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Channel
A
Subchannel
A
Control Unit
A
N
- Device
0
3
A
I
Avai lable
Interruption pending
w;t
W#
2
l;t
N
A
l;t
2
2
A
N
0
3
l;t
=
Interruption pending for a device other than the one
addressed
W
1#
=
Interruption pending for the addressed device
Working
w;t
=
Working with a device other than the one addressed
W#
=
Working with the addressed device
N
*
Not operational
CSW stored
@
In the W;tI#X state, either condition code
1
may be set with the
CSW
stored, or condition code 2 may be set, depending on the
channel and the activity in the channel.
Note: Underscored
condition
codes
pertain to situations that can
OCCUr only on the multiplexer channel.
Condition Codes Set
by
TEST I/O
Programming Notes
1. Disabling the CPU for I/O interruptions
provides the program with a means of
controlling the priority of I/O interruptions
selectively by channels. The priority of devices
attached on a channel cannot be controlled by
the program. The instruction TEST I/O in
some cases permits the program to clear
interruption conditions selectively by I/O
device.
2. When a CSW is stored by the TIO function, the
interface-control-check and channel-control-
check indications may be due to an interruption
condition already existing in the channel or
may be due to an interruption condition created
by the TIO function. Similarly, the unit-check
bit set to one with the channel-end, control-
unit-end, or device-end bits set to zeros may be
due to a situation created by the preceding
operation, the I/O device being not ready, or
an equipment error detected during the
execution of TEST I/O. The instruction TEST
I/O cannot be used to clear an interruption
condition due to the PCI flag while the
subchannel is working.
3. The use of a TEST I/O loop on a multiplexer
channel to retrieve ending status for a channel
program should, in general, be avoided. TEST
I/O loops may be used to return ending status
to a sense command when that command was
initiated by a START I/O that received
condition code O. TEST I/O loops under other
conditions may result in hang conditions.
Input / Output-Instruction-Exception
Handling
Before the channel is signaled to execute an I/O
instruction, the instruction is tested for validity by
the CPU. Exceptional situations detected at this
time cause a program interruption.
The following exception may cause a program
interruption:
Privileged Operation: An I/O instruction is
encountered when the CPU is in the problem state.
The instruction is suppressed before the channel
has been signaled to execute it. The CSW, the
condition code in the PSW, and the state of the
addressed sub channel and I/O device are not
affected by the attempt to execute an I/O
instruction while in the problem state.
Execution of Input/Output Operations
The channel can execute six commands: write,
read, read backward, control, sense, and transfer in
channel. Each command except transfer in channel
initiates a corresponding I/O operation. The term
"I/O operation" refers to the activity initiated by a
command in the I/O device and· associated
subchannel. The subchannel is involved with the
execution of the operation from the initiation of the
command until the channel-end signal is received
Chapter 12. Input/Output Operations
12-27
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