1.
2.
3.
4.
5.
6.When the CCW address is updated during command
chaining, resuming a
suspended channel
program, STARTI/O, or
STARTI/O FAST RELEASE When an initial
selectionsequence resulted in status
including attention,
control-unit end, unit
check, unit exception,
busy, status modifier
(without channel end and
device end), ordevice end (without channel
end)When the control
responds with
status instead of
device address when
channel attempts
select the device
unit
busy
the
the
toWhen command retry is
signaled
When the channel inter
rogates the device in
the process of clearing
an interruption condi
tionWhen the channel signals
the conclusion of the
chain of operations to
the device during
command chaining while
performing the suspend
function101 The command in the current CCW has been accepted, but
data transfer is unpredict
able. This code applies
from the time a device is
logically connected to thechannel until the time it is
determined thata new
sequence code applies. The
code may also be used when a
channel is in the polling or
idle state, and it is not
possible to determine that
code010 or 011 applies.
The code may alsobe used at
other times when achannel cannot distinguish between
code010 or 011. 110 Reserved.
111 Reserved.
Measurement Byte (location 185): A
value is stored at real location 185
whenever anI/O address is stored at
real locations 186-187.Whenever the
channel stores a completeCSW during an
interruption inEC mode and the CSW indicates the conclusion of an operation
initiated via STARTI/O FAST RELEASE executed independent of the device for a
subchannel provided with start-I/O-fast
queuing,the measurement byte (which is otherwise stored as zeros) has the
following format:
Location 185 (real)IDCINPoloool o 2 5 7 The bits of the measurement byte are defined as follows: 0-1 Delay Code This code
indicatesthe condition encount ered by the channel on the first
attempt by the channel to initi
ate theI/O operation at the
device. Delay codes are as
follows:Code Meaning 00 No busy condition
encountered or
no valid code
available01 Channel busy 10 Control unit busy
11 Device busy
2-4Number of Pending Operations (NPO). These bits contain the
binary count of the number of
pendingI/O operations for the
channel at the time the measure
ment byte is stored. A value of
all ones represents seven or
more pendingI/O operations. A
value of all zeros represents
either no pendingI/O operations
or no valid number available.Otherwise, the measurement byte is
stored as zeros.
Errorsdetected during the execution of
anI/O operation do not affect the
validity of the values stored in the
measurement byte unless the channel
control-check condition is indicated in
theCSW. A channel-control-check condi
tion that affects the validity of the
delay code or the number of pendingI/O operations causes the channel to store
zeros in the measurement byte.I/O Address (Locations 186-187): A
two-byte fieldis provided at real
locations 186-187 for storing theI/O address on each I/O interruption in the EC mode, and at the conclusion of a
successful initial-program-Ioading
sequence in theEC mode.
ProgrammingNote I/O-busy conditions result
contention for shared resourcesI/O system. Such contention
from
in the
is notChapter 13. Input/Output Operations 13-83
2.
3.
4.
5.
6.
chaining, resuming a
suspended channel
program, START
START
selection
including attention,
control-unit end, unit
check, unit exception,
busy, status modifier
(without channel end and
device end), or
end)
responds with
status instead of
device address when
channel attempts
select the device
unit
busy
the
the
to
signaled
When the channel inter
rogates the device in
the process of clearing
an interruption condi
tion
the conclusion of the
chain of operations to
the device during
command chaining while
performing the suspend
function
data transfer is unpredict
able. This code applies
from the time a device is
logically connected to the
determined that
sequence code applies. The
code may also be used when a
channel is in the polling or
idle state, and it is not
possible to determine that
code
The code may also
other times when a
code
111 Reserved.
Measurement Byte (location 185): A
value is stored at real location 185
whenever an
real locations 186-187.
channel stores a complete
interruption in
initiated via START
subchannel provided with start-I/O-fast
queuing,
following format:
Location 185 (real)
indicates
attempt by the channel to initi
ate the
device. Delay codes are as
follows:
encountered or
no valid code
available
11 Device busy
2-4
binary count of the number of
pending
channel at the time the measure
ment byte is stored. A value of
all ones represents seven or
more pending
value of all zeros represents
either no pending
or no valid number available.
stored as zeros.
Errors
an
validity of the values stored in the
measurement byte unless the channel
control-check condition is indicated in
the
tion that affects the validity of the
delay code or the number of pending
zeros in the measurement byte.
two-byte field
locations 186-187 for storing the
successful initial-program-Ioading
sequence in the
Programming
contention for shared resources
from
in the
is not