13
14
15
16-23
24-25
and the success or failure of transmission
through previous check stations.
Normally, only one bit will be present in
this field. However, when inter-unit com­
munication cannot be resolved to a single
unit, such as when the interface between
units is at fault, multiple bits (normally two)
may be set in this field. When a reasonable
determination cannot be made, all bits in
this field are set to zero.
If the detect and source fields indicate dif­
ferent units, the interface between them can
also be considered suspect.
Bit 8 -- CPU Bit 9 --Channel
Bit 10 -- Main storage control
Bit 11 --Main storage
Bit 12 --Control unit
Reserved. Stored zero.
Reserved. Stored zero.
Reserved. Stored zero.
Field validity flags. These bits indicate the
validity of the information stored in the des­
ignated fields. When the designated field is
stored by the channel with the correct con­
tents, the validity bit is one. When the des­
ignated field is stored by the channel with
unpredictable contents, the validity bit is
zero. The validity bits for nonstored fields
are meaningless.
The fields designated are:
Bit 16 --Interface address
Bit 17 --(Reserved. Stored zero)
Bit 18 --(Reserved. Stored zero)
Bit 19 --Sequence code
Bit 20 -- Unit status
Bit 21 --Command address and key
Bit 22 --Channel address
Bit 23 --Device address
Type of termination that has occurred is
indicated by these two bits.
This encoded field has meaning only
when a channel control check or an inter­
face control check is indicated in the CSW. When neither of these two checks is indicat­
ed, no termination has been forced by the
channel. 00 Interface disconnect 01 Stop, stack, or normal termination 10 Selective reset
11 System reset
26-27
28
29-31
Reserved. Stored zero. I/O error alert. This bit, when set to one,
indicates that the limited channel logout re­
sulted from the signaling of I/O error alert
on the I/O interface by the indicated unit.
The I/O error alert signal indicates that the
control unit has detected a malfunction
which prevents it from communicating prop­
erly with the channel. The channel, in re­
sponse, performs a malfunction reset and
causes interface control check to be set.
Sequence code identifies the I/O sequence
in progress at the time of error. It is mean­
ingless if stored during the execution of
HALT I/O or HALT DEVICE.
For all cases, the channel program ad­
dress, if validly stored and if nonzero, is the
address of the current CCW + 8.
The sequence code assignments are: 000 A channel-detected error occurred
during the execution of a TEST I/O or
CLEAR I/O instruction. 001 Command-out with a nonzero com­
mand byte on bus-out has been sent
by the channel, but device status has
not yet been analyzed by the channel.
This code is set with a command-out
response to address-in during initial
selection. 010 The command has been accepted by
the device, but no data has been trans­
ferred. This code is set by a service­
out or command-out response to
status-in during an initial selection se­
quence, if the status is either channel
end alone, or channel end and device
end, or channel end, device end, and
status modifier, or all zeros. 011 At least one byte of data has been
transferred over the interface. This
code is set with a service-out response
to service-in and, when appropriate,
may be used when the channel is in an
idle or polling state. 100 The command in the current CCW has
either not yet been sent to the device
or else was sent but not accepted by
the device. This code is set when one
of the following conditions occurs:
1. When the command address is
updated during command chain­
ing or a START I/O. Input/Output Operations 241

Page of GA22-7000-4 Revised September 1, 1975
By TNL: GN22-0498
2. When service-out or command·· out is raised in response to
status-in during an initial selec-·
tion sequence with the status on
bus-in including attention, con·· trol unit end, unit check, unit ex­
ception, busy, status modifier
(without channel end and device
end), or device end (without
channel end).
3. When a short, control-unit-busy
sequence is signaled.
4. When command retry is signaled.
5. When the channel issues a test I/O command rather than the
command in the current CCW"
1 0 1 The command has been accepted, but
data transfer is unpredictable. This eo de applies from the time a device
comes on the interface until the time it
242 System/370 Principles of Operation is determined that a new sequence
code applies. It may thus be used
when a channel goes into the polling
or idle state and it is impossible to de­
termine that code 2 or 3 applies. It
may also be used at other times when
a channel cannot distinguish between
code 2 or 3. 110 Reserved.
111 Reserved. I/O Address (Locations 185-187): A three-byte
field is provided for storing the I/O address on
each I/O interruption in EC mode. Zeros are stor-
ed at location 185, and the channel and device address­
es are stored at locations 186 and 187, respectively.
Locations 160-167, 180-184, and 188-191 are
reserved for future I/O use.