as conditions for the execution of the I/O instruction.
Each composite state is identified in the following dis­
cussion by three alphabetic characters; the first char­
acter position identifies the state of the channel, the
second identifies the state of the subchannel, and the
third refers to the state of the device. Each character
position can contain A, I ,w, or N, denoting the state
of the component. The symbol x in place of a letter
indicates that the state of the corresponding compo­
nent is not significant for the execution of the in­
struction.
Available (AAA): The addressed channel, subchan­
nel, control unit, and I/O device are operational, are
not engaged in the execution of any previously initi­
ated operations, and do not contain any pending in­
terruption conditions.
Interruption Pending in Device (AAI) or Device
Working (AA W): The addressed I/O device or control
unit is executing a previously initiated operation or
contains a pending interruption condition. The ad­
dressed sub channel and channel are available. These
situations are possible:
1. The device is executing an operation after sig­
naling the channel-end condition, such as rewinding
tape or seeking on a disk file.
2. The control unit associated with the device is
executing an operation after signaling the channel­
end condition, such as backspacing file on a magnetic
tape unit.
3. The device or control unit is executing an opera­
tion on another sub channel or channel.
4. The device or control unit contains the device­
end, control-unit-end, or attention condition or, on
the selector channel, the channel-end condition as­
sociated with an operation terminated by HALT I/O. Device Not Operational (AAN): The addressed I/O device is not operational. A device appears not opera­
tional when no control unit recognizes the address.
This occurs when the control unit is not provided in
the system, when power is off in the control unit, or
when the control unit has been logically switched off
the I/O interface. For some types of devices, the not­
operational state is indicated also when the addressed
device is not installed on the control unit. The ad­
dressed subchannel and channel are available.
For devices such as magnetic tape units, the device
appears operational as long as the control unit asso­
ciated with the addressed device is operational. If
the device is not installed or has been logically re­
moved from the control unit, selection of the device
for TEST I/O or a command other than sense causes
the unit-check indication.
Interruption Pending in Subchannel (AIX): An in­
terruption condition is pending in the addressed sub-
channel because of the termination of the portion of
the operation involving the use of channel facilities.
The subchannel has information for a complete csw.
The interruption condition can indicate termination
of an operation at the addressed I/O device or at
another device on the subchannel. In the case of the
multiplexor channel, the channel is available. The
state of the addressed device is not significant, except
when TEST I/O is addressed to the device associated
with the terminated operation. The device associated
with the terminated operation normally is in the in­
terruption pending state. On the selector channel the existence of an inter­
ruption condition in the sub channel immediately
causes the channel to assign to this condition the
highest priority for I/O interruptions and, hence, leads
to the state IIX. Sub channel Working (AWX): The addressed sub­
channel is executing a previously initiated operation
or chain of operations in the multiplex mode and has
not yet reached the channel end for the last opera­
tion. All devices sharing the currently operating con­
trol unit appear in the working state but, for shared
subchannels, the states of devices not attached to the
control unit are not known. The addressed channel is
available.
The sub channel-working state does not occur on the
selector channel since all operations on the selector
channel are executed in the burst mode and cause the
channel to be in the working state ( wwx) .
Subchannel Not Operational (ANX): The addressed
sub channel on the multiplexor channel is not opera­
tional. A subchannel is not operational when it is not
provided in the system. The channel is available.
This state cannot occur on the selector channel.
Interruption Pending in Channel (IXX): The ad­
dressed channel has established which device will
cause the next I/O interruption from this channel.
The state where the channel contains a pending in­
terruption condition is distinguished only by the in­
struction TEST CHANNEL. This instruction does not
cause the subchannel and I/O device to be interro­
gated. The other I/O instructions consider the chan­
nel available when it contains a pending interruption
condition.
Channel Working (WXX): The addressed channel
is operating in the burst mode. In the case of the
multiplexor channel, a burst of bytes is currently
being handled. In the case of the selector channel, an
operation or a chain of operations is currently being
executed, and the channel end for the last operation
has not yet been reached. The states of the addressed
device and, in the case of the multiplexor channel,
of the sub channel are not significant.
