two status-bit combinations by means of a special
communication sequence with the channel. When
immediate retry can be performed, the control unit
signals a channel-end, unit-check, and status-
modifier status-bit combination, together with
device end. When immediate retry cannot be
performed, the presentation of device end is
delayed until the control unit is prepared.
If
device ,
end and no other status bits are signaled, command
retry is performed.
If
device end is accompanied ,
by status modifier, command retry is not
performed, and the channel command-chains to the
CCW following the one for which retry was
signaled. When any other status bit accompany
device end or device end and status modifier, an
interruption condition is generated. In this
situation, the CSW will contain
~he
status
indications causing the interruption condition.
When the channel is not capable of performing
command retry, the retry is suppressed, and an
interruption condition is generated. The CSW will
contain the channel-end, unit-check, and status-
modifier status indications, along /with any other
appropriate status.
//
During command retry, tl}e"channel action is
similar to that taken
whe~ommand
chaining.
Thus, when command yeiry is performed, a START
I/O initiating an immediate operation for which
command chaining is not indicated in the CCW
Chapter 12. Input/Output Operations
12-39
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