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IBM System/360 System Programmer's Guide

IBM System/360
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CHARACTERISTICS
OF
APPENDAGES
An
appendage
is
a
programmer-written
routine
that
provides
additional
control
over
I/O
operations
during
channel
program
execution.
By
providing
appendages,
you
can
examine
the
status
of
I/O
operations
and
determine
the
actions
to
be
taken
for
various
conditions.
An
appendage
may
receive
control
before
a
start
input/output
(SIO)
instruction
is
issued,
or
~hen
one
of
the
following
occurs:
•
Program
controlled
interruption.
•
End
of
extent.
•
Channel
end.
•
Abnormal
encl.
Appendages
are
executed
in
supervisor
state.
However,
you
must
not
issue,
in
an
appendage,
any
SVC
instructions
or
instructions
that
change
the
status
of
the
computing
or
operating
system
(e.g.,
WTO,
LPSW,
SSM,
etc.).
Since
appendages
are
disabled
for
all
types
of
interruptions
except
machine
checks,
you
also
must
not
enter
loops
that
test
for
completion
of
I/O
operations.
An
appendage
must
not
alter
storage
used
by
either
the
supervisor
or
the
input/output
supervisor.
The
identification
of
an
appendage,
which
consists
of
the
last
two
characters
of
its
8-character
name,
~ust
be
specified
in
the
DCB
macro-instruction,
as
described
in
the
section
"EXCP
Programming
Specifications.
'"
When
the
OPEN
macro-instruction
for
the
data
control
block
is
issued,
any
appendages
specified
in
the
DCB
macro-instruction
are
loaded
into
main
storage.
The
appendages
are
linked
to
the
input/output
supervisor
when
their
addresses
are
placed
into
a
table
of
addresses
called
an
appendage
vector
table.
This
table
is
always
constructed
by
the
system
when
OPEN
is
issued;
if
an
appendage
is
not
provided,
the
t~able
contains
the
address
of
a
return
branch
instruction
to
the
input/output
supervisor.
Using
the
appendage
vector
table,
the
input/output
supervisor
branches
and
links
to
an
appendage
at
the
appropriate
timE~.
The
address
of
the
starting
location
of
the
appendage
is
placed
into
register
15.
Parameters
are
passed
to
appendages
by
the
input/output
supervisor.
These
parameters
are
contained
in
registers,
and
are
as
follows:
•
Reqister
1
contains
the
address
of
the
request
element
for
the
channel
program.
The
request
element
contains
the
following
infor-
mation:
Bytes
1
and
2
are
a
field
used
by
the
system
for
linking
the
request
element
into
a
queue.
Bytes
3
and
4
indicate
the
address
of
the
unit
ccntrol
block
(UCB)
for
the
I/O
device.
Byte
5
indicates
the
identification
of
the
task
control
block
(TCB)
for
the
task.
(In
a
multitasking
environment,
this
field
is
not
used.
It
contains
all
zeros
if
the
request
element
is
not
available
and
all
ones
when
the
request
element
is
available.)
Bytes
6,
7,
and
8
indicate
the
address
of
the
input/output
block.
Byte
9
indicates
the
priority
of
the
request.
Execute
Channel
Program
(EXCP)
Macro-Instruction
81

Table of Contents

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IBM System/360 Specifications

General IconGeneral
BrandIBM
ModelSystem/360
CategoryDesktop
LanguageEnglish

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