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magnetic-tape units or some disk-access
mechanisms, that share a control unit. For such
devices, the sharing of the subchannel does not
restrict the concurrency of I/O operations since the
control unit permits only one device to be involved
in a data-transfer operation at a time. I/O devices
may share a control unit without necessarily sharing
a subchannel. For example, each transmission line
attached to the IBM 2702 Transmission Control is
assigned a nonshared subchannel, although all of
the transmission lines share the common control
unit.
Programming Notes
A block-multiplexer channel can be made to
operate as a selector channel by the appropriate
setting of the block-multiplexing-control bit.
However, since a block-multiplexer channel
inherently can interleave the execution of multiple
I/O operations and since the state of the block-
multiplexing-control bit can be changed at any
time, it is possible to have one or more operations
that permit multiplexing and an operation that
inhibits multiplexing being executed simultaneously
by a channel.
Therefore, to ensure complete compatibility with
selector channel operation, all operational
sub channels on the block-multiplexer channel must
be available or operating with multiplexing
inhibited when the use of that channel as a selector
channel is begun. All subsequent operations should
then be initiated with the block-multiplexing-
control bit inhibiting mUltiplexing.
I/O-System Operation
Inputloutput operations are initiated and controlled
by information with two types of formats:
instructions and channel-command words (CCWs).
Instructions are decoded by the CPU and are part
of the CPU program. CCWs are decoded and
executed by the channels and 110 devices and
initiate I/O operations, such as reading and
writing. One or more CCWs arranged for
sequential execution form a channel program. Both
instructions and CCWs are fetched from storage
and their formats are common for all types of I/O
devices, although the modifier bits in the command
code of a CCW may specify device-dependent
operations.
The CPU program initiates 110 operations with
the instruction START I/O or START I/O FAST
RELEASE. These instructions identify the channel
and device and cause the. channel to fetch the
channel-address word (CAW) from a fixed location
in storage. The CAW contains the subchannel key
and designates the location in storage from which
the channel subsequently fetches the first CCW.
The CCW specifies the command to be executed
and the storage area, if any, to be used.
When the CAW has been fetched, some channels
consider the execution of ST ART I/O FAST
RELEASE complete. The results of the execution
of the instruction to that point are indicated by
setting the condition code in the program-status
word (PSW) and, in certain situations, by storing
pertinent information in the channel-status word
(CSW).
If
the channel is not operating in burst mode and
if the subchannel associated with the addressed I/O
device is available, the channel attempts to select
the device by sending the address of the device to
all control units attached to the channel. A control
unit that recognizes the address connects itself
logically to the channel and responds to its
selection by returning the address of the selected
device. The channel subsequently sends the
command-code part of the CCW to the control
unit, and the device responds with a status byte
indicating whether it can execute the command.
At this time, the execution of ST ART 110 and
of ST ART I/O FAST RELEASE, if not previously
considered complete, is completed. The results of
the attempt to initiate the execution of the
command are indicated by setting the condition
code in the PSW and, in certain situations, by
storing pertinent information in the CSW.
If
the I/O operation is initiated at the device
and its execution involves transfer of data, the
sub channel is set up to respond to service requests
from the device and assumes further control of the
operation. In operations that do not require any
data to be transferred to or from the device, the
device may signal the end of the operation
immediately on receipt of the command code.
An I/O operation may involve transfer of data
to one storage area, designated by a single CCW,
or to a number of noncontiguous storage areas. In
the latter case, generally a list of CCWs is used for
execution of the I/O operation, each CCW
designating a contiguous storage area, and the
CCWs are said to be coupled by data chaining.
Data chaining is specified by a flag in the CCW
and causes the channel to fetch another CCW upon
the exhaustion or filling of the storage area
designated by the current CCW. The storage area
designated by a CCW fetched on data chaining
pertains to the 110 operation already in progress at
the I/O device, and the I/O device is not notified
when a new CCW is fetched.
Chapter 12. Input/Output Operations
12-5
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