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Channel
Subchannel
Control Unit
- Device
A
A
A
I
W
N
Available
Interruption pending
Working
*
Not operational
CSW stored
A
N
A
N
A
N
•
When a nonimmediate I/O operation has been initiated,
and the channel is proceeding with its execution,
condition code 0 is set.
•
When an immediate operation has been initiated, and no
command chaining or command retry is taking place, or
the device is not ready, or an error has been detected
by the control unit or device, for the SID function
condition code 1 is set, and the CSW is stored.
Under
the same circumstances, for the SIOF function, condition
code 0 is set, and a deferred-condition-code-1
I/O-interruption condition is generated.
@
The SIOF function may cause condition code 0 to be set, in
which case the other condition code shown will be specified
as a deferred condition code.
Note: Underscored condition codes pertain to situations that can
OCCUr only on the mUltiplexer channel.
Condition Codes Set by START I/O and START I/O FAST
RELEASE
Programming Notes
1. The instruction START I/O FAST RELEASE
has the advantage over START I/O that the
CPU can be released after the CAW is fetched,
rather than after completion of the lengthy
device-selection procedure. Thus, the CPU is
freed for other activity earlier. A disadvantage,
however, is that if a deferred condition code is
presented, the resultant CPU execution time
may be greater than that required in executing
START I/O.
2. When the channel detects a programming error
during execution of the SIO function, when the
addressed device contains an interruption
condition, and when the channel and
sub channel are available, the instruction mayor
may not clear the interruption condition,
depending on the type of error and the model.
If
the instruction has caused the device to be
interrogated, as indicated by the presence of
the busy bit in the CSW, the interruption
condition has been cleared, and the CSW
contains program or protection check, as well
as the status from the device.
3. Two major differences exist between the SIO
and SIOF functions:
a. Unchained immediate commands on certain
channels (that is, those which execute SIOF
independent of the device) result in a
condition code 0 for the SIOF function,
whereas condition code 1 is set for the SIO
function. See also Programming Note 2 in
the section "Command Retry" of this
chapter.
b. Condition code 0 is set by these certain
channels for the SIOF function, even
though the addressed device is not available
or the command is rejected by the device.
The device information will be supplied by
means of an interruption.
STORE CHANNEL ID
STIDC
[S]
8203
82
D2
o
16
20
31
Chapter 12. Input/Output Operations
12-23
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