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support subsystem devices. In addition, each channel is assigned one
channel-shared UCW that is used to present asynchronous interruptions for any
1/0 devices attached to the channel that are not allocated a UCW (defined to the
system).
The UCWs defined for an instruction processing function can be assigned to any of
the channels actually present in the channel group for that instruction processor. A
maximum of 256 UCWs can be assigned to any one channel. The customer
engineer or operator assigns UCWs to specific channel addresses using the operator
console. Each UCW can be designated as shared or nonshared.
A shared UCW can be used by a set of devices, one device at a time. A shared
UCW generally is assigned to a control unit that has multiple devices attached, only
one of which can be in operation at a time. A nonshared UCW is one that is
assigned to only one device. A nonshared UCW is designed for use with a control
unit that has only one 1/0 device attached or that has multiple 1/0 devices
attached that can operate concurrently.
A channel directory for each channel is allocated in auxiliary storage. Each
channel directory has 256 entries, one for each of the possible device addresses for
a channel. A directory entry indicates whether a UCW is assigned to the
associated device address, characteristics of the assigned UCW, and characteristics
of the device assigned the associated device address.
A channel directory entry contains the following:
•
Reference number of the UCW assigned (all device addresses have a UCW
assigned)
•
Assigned bit to indicate whether an 1/0 device is defined for the associated
device address
•
An indication of whether the associated device operates in byte multiplexer
mode
•
An indication of whether the UCW is shared or nonshared (shared bit)
•
An indication of whether the associated device must operate in selector mode
rather than block multiplexer mode
•
An indication of whether the device is attached to a control unit that is to use
the START 1/0 FAST queuing function
•
An indication of whether the associated device is attached to a control unit that
is capable of operating in data streaming mode
An indication of the mode in which channel 5 is to operate (byte or block
multiplexer)
Devices attached to a block multiplexer channel that are capable of block
multiplexing should have the shared and selector mode bits off in their channel
directory entry to indicate allocation of a nonshared UCW that is capable of
disconnecting. For devices attached to a control unit that is capable of data
streaming mode of operation (such as a 3880 storage director), the directory entry
should have the data streaming mode bit on.
Section 30: 4381 Processor Multiprocessor Model Groups
77
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