All unused sense bytes are set to zeros.
Bytes 1 and 2 contain the four-decimal­
digit control-unit type number that
corresponds directly with the control­
unit type number attached to the control
unit.
Byte 3 contains the control-unit model
number, if applicable. If not appli­ cable, byte 3 is a byte of all zeros.
Bytes 4 and 5 contain the four-decimal­
digit I/O-device type number that corre­
sponds directly with the I/O-device type number attached to the I/O device.
Byte 6 contains the I/O-device model
number, if applicable. If not applica­ ble, byte 6 is a byte of all zeros.
Whenever a control unit is not separate­
ly addressable from the attached I/O device or I/O devices, the response to
the sense-ID command is a concatenation
of the control-unit type number and the I/O-device type number.
If a control unit can be addressed sepa­
rately from the attached I/O device or I/O devices, then the response to the
sense-ID command depends on the unit
addressed. If the control unit is
addressed, the response to the sense-ID
command is as follows:
Bytes
o
1,2 3 Contents FF hex
Control-unit type number
Control-unit model number
The response consists of the control­
unit type and model number, with normal
ending status presented after byte 3.
If the I/O device is addressed, the
response to the sense-IO command is as
follows: Contents FF hex I/O-device type number I/O-device model number
The response consists of the I/O-device type and model number, with normal
ending status presented after byte 3.
For communication controllers utilizing
indirect addressing to end devices, and
for cases where the control unit and
device are not distinct, the sense data
source is the same as if a control unit
were being addressed.
A CCW used in a sense 10 operation is
inspected for every flag CO, CC, SLI,
SKIP, S, PCI, and IDA.
Transfer in Channel 1////1
1000 1 CCW Address 0 4 8 31 1/ / / / / / / / / / / / / / / / I 32 63
The next CCW is fetched from the
location in absolute 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 chan­
nel, 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 successive double­
word locations. 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 START I/O. When START I/O FAST RELEASE is executed independent of
the device, the error may cause, depend­
ing on the model, the same indication as
for START I/O or may cause an inter­
ruption condition to be generated.
To address a CCW on integral boundaries
for doublewords, a CCW specifying trans­
fer in channel must contain zeros in bit
positions 29-31. Furthermore, a CCW specifying a transfer in channel must
not be fetched from a location desig­
nated 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 immedi­
ate retry can be performed, the control
unit signals a channel-end, unit-check, Chapter 13. Input/Output Operations 13-53

and status-modifier status-bit combina­
tion, 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 ;s not
performed, and the channel command­
chains to the CCW following the one for
which retry was signaled. When any
other status bits accompany device end
or device end and status modifier, command retry is suppressed, and the
operation is terminated. The resulting CSW contains the status indications that
caused command retry to be suppressed.
When the channel is not capable of
performing command retry, the retry is
suppressed. If command retry is
suppressed during the execution of START I/O or START I/O FAST RELEASE executed as START I/O, the CSW is stored, and
condition code 1 is set. If command
retry is suppressed subsequently, the
operation is terminated, and an inter­
ruption condition is recognized. The CSW will contain the channel-end, unit­
check, and status-modifier status
indications, along with any other appro­
priate status.
During command retry, the channel action
is similar to that taken when command
chaining. Thus, when command retry is
performed, a START I/O initiating an
immediate operation for which command
chaining is not indicated in the CCW causes a condition code 0, rather than a condition code 1, to be set. The subse­
quent termination of the I/O operation
causes an interruption condition to be
generated. During command retry, the CCW may be refetched. Programming Note The following possible results of
command retry must be anticipated by the
program:
1. A CCW with the PCI flag set to one
may, if retried because of command
retry, cause multiple PCI inter­
ruptions to occur.
2. A channel program consisting of a single, unchained CCW specifying an
immediate command may cause a
condition code 0 rather than a condition code 1 to be set. This
setting of the condition code
occurs if the control unit signals
command retry at the time initial
status is signaled to the channel.
An interruption condition is gener­
ated upon completion of the
operation.
13-54 System/370 Principles of Operation 3. If a CCW used in an operation is
changed before that operation has
been successfully completed, the
results are unpredictable. 4. A CSW stored after the initiation
of a retry but before the presenta­
tion of device end, as when an
interruption due to the PCI flag
occurs, contains the address of the
command to be retried plus 8.
5. If a HALT I/O, HALT DEVICE, or CLEAR I/O instruction is issued
after the initiation of a retry but
before the presentation of device end, the CSW contains the address
of the command to be retried plus
8.
6. On a multiplexer channel, chained CCWs which might ordinarily have
been executed in a burst may, upon
the occurrence of command retry, cause multiplexing to occur, with
the result that the channel becomes
unexpectedly available.
7. Command chaining may occur even
though the CCW does not indicate
command chaining. This can occur if command retry is signaled, imme­
diate retry is not requested, and
the control unit or device presents
status consisting solely of device
end and status modifier. CONCLUSION OF INPUT/OUTPUT OPERATIONS When the operation or sequence of oper­
ations initiated by START I/O or START I/O FAST RELEASE is ended, the channel
and the device generate status. Status
can be brought to the attention of the
program by means of an I/O interruption,
by TEST I/O or CLEAR I/O, or, in certain
cases, by START I/O or START I/O FAST
RELEASE. This status, as well as an
address and a count indicating the
extent of the operation sequence, are
presented to the program in the form of a channel-status word (CSW). TYPES OF CONCLUSION Normally an I/O operation at the
subchannel lasts until the device
signals channel end for a CCW for which
command chaining or command retry ;s not
indicated. Channel end can be signaled
during the sequence initiating the oper­
ation, or later. When the channel
detects equipment malfunctioning or an
I/O-system reset is performed, the chan­
nel disconnects the device without
receiving channel end. The program can
force a device to be disconnected prema-