Commalld Reject: The device has detected a pro­
gramming error. A command has been received
which the device is not designed to execute t such as
read backward issued to a direct-access storage de­ vice t or which the device cannot execute because of
its present state t such as write issued to a file­
protected tape unit. Command reject is also indicat­
ed when the program issues an invalid sequence of commands t such as write to a direct-access storage
device without previously designating the data block. Interventlion Required: The last operation could not
be executed because of a condition requiring some
type of ill1tervention at the device. This bit indicates
conditions such as an empty hopper in a card punch
or the printer being out of paper. It is also turned on
when the addressed device is in the not-ready state t is in test mode t or is not provided on the control
unit.
Bus Out Check: The device or the control unit has
received a data byte or a command code with an
invalid parity over the 110 interface. During writing t bus-out check indicates that incorrect data has been
recorded at the device t but the condition does not
cause the operation to be terminated prematurely. Parity errors on command codes and control infor­
mation cause the operation to be immediately termi­
nated and suppresses checking for command reject
and intervention required conditions.
Equipmellit Check: During the last operation t the
device or the control unit has detected equipment
malfunctioning t such as an invalid card hole count or
printer buffer parity error.
Data Check: The device or the control unit has
detected a data error other than those included in
bus-out check. Data check identifies errors associat­
ed with th.e recording medium and includes condi­
tions such as reading an invalid card code or detect­
ing invalid parity on data recorded on magnetic tape. On an input operation, data check indicates that
incorrect data may have been placed in main storage.
The control unit forces correct parity on data sent to
the channel. On writing t this condition indicates that
incorrect data may have been recorded at the device.
Unless the operation is of a type where the error
precludes meaningful continuationt data errors on
reading and writing do not cause the operation to be
terminated prematurely. 220 System/370 Principles of Operation Oven-un: The channel has failed to respond on time
to a request for service from the device. Overrun can
occur when data is transferred to or from a nonbuf­
fered control unit operating with a synchronous me­
dium, and the total activity initiated by the program
exceeds the capability of the channel. When the
channel fails to accept a byte on an input operation t the following data transferred to main storage may
be shifted to fill the gap. On an output operation t overrun indicates that data recorded at the device
may be invalid. The overrun bit is also turned on
when the device receives the new command too late
during command chaining.
All information significant to the use of the device
normally is provided in the first two bytes. Any bit
positions following those used for programming in­
formation contain diagnostic information, which may
extend to as many bytes as needed. The amount and
the meaning of the status information are peculiar to
the type of 110 device and are specified in the SL or
SRL publication for the device.
The basic sense command has zero modifier bits.
This command initiates a sense operation on all de­
vices and cannot cause the command-rejectt
intervention-required, data-check t or overrun bits to
be turned on. If the control unit detects an equip­
ment malfunction, or invalid parity of the sense com­
mand code t the equipment-check or bus-out-check
bits are turned on t and unit check is indicated in the
unit-status byte.
Devices that can provide special diagnostic sense
information or can be instructed to perform other
special functions by use of the sense command, may
define modifier bits for the control of these func­
tions. 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 re­
maining sense command codes may be considered
invalid, thus causing the unit-check indication, or
may cause the same action as the basic sense com­
mand, depending upon the type of device.
The sense information pertaining to the last 1/0 operation or unit action may be reset any time after
the completion of a sense command addressed to
that device. The sense information may also be reset
by any other command addressed to the control unit,
provided the busy bit is not included in the initial
status byte, except where the command is a TEST 110 or a no-operation and is addressed to the device
that causes the sense.
A CCW used in a sense operation is inspected for
everyone of the six flags--CD, CC t SLIt SKIP t PCI, and IDA. Bit positions 0-3 of the CCW contain
modifier bits.

I Transfer in Channel 1000
1 CCWAddress I : o 4 8 31 32 63
The next CCW is fetched from the location in abso­
lute main 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 of the execution of the command. The
purpose of the transfer-in-channel command is to
provide chaining between CCWs not located in adja­
cent 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 may not
specify transfer in channel. When this restriction is
violated, no I/O operation is initiated, and the
program-check condition is generated. The error
causes the status portion of the CSW with the
program-check indication to be stored during the
execution of START I/O or ST ART I/O FAST RELEASE being executed as START I/O. When
ST ART I/O FAST RELEASE is executed inde­
pendently of the device, the error causes an I/O 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 29-31. Further­
more, a CCW specifying a transfer in channel may
not be fetched from a location designated by an
immediately preceding transfer in channel. When
either of these errors is detected or when an invalid
address is specified in transfer in channel, the
program-check condition is generated. When the
transfer-in-channel command designates a CCW in a
location protected against fetching, the protection­
check condition is generated. Detection of these
errors during data chaining causes the operation at
the I/O device to be terminated and an interruption
condition to be generated, whereas during command
chaining it causes only an interruption condition to
be 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 com­
mand retry, a channel and control-unit procedure
that causes a command to be retried without requir­
ing an I/O interruption. This retry is initiated by the
control unit presenting either of two status-bit com­
binations by means of a special I/O interface se­
quence. When immediate retry can be performed, it
presents 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. When the channel is not
capable of performing command retry, or when any
status bit other than device end accompanies the
requested command retry initiation, the retry is sup­
pressed, 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, the channel action is simi­
lar to that taken when command chaining. Thus,
when command retry is performed, a START I/O initiating an immediate operation for which com­
mand chaining is not indicated in the CCW causes a
condition code 0, rather than condition code 1, to be
set. The subsequent termination of the command
execution causes an interruption condition to be
created.
Programming Notes
The following possible results of a command retry
must be anticipated by the program:
1. A CCW containing a PCI may, if reiried be­
cause of command retry, cause multiple PCI interruptions to occur.
2. A channel program consisting of a single, un­
chained CCW specifying an immediate com­
mand may cause a condition code 0 rather than
1 to be set. This setting of the condition code
occurs if the control unit signals command re­
try at the time initial status is presented to the
command. The channel program then causes a
later interruption upon completion of the oper­
ation.
3. If a CCW used in an operation is changed be­
fore that operation has been successfully com­
pleted, the results are unpredictable.
4. A CSW stored after the initiation of a retry but
prior to the presentation of device end, as
when a PCI interruption is taken, contains the
address of the command to be retried + 8.
5. If a HALT I/O, HALT DEVICE, or CLEAR I/O instruction is issued between the initiation
of a retry but prior to the presentation of de-
Input/Output Operations 221