Advertisement
Data Address: Bits 8-31 specify a location in
storage. It is the
first
location referred to in the
area designated by the
CCW.
Chain-Data
(CD)
Flag: Bit 32, when one, specifies
chaining of data. It causes the storage area
designated by the next CCW to be used with the
current operation.
Chain-Command (CC) Flag: Bit 33, when one, and
when the CD flag is zero, specifies chaining of
commands. It causes the operation specified by the
command code in the next CCW to be initiated on
normal completion of the current operation.
Suppress-Length-Indication (SLI) Flag: Bit 34
controls whether incorrect-length is to be indicated
to the program. When this bit is one and the CD
flag is zero, the incorrect-length indication is
suppressed. When both the CC and SLI flags are
one, command chaining takes place regardless of
any incorrect-length situation.
Skip (SKIP) Flag: Bit 35, when one, specifies
suppression of the transfer of information to
storage during a read, read backward, or sense
operation.
Program-Controlled-Interruption (PCI) Flag: Bit
36, when one, causes the channel to generate an
interruption condition when the CCW takes control
of the channel. When bit 36 is zero, normal
operation takes place.
Count: Bits 48-63 specify the number of bytes in
the storage area designated by the CCW.
Bit positions 37-39 of every CCW other than
one specifying transfer in channel must contain
zeros. Otherwise, a program-check condition is
generated. When the first CCW designated by the
CAW does not contain zeros in bit positions 37-39,
the I/O operation is not initiated, and the status
portion of the CSW with the program-check
indication is stored during execution of START
I/O or ST ART I/O FAST RELEASE being
executed as START I/O; Detection of this
condition during data chaining causes the I/O
device to be Signaled to conclude the operation.
When the absence of these zeros is detected during
command chaining or subsequent to the execution
of START I/O FAST RELEASE, the new
operation is not initiated, and an interruption
condition is generated.
The contents of bit positions 40-47 of the CCW
are ignored.
Programming Note
Bit positions 37-39 of the CCW, which presently
must contain zeros, may in the future be assigned
to the control of new functions. It is
recommended, therefore, that these bit positions
not be set to ones for the purpose of obtaining an
intentional program-check indication.
Command Code
The command code, bit positions 0-7 of the CCW,
specifies to the channel and the I/O device the
operation to be performed. A detailed description
of each command appears under "Commands."
The two low-order bits or, when these bits are
00, the four low-order bits of the command code
identify the operation to the channel. The channel
distinguishes among the following four operations:
Output forward (write, control)
Input forward (read, sense)
Input backward (read backward)
Branching (transfer in channel)
The channel ignores the high-order bits of the
command code.
Commands that initiate I/O operations (write,
read, read backward, control, and sense) cause all
eight bits of the command code to be transferred to
the I/O device. In these command codes, the
leftmost bit positions contain modifier bits. The
modifier bits specify to the device how the
command is to be executed. They may, for
example, cause the device to compare data received
during a write operation with data previously
recorded, and they may specify such information as
recording density and parity. For the control
command, the modifier bits may contain the order
code specifying the control function to be
performed. The meaning of the modifier bits
depends on the type of I/O device and is specified
in the SL publication for the device.
The command-code assignment is listed in the
following table. The symbol X indicates that the
bit position is ignored; M identifies a modifier bit.
Code
Command
XXXX 0000
I nval i d
MMMM MMOl
Write
MMMM MM10
Read
MMMM 1100
Read Backward
MMMM MMll
Control
MMMM 0100
Sense
XXXX 1000
Transfer in Channel
Whenever the channel detects an invalid
command code during the initiation of a command,
Chapter 12. Input/Output Operations
12-29
Advertisement
Chapters
