All flags have individual significance,
except that the CC and SLI flags are ignored when the CD flag is set to one.
The SLI flag is ignored on immediate
operations, 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, and the
format is illustrated.
Programming 1. A malfunction that affects the
validity of data transferred 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 storage should not be
destroyed until the program has
verified that no malfunction
affecting the transfer and record­
ing of data was detected.
2. An error condition may be recog­
nized by the channel and the I/O operation terminated when 256 or
more chained commands are executed
with an I/O device and none of the
executed commands result in the
transfer of any data. When this
condition is recognized, program
check is indicated. IMMMMMMOll Data Address
o 8 31 C C 5 P I
D C L / C D 5 o //////// Count I I A
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 storage to the I/O device. Data in storage is fetched in an ascending order of addresses, start­
ing with the address specified in the CCW. A CCW used in a write operation is
inspected for the CD, CC, SLI, 5, PCI, and IDA flags. The setting of the skip
flag is ignored. Bit positions 0-5 of
the CCW contain modifier bits. Programming Note
When writing on devices for which block
length is not defined, such as a
magnetic-tape unit or an inquiry
station, the amount of data written is
controlled only by the count in the CCW. Every operation terminated under count
control causes the incorrect-length
indication, unless the indication is
suppressed by the SLI flag. MMMMMMIO Data Address
o 8 31
s C C S K P I
D C l I C D
S 0 //////// Count I
p
I A
32 40 48 63
A read operation is initiated at the I/O device, and the subchannel is set up to
transfer data from the device to
storage. For devices such as
magnetic-tape units, disk storage, and
card equipment, the bytes of data a block are provided in the same
sequence as written by means of a write
command. Data is placed in storage in
an ascending order of addresses, start­ ing with the address specified in the CCW. A CCW used in a read operation is
inspected for every flag -- CD, CC, SLI, SKIP, S, PCI, and IDA. Bit positions 0-5 of the CCW contain modifier bits. Chapter 13. Input/Output Operations 13-49

Read Backward MMMMII00 Data Address
o 8 31 S C C S K P I 0 C L I C 0 S 0 //////// Count I P I A
32 40 48 63
A read-backward operation is initiated
at the I/O device, and the subchannel is set up to transfer data from the device
to storage. On magnetic-tape units,
read backward causes reading to be performed with the tape moving backward. The bytes of data within a block are
sent to the channel in a sequence oppo­
site to that on writing. The channel
places the bytes in storage in a
descending order of addresses, starting
with the address specified in the CCW. The bits within a 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 flag -- CO, CC, SLI, SKIP, S, PCI, and lOA. Bit posi­
tions 0-3 of the CCW contain modifier
bits. Control Oata Address
o 8 31 C C S P I 0 C L / C 0 S o //////// Count I I A
32 35 40 48 63
A control operation is initiated at the
I/O device, and the subchannel is set up
to transfer data from storage to the
device. The device interprets the data as control information. The control
information, if any, is fetched from
storage in an ascending order of 13-50 System/370 Principles of Operation
addresses, starting with the address
specified in the CCW. A control command
may be used to initiate at the I/O
device an operation not involving trans­
fer of data, such as backspacing or
rewinding magnetic tape or positioning a
disk-access mechanism.
For many control functions, the entire
operation is specified by the modifier
bits in the command code, and the func­
tion is performed as an immediate opera­
tion (see the section "Immediate
Operations" later in this chapter). If
the command code does not specify the
entire control function, the data­
address field of the CCW designates the
location containing the required addi­
tional information. This control
information may include a code further
specifying the operation to be performed
or an external address, such as the disk
address for the seek function, and is
transferred 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
control command 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 oper­
ations and their codes are specified in the SL publication for the device.
A CCW used in a control operation is
inspected for the CD, CC, SLI, S, PCI, and IDA flags. The setting of the skip
flag is ignored. Bit positions 0-5 of
the CCW contain modifier bits.
Programming Note
Since a CCW (other than transfer in
channel) 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 transferred causes the incorrect­
length indication, provided the
operation is not immediate and has not
been rejected during the initiation
sequence. The incorrect-length indi­
cation is suppressed when the SLI flag
is on and the CO flag is off.