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Unit Status
The following status indications are generated by
the I/O device or control unit. The timing and
causes of these status indications for each type of
device are specified in the SL publication for the
device.
When the I/O device is accessible from more
than one channel, status due to channel-initiated
operations is signaled to the channel that initiated
the associated I/O operation. The handling of
status not associated with I/O operations, such as
attention or device end due to transition from the
not-ready to the ready state, depends on the type
of device and situation and is specified in the SL
publication for the device.
Attention
Attention is signaled when the device detects an
asynchronous situation that is significant to the
program. Attention is interpreted by the program
and is not associated with the initiation, execution,
or conclusion of an I/O operation.
The device can signal attention to the channel
when no operation is in progress at the I/O device,
control unit, or subchannel. Attention can be
signaled with device end upon completion of an
operation, and it can be signaled to the channel
during the initiation of a new I/O operation.
Attention along with device end and unit exception
can also be signaled whenever a device changes
from the not-ready to the ready state. The
handling and presentation of attention to the
channel depends on the type of device.
When the device signals attention during the
initiation of an operation, the operation is not
initiated. Attention causes command chaining to
be suppressed.
Status Modifier
Status modifier is generated by the device when the
device cannot provide its current status in response
to TEST I/O, when the control unit is busy, when
the normal sequence of commands has to be
modified, or when command retry is to be initiated.
When status modifier is signaled in response to
TEST I/O and status modifier is the only status bit
that is set to one, this indicates that the device
cannot execute the instruction and has not provided
its current status. The interruption condition,
which may be pending at the device or subchannel,
has not been cleared, and the CSW stored by TEST
I/O contains zeros in the subchannel-key,
CCW -address, and count fields.
12-48
IBM 4300 Processors Principles of Operation
When the status-modifier bit in the CSW is set
to one together with the busy bit, it indicates that
the busy status pertains to the control unit
associated with the addressed I/O device. The
control unit appears busy when it is executing a
type of operation that precludes the acceptance and
execution of any command or the instructions
TEST I/O, HALT I/O, and HALT DEVICE or
when it contains an interruption condition for a
device other than the one addressed. The
interruption condition may be due to control-unit
end, due to channel end following the execution of
CLEAR I/O, or, on the selector channel, due to
channel end following the execution of HALT I/O
or HALT DEVICE. The busy state occurs for
operations such as backspace file, in which case the
control unit remains busy after providing channel
end, for operations concluded by CLEAR I/O, and
for operations concluded on the selector channel by
HAL T I/O or HALT DEVICE, and temporarily
occurs on the 2702 Transmission Control after
initiation of an operation on a device
accommodated by the control unit. A control unit
accessible from two or more channels appears busy
when it is communicating with another channel.
Presence of status modifier and device end
means that the normal sequence of commands must
be modified. The handling of this status
combination by the channel depends on the
operation. If command chaining is specified in the
current CCW and no unusual situations have been
detected, presence of status modifier and device
end causes the channel to fetch and chain to the
CCW whose storage address is 16 higher than that
of the current CCW. If the I/O device signals
status modifier at a time when no command
chaining is specified, or when any unusual
situations have been detected, no action· is taken in
the channel, and the status-modifier bit and any
other status bit presented by the device are set to
ones in the CSW.
Status modifier is set to one in combination with
unit check and channel end to initiate the
command-retry procedure.
Control-Unit End
Control-unit end indicates that the control unit has
become available for use for another operation.
Control-unit end is provided only by control
units shared by I/O devices or control units
accessible by two or more channels, and only when
one or both of the following have occurred:
1. The program had previously caused the control
unit to be interrogated while the control unit
was in the busy state. The control unit is
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