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involving data transfer, the new command always
applies to the next block at the device.
Command chaining takes place and the new
operation is initiated only if no unusual situations
have been detected in the current operation. In
particular, the channel initiates a new I/O
operation by command chaining upon receipt of a
status byte signaling one of the following status
combinations: device end, device end and status
modifier, device end and channel end, device end
and channel end and status modifier. In the former
two cases, channel end must have been signaled
before device end, with all other status bits set to
zeros.
If
status such as attention, unit check, unit
exception, incorrect length, program check, or
protection check has occurred, the sequence of
operations is concluded, and the status associated
with the current operation causes an interruption
condition to be generated. The new CCW in this
case is not fetched. Incorrect length does not
suppress command chaining if the current CCW has
the SLI flag set to one.
An exception to sequential chaining of CCWs
occurs when the I/O device presents status
modifier with device end. When no unusual
conditions have been detected and command
chaining is specified or when command retry has
been previously signaled and an immediate retry
could not be performed, the combination of status
modifier and device end causes the channel to alter
the sequential execution of CCWs.
If
command
chaining was specified, the status causes the
channel to chain to the CCW whose storage
address is 16 higher than that of the CCW that
specified chaining.
If
command retry was
previously signaled and immediate retry could not
be performed, the. status causes the channel to
command-chain to the CCW whose storage address
is 8 higher than that of the CCW for which retry
was initially signaled.
When both command and data chaining are
used, the first CCW associated with the operation
specifies the operation to be executed, and the last
CCW indicates whether another operation follows.
Programming Note
Command chaining makes it possible for the
program to initiate transfer of multiple blocks by
means of a single START I/O or START I/O
FAST RELEASE. It also permits a subchannel to
be set up for the execution of auxiliary functions,
such as positioning the disk-access mechanism, and
for data-transfer operations without interference by
the program at the end of each operation.
Command chaining, in conjunction with the
12-34
IBM 4300 Processors Principles of Operation
status-modifier condition, permits the channel to
modify the normal sequence of operations in
response to signals provided by the I/O device.
Skipping
Skipping is the suppression of storage references
during an I/O operation. It is defined only for
read, read backward, and sense operations and is
controlled by the skip flag, which can be specified
individually for each CCW. When the skip flag is
one, skipping occurs; when zero, normal operation
takes place. The setting of the skip flag is ignored
in all other operations.
Skipping affects only the handling of information
by the channel. The operation at the I/O device
proceeds normally, and information is transferred
to the channel. The channel keeps updating the
count but does not place the information in storage.
Chaining is not precluded by skipping. In the case
of data chaining, normal operation is resumed if the
skip flag in the new CCW is zero.
When the skip flag is set to one, the data address
in the C CW is not checked.
Programming Note
Skipping, when combined with data chaining,
permits the program to place in storage selected
portions of a block from an I/O device.
Program-Controlled Interruption
The program-controlled-interruption (PCI) function
permits the program to cause an I/O interruption
during the execution of an I/O operation. The
function is controlled by the PCI flag in the CCW.
The flag can be on either in the first CCW
specified by START I/O or START I/O FAST
RELEASE or in a CCW fetched during chaining.
Neither the PCI flag nor the associated interruption
affects the execution of the current operation.
Whenever the PCI flag in the CCW is one, an
interruption condition is generated in the channel.
When the first CCW associated with an operation
contains the PCI flag, either initially or upon
command chaining, the interruption may occur as
early as immediately upon the initiation of the
operation. The PCI flag in a CCW fetched on data
chaining causes the interruption to occur after all
data designated by the preceding CCW has been
transferred. The time of the interruption, however,
depends on the model and the current activity in
the system and may be delayed even if I/O
interruptions are allowed. No predictable
relationship exists between the time the
interruption due to the PCI flag occurs and the
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