If device end is not indicated, the
device remains busy, and a subse­
quent device-end condition is
generated by the device.
4. The I/O device is interruption­
pending, or the control unit is
interruption-pending for the
addressed device. The CSW unit­
status field contains one in the
busy-bit position, identifies the
interruption condition, and may
contain other bits provided by the
device or control unit. The inter­
ruption condition is cleared. The I/O operation is not initiated.
The channel-status field indicates
any errors detected by the channel.
5. The I/O device or the control unit
is executing a previously initiated
operation, or the control unit is
interruption-pending for a device
other than the one addressed. The CSW unit-status field contains one
in the busy-bit position or, if the
control unit is busy, the busy and
status-modifier bits are ones. The I/O operation is not initiated.
The channel-status field indicates
any errors detected by the channel,
and the PCI bit is one if specified
in the first ccw.
6. The I/O device or control unit
detected an equipment or program­
ming error during the initiation,
or the addressed device is not
ready. The CSW identifies the
error. The channel-end and busy
bits are zeros, unless the device
was busy, in which case the busy
bit, as well as any bits causing
interruption conditions, are ones.
The interruption conditions indi­
cated in the CSW have been cleared
at the device. The I/O operation
is not initiated. No interruption
conditions are generated at the I/O device or subchannel.
When the SIO or SIOF function cannot be
executed because of a pending logout
which affects the operational capability
of the channel or subchannel, a full CSW is stored. The fields in the CSWare all set to zeros, with the exception of
the logout-pending bit and the channel­
control-check bit, which are set to
ones. No channel logout occurs in this
case. Certain situations encountered during
the execution of SIO cause condition
code 1 to be set. When SIOF is
executed, these same situations may be
encountered after condition code 0 is
set. When the latter occurs, a
deferred-condition-code-1 1/0- interruption condition is generated to
report these situations to the program.
An exception to this may occur when
start-I/O-fast queuing is provided for
the subchannel. With start-I/O-fast
queuing, control-unit-busy or device­
busy conditions encountered while
attempting to initiate the I/O operation
may be handled by the channel instead of
a deferred-condition-code-1 I/O inter­
ruption generated.
When the SIOF function causes condition
code 0 to be set and, subsequently, it
is determined that the device is not
operational, a deferred-condition-code-3
I/O-interruption condition is generated.
In both of the above cases, in the
resulting I/O interruption, a full CSW is stored, and the deferred condition
code appears in the CSW. When start-I/O-fast queuing is provided, I/O operations may remain pending at the
subchannel while the control unit or
device is busy. The control unit or
device signals the end of the busy peri­
od by presenting a status byte
containing control-unit end or device
end, respectively.
When device-end status signals the end
of a previously signaled device-busy
period, and an I/O operation is pending
at the subchannel for the device, the
channel attempts to initiate the pending
operation without causing an I/O inter­
ruption. When the status is control­
unit end, and one or more devices
attached to the control unit have I/O operations pending, the channel attempts
to initiate one of the pending oper­
ations.
If a control unit presents a status byte
and the channel is unable to accept that
status byte because of an I/O operation
that is pending at the associated
subchannel for a different device to
which a busy indication had previously
been presented, then the I/O operation
that is queued at the subchannel is
terminated and the subchannel becomes
interruption-pending. When the associ­
ated interruption occurs, the CSW that
is stored contains the busy indication
in the unit-status byte, and the
deferred condition code is 1.
If the busy indication received by the
channel when the device or control unit
was interrogated while busy was not
presented to the program, the no­
longer-busy indication is not presented
to the program. If the device-busy
indication was presented to the program
and no I/O operation is pending for that
device when the device-end indication is
received, an interruption condition is
recognized, and the device-end indi­
cation is presented to the program.
If the control-unit busy indication was
presented to the program, receipt of the
corresponding control-unit-end (CUE) indication causes the channel to recog­
nize an interruption condition. If the
subchannel corresponding to the unit
address with which the CUE indication is Chapter 13. Input/Output Operations 13-29

associated is available, the subchannel
is made interruption-pending. The CUE status is stored in the unit-status
field of the CSW stored when the inter­
ruption condition is cleared. If the
subchannel corresponding to the unit
address with which the CUE indication is
associated is working, that is, the
subchannel contains a pending I/O opera­
tion or a suspended channel-program
execution, the channel generates the
channel-available-interruption (CAl) condition instead, and the control­
unit-end status is not made available to
the program. When the CAl condition
replaces the CUE condition, the state of
the associated subchannel is not
affected. (See the section, "Channel­ Available Interruption," for the
detailed description of CAl.) On the byte-multiplexer channel, both
the SIO and SIOF functions cause the
addressed device to be selected and the
operation to be initiated only after the 13-30 System/370 Principles of Operation
channel has serviced all outstanding
requests for data transfer for previous­
ly initiated operations.
Program Exceptions:
Privileged operation
Resulting Condition Code: OSlO or SIOF function has been
accepted
1 CSW stored
2 Channel or subchannel busy
3 Not operational
The condition code set by START I/O and
START I/O FAST RELEASE for all possible
states of the I/O system ;s shown in the
figure "Condition Codes Set by START I/O
and START I/O FAST RELEASE." See the
section "States of the Input/Output
System" in this chapter for a detailed
definition of the A, I, W, and N states.