magnetic-tape unit, after signaling channel end.
3. The device or control unit is executing an operation with another
subchannel or channel.
4. The device or control unit contains
the device-end, control-unit-end,
or attention condition, or a
channel-end condition associated with a terminated operation.
Device Not Operational (AAN): The
addressed channel and subchannel are
available. The addressed I/O device is not operational. A device appears not
operational when no control unit recog­
nizes 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 removed from the channel. The
not-operational state is indicated also
when the control unit is provided and is
designed to attach the device, but the
device has not been installed and the address has not been assigned to the control unit. (See also the section
"Input/Output Device Addressing" in this
chapter.)
Interruption Pending in Subchannel
(AIX): The addressed channel is avail­
able. An interruption condition is
pending in the addressed subchannel. The subchannel is able to provide infor­
mation for a CSW. The interruption
information indicates status associated
with the addressed I/O device or another I/O device associated with the subchan­
nel. The state of the addressed device
is not significant, except when the address specified by TEST I/O is the same as the address of the I/O device
for which the subchannel is
interruption-pending, in which case the CSW contains status information that has
been provided by the device. The state AIX does not occur on the
selector channel. On the selector chan­
nel, the existence of an interruption
condition in the subchannel immediately
causes the channel to assign to this
condition the highest priority for I/O interruptions and, hence, leads to the
state IIX.
Subchannel Working (AWX): The addressed
channel is available. The addressed
subchannel is executing a previously
initiated START I/O (510) or START I/O FAST RELEASE (SIOF) function. The
addressed subchannel enters the working
state when condition code 0 is set for 510 or SIOF. The addressed subchannel
remains in the working state until the 510 or SIOF function is concluded at the
subchannel. Usually the conclusion of
the 510 or SIOF function occurs when the I/O operation or chain of operations
receives channel end for the last opera­
tion.
The state of the addressed device is not
significant, except when HALT I/O or
HALT DEVICE is issued. During the
execution of HALT I/O and HALT DEVICE, the state of the device may be interro­
gated and will then be indicated in
either the CSW or the condition code.
HALT DEVICE issued to a subchannel that
has a pending or suspended I/O operation
considers the channel to be busy. In
this case, the I/O system appears to be in the channel-working state (WXX) rath­
er than the subchannel-working state
(AWX).
The subchannel-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 channel is available. The
addressed subchannel on the multiplexer
channel is not operational. A subchan­
nel is not operational when it is not
provided in the channel. This state
cannot occur on the selector channel.
Interruption Pending in Channel (IXX):
The addressed channel is not working and
has established which device will cause
the next I/O interruption from this
channel. The state in which the channel
contains an interruption condition is
distinguished only by the instruction
TEST CHANNEL. This instruction does not
cause the subchannel and I/O device to
be interrogated. The other I/O instructions, with the exception of
STORE CHANNEL ID, consider the channel
available when it contains an inter­
ruption condition. A channel with an
interruption condition may be considered
to be working by the instruction STORE CHANNEL 10. When the channel assigns
priority for interruptions among
devices, the interruption condition is
preserved in the I/O device or subchan­
nel. (See the section "Interruption Conditions" in this chapter.) Channe! Working (WXX): The addressed
channel is operating in the burst mode.
In the multiplexer channel, a burst of
bytes is currently being handled. In 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 signaled. The states of the addressed
device and, in the mu!tiplexer channel,
of the subchannel are not significant.
In addition, because of internal activ­
ity, some block-mu!tiplexer channels may
at times appear to be working even
though they are not operating in burst
mode. Depending on the model and the
channel type, TEST I/O, CLEAR I/O, START I/O FAST RELEASE, and HALT DEVICE may
consider the channel to be available Chapter 13. Input/Output Operations 13-11

when the channel is working with a device other than the addressed device. Channel Not Operational (NXX): The
addressed channel is not operational. A
channel is not operational when it is
not available in the configuration, when
power is off in the channel, when it is
not connected to the CPU, or when it
detects a channel-check-stop condition.
As long as a channel-cheek-stop condi­
tion persists, the channel performs no I/O instructions, with the exception of CLEAR CHANNEL (which may be executed,
depending on the system model); performs
no I/O interruptions; executes no chan­
nel programs; and suspends all
I/O-interface activity. When a channel
is not operational, the states of the
addressed I/O device and subchannel are
not significant.
RESETTING OF THE INPUT/OUTPUT SYSTEM Two types of resetting can occur in the I/O system: an I/O-system reset and an I/O selective reset. The response of
each type of I/O device to the two types
of reset is specified in the SL publica­
tion for the device. I/O-System Reset
I/O-system reset is performed in the
channel and on the associated I/O inter­
face when the CPU to which the channel
is connected executes the instruction CLEAR CHANNEL or a program reset,
initial-program reset, clear reset, or
power-on reset is performed, when a power-on sequence is performed by the
channel, and, under certain conditions
on some models, when a channel detects
equipment malfunctions and the
recovery-extension facility is not
installed.
I/O-system reset causes the channel to
conclude operations on all subchannels.
Status information and all interruption
conditions in all subchannels are reset,
and all operational subchannels are
placed in the available state. The
channel signals system reset to all I/O devices attached to it. I/O Selective Reset I/O selective reset is performed by some
channels when they detect certain equip­
ment malfunctions. I/O selective reset causes the channel
to signal selective reset to the device
that is connected to the channel at the
13-12 System/370 Principles of Operation
time the malfunction is detected.
subchannels are reset.
Effect of Reset QU g Working Device
No
With either type of reset, if the device
is currently communicating with a chan­
nel, the device immediately disconnects
from the channel. Data transfer and any
operation using the facilities of the
control unit are immediately concluded,
and the I/O device is not necessarily
positioned at the beginning of a block.
Mechanical motion 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
appears in the working state until the
termination of mechanical motion or the
inherent cycle of operation, if any,
whereupon it becomes available. Status
information in the device and control
unit is reset, but an interruption
condition may be generated when any
mechanical operation is completed.
Reset Upon Malfunction
When a malfunction occurs and the
program is alerted by an I/O inter­
ruption, or when a malfunction occurs
during the execution of an I/O instruc­
tion and the program is alerted by the
setting of a condition code, then an I/O selective reset may have been performed.
A CSW is stored identifying the cause of
the malfunction.
The device addressed by the I/O instruc­
tion is not necessarily the device that
is reset.
When a malfunction occurs and the
program is alerted by a machine-check
interruption, then an I/O selective
reset or, on some models, I/O-system
reset may have been performed. This may
or may not be accompanied by an I/O interruption. 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 instruction to set one of four
condition codes in the PSW. The condi­
tion code is set at the time the
execution of the instruction is
concluded, that is, the time the CPU is
released to proceed with the next
instruction. The condition code ordi­
narily indicates whether or not the