any interruption conditions while an operation is in
progress.
As soon as the selector channel has cleared the in
terruption conditions associated with data transfer, it
starts scanning devices for attention, control-unit-end,
and device-end conditions and for the channel-end
condition associated with operations terminated by
HALTI/O. The highest priority is assigned to the I/O device that first identifies itself on the interface. On the multiplexor channel the priority among re
quests for interruption is based only on the response
to scanning. The multiplexor channel continuously
scans itsI/O devices. The highest priority is assigned
to the device that first responds with an interruption
condition or that requests service for data transfer and
contains thePCI condition in the subchannel. The PCI, as well as any other condition in the subchannel, can
not cause anI/O interruption unless the device initiates
a reference to the subchannel.
Except for conditions associated with termination of
data transfer, the current assignment of priority for
interruption among devices on a channel may be can
celed whenSTART I/O or TEST I/O is issued to the chan
nel. Whenever the assignment is canceled, the channel
resumes scanning for interruption conditions and re
assigns the priority on completion of the activity as
sociated with theI/O instruction.
The assignment of priority among requests for inter
ruption from channels is based on the type of channel.
The priorities of selector channels are in the order of
their addresses, with channel 1 having the highest
priority. The interruption priority of the multiplexor
channel is not fixed and depends on the model and on
the current activity in the channel. Its priority may
be above, below, or between those of the selector
channels.
Interruption Action
AnI/O interruption can occur only when the channel
accommodating the device is not masked and after the
execution of the current instruction in theCPU has
been terminated. If a channel has established the
priority among requests for interruption from devices
while it is masked, the interruption occurs immediate
ly after the termination of the instruction removing
the mask and before the next instruction is executed.
This interruption is associated with the highest priority
condition on the channel. If more than one channel is
unmasked concurrently, the interruption occurs from
the channel having the highest priority among those
requesting interruption.
If the priority among interruption conditions has not
yet been established in the channel by the time the
mask is removed, the interruption does not necessarily108 occur immediately after the termination of the instruc
tion removing the mask. This delay can occur regard
less of how long the interruption condition has existed
in the device or the subchannel.
The interruption causes the current program status
word (psw) to be stored as the old psw at location 56
and causes the csw associated with the interruption to
be stored at location 64. Subsequently, a new psw is
loaded from location120, and processing resumes in
the state indicated by this psw. TheI/O device causing
the interruption is identified by the channel address
in bit positions 21-23 and by the device address in bit
positions 24-31 of the old psw. The csw associated
with the interruption identifies the condition respon
sible for the interruption and provides further details
about the progress of the operation and the status of
the device.
Programming Note
When a number ofI/O devices on a shared control unit
are concurrently executing operations such as rewind
ing tape or positioning a disk access mechanism, the
initial device-end signals generated on completion of
the operations are provided in the order of generation,
unless command chaining is specified for the operation
last initiated. In the latter case, the control unit pro
vides the device-end signal for the last initiated op
eration first, and the other signals are delayed until
the subchannel is freed. Whenever interruptions due
to the device-end signals are delayed either because
the channel is masked or the subchannel is busy, the
original order of the signals is destroyed.Channel Status Word
The channel status word (csw) provides to the pro
gram the status of anI/O device or the conditions
under which anI/O operation has been terminated.
The csw is formed, or parts of it are replaced, in the
process ofI/O interruptions and during execution of START I/O, TEST I/O, and HALT I/O. The csw is placed in
main storage at location 64 and is available to the pro
gram at this location until the time the nextI/O inter
ruption occurs or until anotherI/O instruction causes
its content to be replaced, whichever occurs first.
When the csw is stored as a result of anI/O inter
ruption, theI/O device is identified by the I/O address
in the old psw. The information placed in the csw bySTART I/O, TEST I/O, or HALT I/O pertains to the device
addressed by the instruction.
The csw has the following format: __ __________ C_om_m_a_n_d_A __ dd_r_es_s __ ---------.J 34 7 8 31
L
StatusCount 4748 63
progress.
As soon as the selector channel has cleared the in
terruption conditions associated with data transfer, it
starts scanning devices for attention, control-unit-end,
and device-end conditions and for the channel-end
condition associated with operations terminated by
HALT
quests for interruption is based only on the response
to scanning. The multiplexor channel continuously
scans its
to the device that first responds with an interruption
condition or that requests service for data transfer and
contains the
not cause an
a reference to the subchannel.
Except for conditions associated with termination of
data transfer, the current assignment of priority for
interruption among devices on a channel may be can
celed when
nel. Whenever the assignment is canceled, the channel
resumes scanning for interruption conditions and re
assigns the priority on completion of the activity as
sociated with the
The assignment of priority among requests for inter
ruption from channels is based on the type of channel.
The priorities of selector channels are in the order of
their addresses, with channel 1 having the highest
priority. The interruption priority of the multiplexor
channel is not fixed and depends on the model and on
the current activity in the channel. Its priority may
be above, below, or between those of the selector
channels.
Interruption Action
An
accommodating the device is not masked and after the
execution of the current instruction in the
been terminated. If a channel has established the
priority among requests for interruption from devices
while it is masked, the interruption occurs immediate
ly after the termination of the instruction removing
the mask and before the next instruction is executed.
This interruption is associated with the highest priority
condition on the channel. If more than one channel is
unmasked concurrently, the interruption occurs from
the channel having the highest priority among those
requesting interruption.
If the priority among interruption conditions has not
yet been established in the channel by the time the
mask is removed, the interruption does not necessarily
tion removing the mask. This delay can occur regard
less of how long the interruption condition has existed
in the device or the subchannel.
The interruption causes the current program status
word (psw) to be stored as the old psw at location 56
and causes the csw associated with the interruption to
be stored at location 64. Subsequently, a new psw is
loaded from location
the state indicated by this psw. The
the interruption is identified by the channel address
in bit positions 21-23 and by the device address in bit
positions 24-31 of the old psw. The csw associated
with the interruption identifies the condition respon
sible for the interruption and provides further details
about the progress of the operation and the status of
the device.
Programming Note
When a number of
are concurrently executing operations such as rewind
ing tape or positioning a disk access mechanism, the
initial device-end signals generated on completion of
the operations are provided in the order of generation,
unless command chaining is specified for the operation
last initiated. In the latter case, the control unit pro
vides the device-end signal for the last initiated op
eration first, and the other signals are delayed until
the subchannel is freed. Whenever interruptions due
to the device-end signals are delayed either because
the channel is masked or the subchannel is busy, the
original order of the signals is destroyed.
The channel status word (csw) provides to the pro
gram the status of an
under which an
The csw is formed, or parts of it are replaced, in the
process of
main storage at location 64 and is available to the pro
gram at this location until the time the next
ruption occurs or until another
its content to be replaced, whichever occurs first.
When the csw is stored as a result of an
ruption, the
in the old psw. The information placed in the csw by
addressed by the instruction.
The csw has the following format:
L
Status