The following table lists the conditions that are
identified and the corresponding condition codes for
each instruction. The states of the system and their
abbreviations are defined in "States of the Input/Out­ put System." The digits in the table represent the
numeric value of the code. The instruction START I/O can set code 0 or 1 for the AAA state, depending on
the type of operation that is initiated.
Available AAA 0 0 0 Interruption pend. in device AAI 0 0 Device working AAW 0 0 Device not operational AAN 3 3 0 0 Interruption pend. in sub channel AIX
For the addressed device 2 0 0 For another device 2 2 0 0 Subchannel working AWX 2 2 0 Subchannel not operational ANX 3 3 3 0 Interruption pend. in channel IXX sce note below 1 Channel working WXX 2 2 2 2 Channel not operational NXX 3 3 3 3 Error Channel equipment error P P Channel programming error Device error CSW or its status portion is stored at location 64 during
execution of the instruction.
- The condition cannot be identified during execution of the in­
struction. NOTE: For the purpose of executing START I/O, TEST I/O,
and HALT I/O, a channel containing a pending interruption
condition appears the same as an available channel, and the
condition codes for the IXX state are the same as for the AXX
state, where the X's represent the states of the subchannel and
the device. As an example, the condition eode for the IAA
state is the same as for the AAA state.
The available condition is indicated only when no
errors are detected during the execution of the I/O instruction. When a programming error occurs in the
information placed in the CAW or ccw and the ad­
dressed channel or subchannel is working, either con­
dition code 1 or 2 may be set, depending upon the
model. Similarly, either code 1 or 3 may be set when
a programming error occurs and a part of the ad­
dressed I/O system is not operational.
When a subchannel on the multiplexor channel con­
tains a pending interruption condition (state AIX),
the I/O device associated with the terminated opera­
tion normally is in the interruption-pending state.
When the channel detects during execution of TEST I/O that the device is not operational, condition code
3 is set. Similarly, condition code 3 is set when HALT I/O is addressed to a subchannel in the working state
and operating in the multiplex mode (state A WX ) ,
but the device turns out to be not operational. The
not-operational state in both situations can be caused
by operator intervention or by equipment malfunction
and, for HALT I/O, may occur when the isntruction is
addressed to a control unit other than the one cur­
rently operating.
The error conditions listed in the preceding table
include all equipment or programming errors detected
by the channel or the I/O device during execution of
the I/O instruction. Except for channel equipment er­
rors, in which case no csw may be stored, the status
portion of the csw identifies the error. Three types of
errors can occur:
Channel Equipment Error: The channel can detect
the following equipment errors during execution of START r/o, TEST I/O, and HALT r/o:
1. The device address that the channel received on
the interface during initial selection either has a
parity error or is not the same as the one the channel
sent out. Some device other than the one addressed
may be malfunctioning.
2. The unit-status byte that the channel received on
the interface during initial selection has a parity error.
3. A signal from the I/O device occurred during
initial selection at an invalid time or had invalid
4. The channel detected an error in its control
The channel may perform the malfunction-reset
function, depending on the type of error and the
model. If a csw is stored, channel control check or
interface control check is indicated, depending on the
type of error.
Channel Programming Error: The channel can de­
tect the following programming errors during execu­
tion of START I/O: 1. Invalid ccw address in CAW 2. Invalid ccw address specification in CAW 3. Invalid storage protection key in CAW 4. Invalid CAW format
5. First ccw specifies transfer in channel
6. Invalid command code in first ccw
7. Initial data address exceeds addressing capacity
of model
8. Invalid count in first ccw
9. Invalid format of first ccw
The csw indicates program check.
Device Error: Programming or equipment crrors
detected by the device during the execution of START I/O are indicated by unit check or unit exception in
the csw. The instruction TEST I/O can cause unit check
to be generated.
The conditions responsible for unit check and unit
exception for each type of 1/0 device are detailed in
the SRL publication for the device.
[nput/Output Operations 91
Instruction Format
All I/O instructions use the following SI format: OpCode B1 I 78 1516 1920 31
Bit positioIiS 8-15 of the instruction are ignored.
The content of the Bl field designates a register. The
sum obtained by the addition of the content of reg­
ister Bl and content of the Dl field identifies the
channel and the I/O device. This sum has the format:
Bit positions 0-7 are not part of the address. Bit
positions 8-20, which constitute the high-order portion
of the address, are ignored. Bit positions 21-23 of the
sum contain the channel address, while bit positions
24-31 identify the device on the channel and, on the
multiplexor channel, the subchannel. Instructionls The mnemonics, format, and operation codes of the I/O instructions follow. The table also indicates that
all I/O instructions cause program interruption when
they are encountered in the problem state, and that
all I/O instructions set the condition code.
Test 110 TIO SI, C M 9D
Halt 110 HIO SI, C M 9E
Test Channel TCH SI, C M 9F NOTES C Condition code is set
M Privileged-operation exception
Programming Note
The instructions START I/O, TEST I/O, and HALT I/O may cause a csw to be stored. To prevent the con­
tents of the csw stored by the instruction from being
destroyed by an immediately following I/O interrup­
tion, all channels must be masked before issuing START I/O, TEST I/O, or HALT I/O and must remain
masked until the information in the csw provided by
the instruction has been acted upon or stored else­
where for later use.
92 Start I/O 5/0 51 9C 7 8 1516 1920 31
A write, read, read backward, control or sense opera­
tion is initiated at the addressed I/O device and sub­
channel. The instruction START I/O is executed only
when the CPU is in the supervisor state.
Bit positions 21-31 of the sum formed by the addi­
tion of the content of register Bl and the content of
the Dl field identify the channel, sub channel, and I/O device to which the instruction applies. The CAW at
location 72 contains the protection key for the sub­
channel and the address of the first ccw. The ccw so
designated specifies the operation to be performed,
the main-storage area to be used, and the action to
be taken when the operation is completed.
The I/O operation specified by START I/O is initiated
if the addressed I/O device and subchannel are avail­
able, the channel is available or is in the interruption­
pending state, and errors or exceptional conditions
have not been detected. When the addressed part of
the I/O system is in any other state or when the chan­
nel or device detect any error or exceptional condition
during exccution of the instruction, the I/O operation
is not initiated.
When any of the following conditions occurs, START I/O causes the status portion, bit positions 32-47, of
the csw at location 64 to be replaced by a new set of
status bits. The status bits pertain to the device ad­ dressed by the instruction. The contents of the other fields of the csw at location 64 are not changed.
1. An immediate operation was executed, and no
command chaining is taking place. An operation is
called immediate when the I/O device signals the
channel-end condition immediately on receipt of the
command code. The csw contains the channel-end bit
and any other indications provided by the channel or
the device. The busy bit is off. The I/O operation has
been initiated, but no information has been transferred
to or from the storage area designated by the ccw. No
interruption conditions are generated at the device or
snbchannel, and the sub channel is available for a
new I/O operation.
2. The I/O device contains a pending interruption
condition due to device end or attention, or the con-
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