<s>
In	O
MVS/XA	O
and	O
successor	O
versions	O
of	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
,	O
Start	B-Application
Input/Output	I-Application
(	O
STARTIO	B-Application
)	O
is	O
a	O
macro	O
instruction	O
and	O
a	O
"	O
branch	O
entry	O
"	O
for	O
low-level	O
device	O
access	O
,	O
where	O
the	O
programmer	O
is	O
responsible	O
for	O
providing	O
a	O
list	O
of	O
device-specific	O
CCWs	O
,	O
that	O
is	O
,	O
a	O
channel	O
program	O
,	O
to	O
be	O
executed	O
by	O
I/O	O
channels	O
,	O
control	O
units	O
and	O
devices	O
and	O
a	O
number	O
of	O
"	O
exits	O
"	O
,	O
several	O
of	O
which	O
may	O
be	O
immediate	O
returns	O
to	O
the	O
Input/Output	B-Application
Supervisor	I-Application
(	O
IOS	B-Application
)	O
.	O
</s>
<s>
Invokers	O
of	O
STARTIO	B-Application
must	O
be	O
in	O
supervisor	O
mode	O
and	O
key	O
0	O
.	O
</s>
<s>
STARTIO	B-Application
interfaces	O
directly	O
with	O
the	O
IOS	B-Application
component	O
of	O
MVS	O
.	O
</s>
<s>
Start	B-Application
Input/Output	I-Application
differs	O
from	O
EXCP	B-Application
(	O
including	O
XDAP	B-Application
,	O
which	O
is	O
simply	O
a	O
DASD-only	O
subset	O
of	O
EXCP	B-Application
)	O
and	O
EXCPVR	B-Application
in	O
the	O
following	O
fundamental	O
way	O
:	O
Start	B-Application
Input/Output	I-Application
is	O
a	O
low	O
level	O
facility	O
that	O
supports	O
,	O
e.g.	O
,	O
selection	O
of	O
channel	O
paths	O
,	O
selection	O
of	O
exposures	O
while	O
Execute	B-Application
Channel	I-Application
Program	I-Application
is	O
a	O
high	O
level	O
facility	O
that	O
supports	O
,	O
e.g.	O
,	O
CCW	O
translation	O
,	O
page	O
fixing	O
,	O
serialization	O
of	O
related	O
requests	O
,	O
VIO	O
.	O
</s>
<s>
Callers	O
of	O
Execute	B-Application
Channel	I-Application
Program	I-Application
must	O
be	O
in	O
TCB	O
mode	O
.	O
</s>
<s>
While	O
IBM	O
documents	O
EXCPVR	B-Application
as	O
being	O
privileged	O
,	O
some	O
unprivileged	O
access	B-Application
method	I-Application
code	O
uses	O
it	O
for	O
SAM-E	O
processing	O
of	O
DASD	O
data	O
sets	O
.	O
</s>
<s>
Except	O
for	O
some	O
special	O
cases	O
,	O
an	O
application	O
may	O
only	O
use	O
Execute	B-Application
Channel	I-Application
Program	I-Application
for	O
a	O
Data	B-Application
Control	I-Application
Block	I-Application
(	O
DCB	O
)	O
that	O
it	O
has	O
OPENed	O
(	O
that	O
is	O
,	O
it	O
has	O
been	O
made	O
accessible	O
to	O
the	O
application	O
by	O
the	O
system	O
's	O
OPEN/CLOSE/END	O
-OF-VOLUME	O
supervisor	O
services	O
,	O
SVCs	O
19	O
,	O
20	O
and	O
55	O
,	O
respectively	O
)	O
.	O
</s>
<s>
Callers	O
of	O
STARTIO	B-Application
must	O
be	O
privileged	O
,	O
must	O
page	O
fix	O
all	O
storage	O
areas	O
related	O
to	O
the	O
request	O
and	O
must	O
use	O
global	O
storage	O
for	O
the	O
SRB/IOSB	O
.	O
</s>
<s>
STARTIO	B-Application
's	O
initiation	O
sequence	O
,	O
usually	O
referred	O
to	O
as	O
its	O
"	O
front	O
end	O
"	O
,	O
may	O
be	O
in	O
TCB	O
mode	O
or	O
in	O
SRB	B-Application
mode	O
.	O
</s>
<s>
STARTIO	B-Application
's	O
termination	O
sequence	O
,	O
usually	O
referred	O
to	O
as	O
its	O
"	O
back	O
end	O
"	O
,	O
is	O
always	O
in	O
SRB	B-Application
mode	O
.	O
</s>
<s>
STARTIO	B-Application
always	O
utilizes	O
an	O
SRB/IOSB	O
pair	O
.	O
</s>
<s>
The	O
SRB	B-Application
(	O
Service	B-Application
Request	I-Application
Block	I-Application
)	O
is	O
utilized	O
to	O
schedule	O
processor	O
activity	O
,	O
as	O
required	O
,	O
on	O
any	O
available	O
processor	O
,	O
in	O
connection	O
with	O
the	O
related	O
I/O	O
request	O
,	O
as	O
the	O
I/O	O
request	O
is	O
not	O
necessarily	O
run	O
by	O
