Name Abbreviation and Definition Channel Available A None of the following states I nterruption pending
Working W
Interruption immediately available from channel Channel operating in burst mode
Not operational N Channel not operational Subchannel Available A None of the following states
Interruption pending
Working
Not operational I W
N
Information for CSW available in subchannel Subchannel executing an operation Subchannel not operational I/O Device Available Interruption pending
Working
A
W
None of the following states
Interruption condition pending in device
Device executing an operation
Not operational N
Device not operational
Input/Output System States
ing in the subchannel, the channel also is in the same
state. The channel and subchannel, however, are not
synonymous, and an interruption condition not asso-,
ciated with data transfer, such as attention, does not
affect the state of the sub channel. Thus, the sub­
channel may be available when the channel has an
interruption condition pending. Consistent distinc­
tion between the subchannel and channel permits
selector and multiplexer channels to be covered uni­
formly by a single description.
The device referred to in the preceding table in­
cludes both the device proper and its control unit.
For some types of devices, such as magnetic tape
units, the working and the interruption-pending
states can be caused by activity in the addressed
device or control unit. A "not available" shared con­
trol unit imposes its state on all devices attached to
the control unit. The states of the devices are not
related to those of the channel and sub channel.
When the response to an I/O instruction is deter­
mined on the basis of the states of the channel and
subchannel, the components further removed are not
interrogated. Thus, ten composite states are identi­
fied as conditions for the execution of the I/O in­
struction. Each composite state is identified in the
following discussion by three alphabetic characters; the first character position identifies the state of the
channel, the second identifies the state of the sub­
channel, and the third refers to the state of the de­
vice. 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 component is not significant for the
execution of the instruction.
Available (AAA): The addressed channel, sub chan­
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
interruption conditions.
Interruption Pending in Device (AAI) or Device
Working (AA W): The addressed channel and sub­
channel are available. The addressed control unit or
I/O device is executing a previously initiated opera­
tion or contains a pending interruption condition.
These situations are possible:
1. The device is executing an operation, such as
rewinding tape or seeking on a disk file, after
signaling the channel-end condition.
2. The control unit associated with the device is
executing an operation, such as backspacing
file on a magnetic tape unit, after signaling the
channel-end condition.
3. The device or control unit is executing an oper­
ation on 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 ter­
minated operation.
Device Not Operational (AAN): The addressed
channel and sub channel are available. The addressed
I/O device is not operational. A device appears not
operational when no control unit recognizes the ad­
dress. This occurs when the control unit is not pro­
vided 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. The not-operational
state is indicated also when the control unit is pro­
vided 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 (for example, the
second set of lines on the IBM 2702 Transmission
Input/Output Operations 193
Control). See also "Input/Output Device Address-
. " mg.
If the addressed device is not installed or has been
logically removed from the control unit, but the as­
sociated control unit is operational and the address
has been assigned to the control unit (for example,
access mechanism 7 on the IBM 3830 Storage Con­
trol that has only access mechanisms 0-3 installed)
the device is said to be not-ready. When an instruc­
tion is addressed to a device in the not-ready state,
the control unit responds to the selection and indi­
cates unit-check whenever the not-ready state pre­
cludes a successful execution of the operation. See "Unit Check. " Inte"uption Pending in Subchannel (AIX): The
addressed channel is available. An interruption con­
dition is pending in the addressed subchannel be­
cause of the concluding of the portion of the opera­
tion involving the use of channel facilities. The sub­
channel is in a position to provide information for a
complete CSW. The interruption condition can indi­
cate concluding of an operation at the addressed I/O device or at another device on the subchannel. The
state of the addressed device is not significant, ex­
cept when TEST I/O is addressed to the device as­
sociated with the concluded operation, in which case
the CSW contains status information provided by
the device. The state AIX does not occur on the selector
channel. On the selector channel, the existence of an
interruption 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 tlhe state IIX.
Subchannel Working (A WX): The addressed chan­
nel is available. The addressed subchannel is execut­
ing a previously initiated operation or chain of oper­
ations and has not yet received the channel end for
the last operation. The state of the addressed device
is not significant, except when HALT I/O or HALT
DEVICE is issued. During HALT I/O and HALT
DEVICE, the state of the device may be interrogat­
ed and will then be indicated in either the CSW or
the condition code.
The subchannel-working state does not occur on
the selector channel since all operations on the selec­
tor channel are executed in the burst mode and
cause the channel to be in the working state
(WWX).
194 System/370 Principles of Operation Subchannel Not Operational (ANX): The addressed
channel is available. The addressed sub channel on
the multiplexer channel is not operational. A sub­
channel is not operational when it is not provided in
the system. This state cannot occur on the selector
channel.
Interruption Pending in Channel (IXX): The ad­
dressed channel is not working and has established
which device will cause the next I/O interruption
from this channel. The state where the channel con­
tains a pending interruption condition is distin­
guished only by the instruction TEST CHANNEL.
This instruction does not cause the sub channel and I/O device to be interrogated. The other I/O in­
structions, with the exception of STORE CHAN­
NEL ID, consider the channel available when it con­
tains a pending interruption condition. A channel
with a pending interruption condition may be con­
sidered to be working by the instruction STORE CHANNEL ID. When the channel assigns priority
for interruptions among devices, the interruption
condition is preserved in the I/O device or subchan­
nel. (See "Interruption Conditions. ") Channel Working (WXX): The addressed channel
is operating in the burst mode. In the case of the
multiplexer 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 ad­
dressed device and, in the case of the multiplexer
channel, of the subchannel are not significant. De­
pending on the channel type and system model,
TEST I/O and HALT DEVICE may consider the
channel to be available 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 opera­
tional when it is not provided in the system, when
power is off in the channel, or when it is not config­
ured to the CPU. The states of the addressed I/O device and sub channel 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 kinds
of reset is specified in the SL and SRL publications
for the device.
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