SLI, SKIP, PCI, and IDA. Bit positions 0-3 of the
CCW contain modifier bits.
Control
Data Address
o 8 Count 32 40 48 63
A control operation is initiated at the I/O device,
and the sub channel is set up to transfer data from
main storage to the device. The device interprets the
data as control information. The control informa­
tion, if any, is fetched from storage in an ascending
order of addresses, starting with the address speci­
fied in the CCW. A control command may be used
to initiate at the I/O device an operation not involv­
ing transfer of data --such as backspacing or rewind­
ing magnetic tape or positioning a disk-access mech­
anism.
For many control functions, the entire operation
is specified by the modifier bits in the command
code, and the function is performed over the I/O interface as an immediate operation (see "Immediate Operations"). If the command code does not specify
the entire control function, the data-address field of
the CCW designates the location containing the
required additional information. This control informa­
tion may include an order code further specifying
the operation to be performed or an address, such as
the disk address for the seek function, and is trans­
ferred in response to requests by the device.
A control command code containing zeros for the
six modifier bits is defined as a no-operation. The
no-operation order causes the addressed device to
respond with channel end and device end without
causing any action at the device. The order can be
executed as an immediate operation, or the device
can delay the status until after the initial selection
sequence is completed. Other operations that can be
initiated by means of the control command depend
on the type of I/O device. These operations and
their codes are specified in the SL or SRL publica­
tion for the device.
A CCW used in a control operation is inspected
for the CD, CC, SLI, PCI, and IDA flags. The set­
ting of the skip flag is ignored. Bit positions 0-5 of
the CCW contain modifier bits. Since a CCW with a count of zero is invalid, the
program cannot use the CCW count field to specify
that no data be transferred to the I/O device. Any
operation terminated before data has been trans­
ferred causes the incorrect-length indication, provided
the operation is not immediate and has not been
rejected during the initiation sequence. The
incorrect-length indication is suppressed when the SLI flag is on.
Sense I MMMM0100 Data Address I : 0 8 31 C C S S P I D C L K C D 00 Count I I A
P
32 40 48 63
A sense operation is initiated at the I/O device, and
the sub channel is set up to transfer data from the
device to main storage. The data is placed in storage
in an ascending order of addresses, starting with the
address specified in the CCW.
Data transferred during a sense operation pro­
vides information concerning both unusual condi­
tions detected in the last operation and the status of
the device. The status information provided by the
sense command is more detailed than that supplied
by the unit-status byte and may describe reasons for
the unit-check indication. It may also indicate, for
example, if the device is in the not-ready state, if the
tape unit is in the file-protected state, or if magnetic
tape is positioned beyond the end-of-tape mark.
For most devices, the first six bits of the sense
data describe conditions detected during the last
operation. These bits are common to all devices hav­
ing this type of information and are designated as
follows:
Bit Designation 0 Command reject
1 Intervention requ ired
2 Bus-out check
3 Equipment check
4 Data check
5 Overrun The following is the meaning of the first six bits:
Input/ Output Operations 219

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.