the	O
processor	O
in	O
which	O
the	O
caller	O
is	O
running	O
(	O
indeed	O
,	O
in	O
most	O
cases	O
it	O
will	O
not	O
be	O
)	O
.	O
</s>
<s>
The	O
IOSB	B-Application
(	O
Input/Output	B-Application
Supervisor	I-Application
Block	I-Application
)	O
describes	O
the	O
related	O
I/O	O
request	O
,	O
and	O
passes	O
parameters	O
to	O
and	O
receives	O
responses	O
from	O
IOS	B-Application
.	O
</s>
<s>
IOS	B-Application
generally	O
calls	O
appendages	O
in	O
enabled	O
SRB	B-Application
mode	O
,	O
but	O
in	O
special	O
cases	O
calls	O
an	O
appendage	O
as	O
a	O
Disabled	O
Interrupt	O
Exit	O
(	O
DIE	O
)	O
.	O
</s>
<s>
The	O
Unit	B-Application
Control	I-Application
Block	I-Application
(	O
UCB	O
)	O
for	O
each	O
device	O
points	O
to	O
a	O
Device	O
Descriptor	O
Table	O
(	O
DDT	O
)	O
,	O
which	O
identified	O
routines	O
needed	O
for	O
device	O
dependent	O
processing	O
,	O
e.g.	O
,	O
handling	O
of	O
Attention	O
interrupts	O
,	O
appending	O
of	O
RESERVE	O
CCWs	O
,	O
appending	O
of	O
Set	O
File	O
Mask	O
and	O
Seek	O
.	O
</s>
<s>
Although	O
use	O
of	O
STARTIO	B-Application
by	O
customers	O
is	O
not	O
supported	O
by	O
IBM	O
,	O
nevertheless	O
its	O
macro	O
instruction	O
and	O
its	O
associated	O
"	O
branch	O
entry	O
"	O
have	O
remained	O
remarkably	O
stable	O
throughout	O
the	O
forty-year-long	O
(	O
as	O
of	O
2014	O
)	O
transition	O
from	O
MVS/370	O
to	O
z/OS	O
,	O
as	O
STARTIO	B-Application
is	O
the	O
sole	O
method	O
of	O
performing	O
lowest-level	O
physical	O
I/O	O
operations	O
on	O
channel-attached	O
devices	O
on	O
MVS/370	O
and	O
subsequent	O
instances	O
of	O
the	O
OS	O
.	O
</s>
<s>
In	O
particular	O
,	O
VSAM	O
and	O
VTAM	B-Application
use	O
STARTIO	B-Application
,	O
as	O
,	O
of	O
course	O
,	O
do	O
EXCP	B-Application
,	O
XDAP	B-Application
and	O
EXCPVR	B-Application
,	O
and	O
certain	O
instances	O
of	O
IMS/VS	B-Application
and	O
JES	B-Application
,	O
most	O
particularly	O
where	O
Channel-to-Channel	O
operations	O
are	O
involved	O
.	O
</s>
<s>
Should	O
a	O
catastrophic	O
error	O
occur	O
during	O
IOS	B-Application
's	O
processing	O
of	O
the	O
related	O
I/O	O
request	O
,	O
it	O
is	O
this	O
TCB	O
,	O
usually	O
the	O
caller	O
's	O
"	O
job	O
step	O
"	O
TCB	O
,	O
which	O
will	O
be	O
abnormally	O
terminated	O
,	O
not	O
whichever	O
TCB	O
happened	O
to	O
be	O
active	O
when	O
the	O
error	O
was	O
detected	O
,	O
which	O
,	O
by	O
the	O
basic	O
design	O
of	O
IOS	B-Application
,	O
which	O
is	O
run	O
asynchronously	O
to	O
all	O
tasks	O
and	O
all	O
address	O
spaces	O
,	O
could	O
be	O
any	O
TCB	O
in	O
any	O
address	O
space	O
,	O
or	O
,	O
indeed	O
,	O
no	O
TCB	O
at	O
all	O
,	O
e.g.	O
,	O
an	O
SRB	B-Application
,	O
or	O
,	O
perhaps	O
more	O
likely	O
,	O
the	O
System	O
Wait	O
Task	O
(	O
which	O
runs	O
in	O
the	O
OS	O
Master	O
Scheduler	O
's	O
address	O
space	O
)	O
.	O
</s>
<s>
Perhaps	O
the	O
best	O
description	O
of	O
STARTIO	B-Application
and	O
its	O
application	O
is	O
a	O
"	O
white	O
paper	O
"	O
entitled	O
"	O
The	O
[	O
Start	B-Application
Input/Output	I-Application
Interface	O
]	O
of	O
MVS	O
"	O
,	O
written	O
by	O
Peter	O
Haas	O
,	O
then	O
with	O
Amdahl	O
Corp	O
(	O
subsequently	O
liquidated	O
)	O
.	O
</s>
<s>
Haas	O
's	O
"	O
white	O
paper	O
"	O
contained	O
an	O
application	O
program	O
example	O
which	O
illustrated	O
the	O
required	O
and	O
optional	O
parameters	O
and	O
exits	O
employed	O
by	O
the	O
STARTIO	B-Application
interface	O
.	O
</s>
<s>
A	O
more	O
detailed	O
description	O
of	O
EXCP[VR]	O
and	O
STARTIO	B-Application
is	O
available	O
in	O
the	O
logic	O
manual	O
.	O
</s>
