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HAL,T
110
HID
[5]
9EOO
o
16
20
31
Execution of. the current I/O operation at the
addressed I/O device, subchannel, or channel is
terminated. The subsequent state of the
subchannel depends on the type of channel. Bits
8-14 of the instruction are ignored.
Bits 16-31 of the second-operand address
identify the channel and, when the channel is not
working, identify the sub channel and the I/O
device to which the instruction applies.
When the channel is either available or
interruption-pending, with the subchannel either
available or working, HALT I/O causes the
addressed device to be selected and to be signaled
to terminate the current operation, if any.
If
the
subchannel is available, its state is not affected. If,
on the byte-multiplexer channel, the subchannel is
working, data transfer is immediately terminated,
but the subchannel remains in the working state
until the device provides the next status byte,
whereupon the subchannel is placed in the
interruption-pending state.
When HALT I/O is issued to a channel
operating in the burst mode, data transfer for the
burst operation is terminated, and the device
performing the burst operation is immediately
disconnected from the channel. The subchannel
and I/O-device address in the instruction, in this
case, is ignored.
The termination of a burst operation by HALT
I/O on the selector channel causes the channel and
sub channel to be placed in the interruption-pending
state. Generation of the interruption condition is
not contingent on the receipt of a status byte from
the device. When HALT I/O causes a burst
operation on the byte-multiplexer channel to be
terminated, the subchannel associated with the
burst operation remains in the working state until
the device signals channel end, whereupon the
subchannel enters the interruption-pending state.
The termination of a burst operation by HALT I/O
on a block-multiplexer channel may, depending on
the model and the type of subchannel, take place as
for a selector channel or may allow the sub channel
to remain in the working state until the device
provides ending status.
O;nJhe.hyte,~multiplexer,
channel .operating in.::tIm
byte~multiplex
mode, the device· is selected and-the
instruction executed only after the channel has
serviced all outstanding requests for data transfer
for previously initiated operations, including the
operation to be halted. If the control unit does not
accept the HALT I/O signal because it is in the
not-operational or control-unit-busy state, the
sub channel, if working, is set up to signal
termination of device operation the next time the
device requests or offers a byte of data.
If
command chaining is indicated in the subchannel
and the device presents status next, chaining is
suppressed.
When the addressed subchannel is interruption-
pending, with the channel available or
interruption-pending, HALT I/O does not cause
any action.
When any of the following conditions occurs,
HAL T I/O causes the status portion, bits 32-47, of
the CSW to be replaced by a new set of status bIts.
The contents of the other fields of the CSW are not
changed. The CSW stored by HALT I/O pertains
only to the execution of HALT I/O and does not
describe the I/O operation, at the addressed
subchannel, that is terminated. The extent of data
transfer, and the status at the termination of the
operation at the subchannel, are provided in the
CSW associated with the interruption condition due
to the termination.
1. The addressed device was selected and signaled
to terminate the current operation. The CSW
contains zeros in the status field unless an
equipment error is detected.
2. The channel attempted to select the addressed
device, but the control unit could not accept
the HALT I/O signal because it is executing a
previously initiated operation or had an
interruption condition associated with a device
other than· the one addressed. The signal to
terminate the operation has not been
transmitted to the device, and the subchannel,
if in the working state, will signal termination
the next time the device identifies itself. The
CSW unit-status field contains ones in the busy
and status-modifier bit positions. The
channel-status field contains zeros unless an
equipment error is detected.
3. The channel detected an equipment
malfunction during the execution of HALT
I/O. The status bits in the CSW identify the
Chapter 12. Input/Output Operations
12-19
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