Termination of Input/Output Operations
When the operation or sequence of operations initiated
bySTART I/O is terminated, the channel and the device
generate status conditions. These conditions can be
brought to the attention of the program by theI/O interruption mechanism, by TEST I/O, or, in certain
cases, bySTART I/O. The status conditions, as well as
an address and a count indicating the extent of the op
eration sequence, are presented to the program in the
form of a csw.
Types of Termination
Normally anI/O operation at the subchannellasts until
the device signals channel end. The channel-end con
dition can be signaled during the sequence initiating
the operation, or later. When the channel detects
equipment malfunctioning or a system reset is per
formed, the channel disconnects the device without
receiving channel end. The program can force a de
vice on the selector channel to be disconnected pre
maturely by issuing HALTI/O. Termination at Operation Initiation
After the addressed channel and subchannel have been
verified to be in a state whereSTART I/O can be exe
cuted, certain tests are performed on the validity of
the information specified by the program and on the
availability of the addressed control unit andI/O de
vice. This testing occurs both during the execution ofSTART I/O and during command chaining.
A data-transfer operation is initiated at the subchan
nel and device only when no programming or equip
ment errors are detected by the channel and when the
device responds with zero status during the initiation
sequence. When the channel detects or the device sig
nals anyunusual condition during the initiation of an
operation, but channel end is off, the command is said
to be rejected.
Rejection of the command during the execution ofSTART I/O is indicated by the setting of the condition
code in the psw. Unless the device is not operational,
the conditions that precluded the initiation are de
tailed by the portion of the csw stored bySTART I/O. The device is not started, no interruption conditions
are generated, and the subchannel is not tied up be
yond the initiation sequence. The device is immedi
ately available for the initiation of another operation,
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provided the command was not rejected because of
the busy or non-operational condition.
When an unusual condition causes a command to be
rejected during initiation of anI/O operation by com
mand chaining, an interruption condition is generated,
and the subchannel is not available until the condition
is cleared. The not-operational state on command
chaining is indicated by means of interface control
check; the other conditions are identified by the cor
responding status bits in the associated csw. The new
operation at theI/O device is not started.
Immediate Operations
Instcad of accepting or rejecting a command, theI/O device can signal the channel-end condition immedi
atcly upon receipt of the command code. AnI/O op
eration causing the channel-end condition to be sig
nalcd during the initiation sequence is called an"immediate operation." When the first ccw designated by the CAW initiates
an immediate operation, no interruption condition is
generated. If no command chaining occurs, the chan
nel-end condition is brought to the attention of the
program by causingSTART I/O to store the csw status
portion, and the subchannel is immediately made avail
able to the program. TheI/O operation, however, is
initiated, and, if channel-end is not accompanied by
device end, the devicc remains busy. Device end, when
subsequently provided by the device, causes an inter
ruption condition to be generated.
When command chaining is specified after an im
mediate operation and no unusual conditions have
been detected during the execution,START I/O does not
cause storing of csw status. The subsequent commands
in the chain are handled normally, and the channel-end
condition for the last operation generates an inter
ruption condition even if the device provides the sig
nal immediately upon receipt of the command code.
Whenever immediate completion of anI/O oper
ation is signaled, no data have been transferred to or
from the device. The data address in the ccw is not
checked for validity, except that it may not exceed
the addressing capacity of the model.Since a count of zero is not valid, any ccw specifying
an immediate operation must contain a nonzero count.
When an immediate operation is executed, however,
incorrect length is not indicated to the program, and
command chaining is not suppressed.
When the operation or sequence of operations initiated
by
generate status conditions. These conditions can be
brought to the attention of the program by the
cases, by
an address and a count indicating the extent of the op
eration sequence, are presented to the program in the
form of a csw.
Types of Termination
Normally an
the device signals channel end. The channel-end con
dition can be signaled during the sequence initiating
the operation, or later. When the channel detects
equipment malfunctioning or a system reset is per
formed, the channel disconnects the device without
receiving channel end. The program can force a de
vice on the selector channel to be disconnected pre
maturely by issuing HALT
After the addressed channel and subchannel have been
verified to be in a state where
cuted, certain tests are performed on the validity of
the information specified by the program and on the
availability of the addressed control unit and
vice. This testing occurs both during the execution of
A data-transfer operation is initiated at the subchan
nel and device only when no programming or equip
ment errors are detected by the channel and when the
device responds with zero status during the initiation
sequence. When the channel detects or the device sig
nals any
operation, but channel end is off, the command is said
to be rejected.
Rejection of the command during the execution of
code in the psw. Unless the device is not operational,
the conditions that precluded the initiation are de
tailed by the portion of the csw stored by
are generated, and the subchannel is not tied up be
yond the initiation sequence. The device is immedi
ately available for the initiation of another operation,
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provided the command was not rejected because of
the busy or non-operational condition.
When an unusual condition causes a command to be
rejected during initiation of an
mand chaining, an interruption condition is generated,
and the subchannel is not available until the condition
is cleared. The not-operational state on command
chaining is indicated by means of interface control
check; the other conditions are identified by the cor
responding status bits in the associated csw. The new
operation at the
Immediate Operations
Instcad of accepting or rejecting a command, the
atcly upon receipt of the command code. An
eration causing the channel-end condition to be sig
nalcd during the initiation sequence is called an
an immediate operation, no interruption condition is
generated. If no command chaining occurs, the chan
nel-end condition is brought to the attention of the
program by causing
portion, and the subchannel is immediately made avail
able to the program. The
initiated, and, if channel-end is not accompanied by
device end, the devicc remains busy. Device end, when
subsequently provided by the device, causes an inter
ruption condition to be generated.
When command chaining is specified after an im
mediate operation and no unusual conditions have
been detected during the execution,
cause storing of csw status. The subsequent commands
in the chain are handled normally, and the channel-end
condition for the last operation generates an inter
ruption condition even if the device provides the sig
nal immediately upon receipt of the command code.
Whenever immediate completion of an
ation is signaled, no data have been transferred to or
from the device. The data address in the ccw is not
checked for validity, except that it may not exceed
the addressing capacity of the model.
an immediate operation must contain a nonzero count.
When an immediate operation is executed, however,
incorrect length is not indicated to the program, and
command chaining is not suppressed.