All flags have individual significance, except that
the CC and SLI flags are ignored when the CD flag
is on. The SLI flag is ignored on immediate opera­
tions, in which case the incorrect-length indication is
suppressed regardless of the setting of the flag. The PCI flag is ignored during initial program loading.
Each command is described below with an illustra­
tion of its CCW format.
Programming Note
A malfunction that affects the validity of data trans­
ferred in an I/O operation is signaled at the end of
the operation by means of unit check or channel
data check, depending on whether the device
(control unit) or the channel detected the error. In
order to make use of the checking facilities provided
in the system, data read in an input operation should
not be used until the end of the operation has been
reached and the validity of the data has been
checked" Similarly, on writing, the copy of data in
main storage should not be destroyed until the pro­
gram has verified that no malfunction affecting the
transfer and recording of data was detected.
Write
Read I MMMMMM10 I o 8
C
C SSp I D
C
L K C
D 00 I I P I A
32 [ Data Address
31
Count
63
A read operation is initiated at the I/O device, and
the sub channel is set up to transfer data from the
device to main storage. For devices such as magnetic
tape units, disk storage, and card equipment, the
bytes of data within a block are provided in the same
sequence as written by means of a write command.
Data in storage is placed in an ascending order of
addresses, starting with the address specified in the
CCW.
A CCW used in a read operation is inspected for
every one of the six flags--CD, CC, SLI, SKIP, PCI, and IDA. Bit positions 0-5 of the CCW contain
modifier bits. Data Address J Read Backward
o 8
31
1 r-- M-MM-M-110-0 .------O-ata-Ad-dre-ss ---r-II :: 8 31
32 35 40 48 63 A write operation is initiated at the I/O device, and
the subchannel is set up to transfer data from main
storage to the I/O device. Data in storage is fetched
in an ascending order of addresses, starting with the
address specified in the CCW.
A CC'W used in a write operation is inspected for
the CD, CC, SLI, PCI, and IDA flags. The setting of
the Skip nag is ignored. Bit positions 0-5 of the
CCW contain modifier bits. Progranruoodng Note
When writing on devices for which block length is
not defined, such as a magnetic tape unit or an in­
quiry station, the amount of data written is con­
trolled only by the count in the CCW. Every opera­
tion terminated under count control causes the
incorrect·-Iength indication, unless the indication is
suppressed by the SLI flag.
218 Principles of Operation C
C SSp I D C L K C D 00 I I P I A
32
Count
63
A read-backward operation is initiated at the I/O device, and the sub channel is set up to transfer data
from the device to main storage. On magnetic tape
units, read backward reading to be performed
with the tape moving backwards. The bytes of data
within a block are sent to the channel in a sequence
opposite to that on writing. The channel places the
bytes in storage in a descending order of address,
starting with the address specified in the CCW. The
bits within an eight-bit byte are in the same order as
sent to the device on writing.
A CCW used in a read-backward operation is
inspected for every one of the six flags--CD, CC,

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