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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-CD, CC, SLI, SKIP, and
PCl. Bit positions 0-3 of the CCW contain
modifier bits.
Control
MMMMMM 11
Data Address
o
8
31
C C
S
P
D
C
L
I
C 000
IIIIIIII
Count
I
I
32
35
40
48
63
A control operation is initiated at the I/O device,
and the sub channel 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 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 transfer 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 function is performed as an
immediate operation (see the section "Immediate
Operations
11
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 additional
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 operations 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, and PCI 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 indication is suppressed when the
SLI flag is on.
Sense
MMMM0100
Data Address
o
8
31
s
C C
S
K P
D
C
L I
C 000
IIIIIIII
Count
I P I
32
40
48
63
A sense operation is initiated at the I/
a
device,
and the sub channel is set up to transfer data from
the device to 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
provides information concerning both unusual
conditions 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 in the
CSW 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.
Chapter 12. Input/Output Operations
12-37
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