1.
2.
3.
4.
5.
6. When the CCW address is updated during command
chaining, resuming a
suspended channel
program, START I/O, or
START I/O FAST RELEASE When an initial
selection sequence resulted in status
including attention,
control-unit end, unit
check, unit exception,
busy, status modifier
(without channel end and
device end), or device end (without channel
end) When the control
responds with
status instead of
device address when
channel attempts
select the device
unit
busy
the
the
to When command retry is
signaled
When the channel inter­
rogates the device in
the process of clearing
an interruption condi­
tion When the channel signals
the conclusion of the
chain of operations to
the device during
command chaining while
performing the suspend
function 101 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 the channel until the time it is
determined that a 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
code 010 or 011 applies.
The code may also be used at
other times when a channel cannot distinguish between
code 010 or 011. 110 Reserved.
111 Reserved.
Measurement Byte (location 185): A
value is stored at real location 185
whenever an I/O address is stored at
real locations 186-187. Whenever the
channel stores a complete CSW during an
interruption in EC mode and the CSW indicates the conclusion of an operation
initiated via START I/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
indicates the condition encount­ ered by the channel on the first
attempt by the channel to initi­
ate the I/O operation at the
device. Delay codes are as
follows: ~ Code Meaning 00 No busy condition
encountered or
no valid code
available 01 Channel busy 10 Control unit busy
11 Device busy
2-4 Number of Pending Operations (NPO). These bits contain the
binary count of the number of
pending I/O operations for the
channel at the time the measure­
ment byte is stored. A value of
all ones represents seven or
more pending I/O operations. A
value of all zeros represents
either no pending I/O operations
or no valid number available. Otherwise, the measurement byte is
stored as zeros.
Errors detected during the execution of
an I/O operation do not affect the
validity of the values stored in the
measurement byte unless the channel­
control-check condition is indicated in
the CSW. A channel-control-check condi­
tion that affects the validity of the
delay code or the number of pending I/O operations causes the channel to store
zeros in the measurement byte. I/O Address (Locations 186-187): A
two-byte field is provided at real
locations 186-187 for storing the I/O address on each I/O interruption in the EC mode, and at the conclusion of a
successful initial-program-Ioading
sequence in the EC mode.
Programming Note I/O-busy conditions result
contention for shared resources I/O system. Such contention
from
in the
is not Chapter 13. Input/Output Operations 13-83

apparent to the program to the extent that I/O-busy conditions are handled by
channels when start-I/O-fast queuing is
provided. In order to provide some
indication of I/O-busy conditions
13-84 System/370 Principles of Operation handled by channels, the measurement
byte is provided in systems that provide start-I/O-fast queuing and are operating
in EC mode.