160 164
168 Channel 10 172 I 10El Pointer 176 Limited Channel logout (lCU 180 1000000001 I/O AddrElsS --
184
188 I/O Communications Area Chanm.!1 ID (Locations 168-171): Locations 168-
171, when stored during the execution of a STORE CHANNEL 10 instruction, contain information
which describes the addressed channel. (See STORE CHANNEL 10.) I/O Extended Logout Pointer (Locations 173-
175): The I/O Extended Logout (IOEL) pointer
(locations 173-175) is program-set to designate an
area to be used by channels not capable of storing or
not choosing to store the channel logout information
in the fixed logout area (locations 256-351). The
low-order three bits of the pointer are reserved and
are ignored by the channel so that the I/O extended
logout always begins on a doubleword boundary. Channel logout information may be stored in the 10EL area only when the 10EL mask bit (control
register 14, bit 2) of the CPU to which the channel
is configured is one.
Whether the 10EL facility is used depends on the
channel type and model number. Channels with a
logout length not exceeding 96 bytes use either the 10EL area or locations 256-351 as the logout area.
Channels with a logout length exceeding 96 bytes
use the 10EL area.
Programming Note
The extent of the channel-extended-Iogout area dif­
fers among channels and, for any particular channel,
may depend on the features or engineering changes
installed. In order to provide for such variations, the
program should determine the extent of the logout
by means of STORE CHANNEL 10 whenever a
storage area for the extended logout is to be as­
signed. 240 System/370 Principles of Operation
Limited-Channel Logout (Locations 176-179): The
limited-channel-logout field (locations 176-179)
contains model-independent information related to
equipment errors detected by the channel. This in­
formation is used to provide detailed machine status
when errors have affected I/O operations.
The limited-channel-logout facility may not be
available on all channels. The field, if stored, may
be stored only when the CSW or a portion of the
CSW is stored and mayor may not be accompanied
by the full channel logout. Channels which do not
store the limited-channel-logout field instead usually
store equivalent information in the full channel log­
out.
The bits of the field are defined as follows:
o This bit will always be stored as a zero .when
a limited channel logout (LCL) is stored. If
the program ensures that this bit is set to
one and any channel control check, inter­
face control check, or channel data check
occurs, a test of this bit can determine if the
LCL was stored by the channel. The LCL
cannot be stored by a channel unless one of
these three channel status bits is set.
1-3 Identity of the storage control unit (SC U) through which storage references were di­
rected when an error was detected. This
identity is not necessarily the identity of the
storage unit involved with the transfer.
When only one physical path exists between
channel and storage, the storage control unit
has the identity of the CPU. If more than
one path exists, the storage control unit has
its own identity.
When bit 3 is zero, bits 1 and 2 are mean­
ingless. In this case, the SCU identity is im­
plied to be the same as the CPU identity.
When bit 3 is one, the binary value of bits 1
and 2 identifies a physical SCU. Each SCU in the system has a unique identity.
4-7 Detect field identifies the type of unit that
detected the error. At least one bit is pres­
ent in this field, and mUltiple bits may be set
when more than une unit detects the error.
Bit 4 -- CPU Bit 5 --Channel
Bit 6 --Main-storage control
Bit 7 --Main storage
8-12 Source field indicates the most likely
source of the error. The determination is
made by the channel on the basis of the
type of error check, the location of the
checking station, the information flow path,

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