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ERROR RECOVERY PROCEDURE
Error handling
usually involves storage control
and
system-invoked recovery
actions.
These
recovery
actions
can vary depending
on
how
and
to
what system
the
3340/3344
is
attached.
The
following
topics are associated
with recovery
actions involving the
3830 Model
2,
or
ISC
(storage
controls),
and
the
3340/3344:
•
Error Correction Function
•
Error Condition Table
•
Errory
Recovery
Action
ERROR
CORRECTION FUNCTION
The
error correction function
(ECF)
is
part of the
recovery
action
procedure.
The
ECF
algorithms
and
the related
procedure
are
fully
described
in
the
3830
Model
2
and ISC
reference manuals.
ERROR
CONDITION TABLE
The
Error Condition Table
(Figure 11)
identifies
unique
configurations of sense
bits set
by
the storage control
in
sense bytes
0,
1
,
and
2.
In
addition,
it
refers to
each
of these configurations
in
a
specific
recovery action
to
be invoked
by
the system.
ERROR RECOVERY
ACTION
The 3340/3344
Recovery Action
Table
(Figure 12)
specifies
actions to
be taken
for error conditions
listed
in the
Error Condition Table
.
A
necessary
part of the
recovery
action
is
the construction of Restart
Channel
Command
Words
1
and
2.
Construction of Restart
CCWs
If
Operation Incomplete
(byte
1,
bit
7)
is
set
in
the
sense information,
it
indicates that
an
error
or
unusual
condition occurred during
a
logical
operation
after
data
transfer
had been
initiated.
By
constructing
Restart
Channel
Command
Words,
the error
recovery
procedures can
correct the
unusual condition
and
continue
the
operation
in
progress
from
the point of
interruption to the
normal ending
point.
RESTART
CCW
1
Restart
CCW
1 is
constructed
as follows:
1
The
command
code
byte
is
provided
in
sense
byte
3.
2.
The
data address
is
that of
the
interrupted
CCW,
plus
the
count of
that
CCW,
minus
the residual
count
in
the
channel
status
word (CSW).
3
The
flags,
except
Program
Controlled
Interrupt
(PCI), are those of the interrupted
CCW.
4.
The
count
is
the
residual
count
in
the
CSW.
If
the residual
count
is
zero, a
count
of
one must be
used.
If
a
Write
command
is
in progress, the
data
address should
specify
a byte
containing
'
00
'
If
a
Read
command
is
in progress, turn
on
the
skip
bit.
RESTART
CCW
2
Restart
CCW
2
is
constructed
as follows:
1.
The
command
code
is
provided
in
sense byte
3.
2.
The
count
is
constructed
as follows:
a.
Fetch
the
count
of the
CCW
designated
by
CSW-8,
and
set
a pointer
to
this
CCW.
b.
Subtract the
restart
displacement
from
the
count obtained
in
step
a.
If this
result
is
positive,
go
to step
f
;
otherwise
go
to step
c.
c.
Check
the
chain data
flag
of the
CCW
desig-
nated
by
the
pointer.
If
the
flag
is
not
set,
go
to
step
e;
otherwise
go
to step
d.
d.
Advance
the pointer
to
the next
non-Transfer
in
Channel (non-TIC)
CCW
in
the
data chain
and add
the
count
of
this
CCW
to the
counts
of
all
preceding
non-TIC
CCWs
in
the data
chain.
Return
to step
b.
e.
Truncation
occurs. Set the
restart
CCW
2
count equal
to
1.
Go
to step 3
and
include the
skip
bit in
the Restart
CCW
flags.
f
Set the
Restart
CCW
2 count
equal
to the
result
of the subtraction
in
step
b.
Go
to
Step
3.
1
8
IBM
3340/44
Reference
Manual
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