Input/Output Operations 89
Channel Not Operational (NXX): The addressed
channel is not operational, or the channel address in
the instruction is invalid. A channel is not operational
when it is not provided in the system or when it has
been switched to the test mode. The states of the
addressed I/O device and sub channel are not signifi­
cant.
Resetting of the Input/Output System
Two types of resetting can occur in the I/O system.
The reset states overlap the hierarchy of states distin­
guished for the purpose of responding to the CPU during the execution of I/O instructions. Resetting ter­
minates the current operation, disconnects the device
from the channel, and may place the device in cer­
tain modes of operation. The meaning of the two
reset states for each type of I/O device is specified in
the Systems Reference Library (SRL) publication for
the device.
System Re,set The system-reset function is performed when the
system-reset key is pushed, when initial program
loading is performed, or when a system power-on
sequence is completed. System reset causes the channel to terminate opera­
tions on all subchannels. Status information and in­
terruption conditions in the subchannels are reset, and
all operational subchannels are placed in the available
state. The channel sends the system-reset signal to all I/O devices attached to it.
If the device is! currently communicating over the I/O interface, the device immediately disconnects
from the channel. Data transfer and any operation
using the facilities of the control unit are immediately
terminated, and the I/O device is not necessarily posi­
tioned at the beginning of a block. Mechanical mo­
tion not involving the use of the control unit, such as
rewinding magnetic tape or positioning a disk access
mechanism, proceeds to the normal stopping point, if
possible. The device remains unavailable until the
termination of mechanical motion or the inherent
cycle of operation, if any, whereupon it becomes available" Status information in the device and con­
trol unit :is reset, and no interruption condition is gen­
erated upon completing the operation.
A control unit accessible by more than one channel
is reset if it is currently associated with a channel on
the CPU generating the reset.
Malfunction Reset
The malfunction-reset function is performed when the
channel detects equipment malfunctioning. 90 Execution of malfunction reset in the channel de­
pends on the type of error and the model. It may
cause all operations in the channel to be terminated
and all operational subchannels to be reset to the
available state. The channel may send either the mal­
function-reset signal to the device connected to the
channel at the time the malfunctioning is detected,
or channels sharing common equipment with the CPU may send the system-reset signal to all devices
attached to the channel.
When the channel signals malfunction reset over
the interface, the device immediately disconneots
from the channel. Data transfer and any operation
using the facilities of the control unit are immediately
terminated, and the I/O device is not necessarily po­
sitioned at the beginning of a block. Mechanical mo­
tion not involving the control unit, such as rewinding
magnetic tape or positioning a disk access mechanism,
proceeds to the normal stopping point, if possible.
The device remains unavailable until the termination
of mechanical motion or the inherent cycle of opera­
tion, if any, whereupon it becomes available. Status information associated with the addressed device is
reset, but an interruption condition may be generated
upon completing any mechanical operation.
When a malfunction reset occurs, the program is
alerted by an I/O interruption or, when the malfunc­
tion is detected during the execution of an I/O instruc­
tion, by the setting of the condition code. In either
case the csw identifies the condition. The device ad­
dressed by the I/O instruction or the device identified
by the I/O interruption, however, is not necessarily
the one placed in the malfunction-reset state. In chan­
nels sharing common equipment with the CPU, mal­
functioning detected by the channel may be indicated
by a machine-check interruption, in which case a csw
is not stored and a device is not identified. The
method of identifying malfunctioning depends upon
the model. Condition Code The results of certain tests by the channel and device,
and the original state of the addressed part of the I/O system are used during the execution of an I/O in­
struction to set one of four condition codes in bit
positions 34 and 35 of the psw. The condition code
is set at the time the execution of the instruction is
completed, that is, the time the CPU is released to
proceed with the next instruction. The condition code
indicates whether or not the channel has performed
the function specified by the instruction and, if not,
the reason for the rejection. The code can be used for
decision-making by subsequent branch-on-condition
operations.
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