<s>
In	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
,	O
Execute	B-Application
Channel	I-Application
Program	I-Application
in	I-Application
Real	I-Application
Storage	I-Application
(	O
EXCPVR	B-Application
)	O
is	O
a	O
macro	O
generating	O
a	O
system	B-Operating_System
call	I-Operating_System
,	O
implemented	O
as	O
a	O
Supervisor	B-Operating_System
Call	I-Operating_System
instruction	I-Operating_System
,	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
.	O
</s>
<s>
Additionally	O
,	O
the	O
programmer	O
is	O
expected	O
and	O
required	O
to	O
ensure	O
that	O
the	O
input/output	O
data	O
areas	O
are	O
in	O
real	O
storage	O
and	O
are	O
"	O
fixed	O
"	O
,	O
either	O
prior	O
to	O
invoking	O
EXCPVR	B-Application
or	O
via	O
the	O
page	O
fix	O
appendage	O
.	O
</s>
<s>
EXCPVR	B-Application
can	O
in	O
some	O
situations	O
provide	O
improved	O
performance	O
compared	O
to	O
EXCP	B-Application
.	O
</s>
<s>
EXCPVR	B-Application
may	O
be	O
viewed	O
,	O
historically	O
,	O
as	O
a	O
V=	O
R	O
(	O
i.e.	O
,	O
Virtual	O
=	O
Real	O
)	O
version	O
of	O
EXCP	B-Application
.	O
</s>
<s>
However	O
,	O
EXCPVR	B-Application
is	O
not	O
restricted	O
to	O
V=	O
R	O
applications	O
.	O
</s>
<s>
Indeed	O
,	O
EXCPVR	B-Application
may	O
refer	O
to	O
non-V	O
=	O
R	O
data	O
areas	O
provided	O
such	O
data	O
areas	O
are	O
"	O
fixed	O
"	O
and	O
the	O
channel	O
command	O
words	O
which	O
reference	O
such	O
data	O
areas	O
have	O
been	O
translated	O
from	O
virtual	O
to	O
real	O
addresses	O
by	O
the	O
programmer	O
using	O
the	O
LRA	O
[	O
Load	O
Real	O
Address	O
]	O
privileged	O
instruction	O
,	O
as	O
the	O
data	O
channels	O
deal	O
only	O
with	O
real	O
addresses	O
,	O
not	O
virtual	O
addresses	O
.	O
</s>
<s>
EXCPVR	B-Application
was	O
first	O
introduced	O
in	O
SVS	O
and	O
was	O
continued	O
in	O
MVS/370	O
.	O
</s>
<s>
EXCPVR	B-Application
's	O
front-end	O
is	O
always	O
in	O
TCB	O
mode	O
as	O
EXCPVR	B-Application
is	O
a	O
Type	O
1	O
SVC	O
.	O
</s>
<s>
In	O
MVS/370	O
and	O
subsequent	O
instances	O
of	O
the	O
OS	O
,	O
EXCPVR	B-Application
invokes	O
STARTIO	B-Application
to	O
schedule	O
execution	O
of	O
the	O
channel	O
program	O
,	O
hence	O
EXCPVR	B-Application
's	O
back-end	O
is	O
always	O
in	O
SRB	O
mode	O
,	O
but	O
the	O
back-end	O
contains	O
emulation	O
code	O
which	O
allows	O
the	O
appendages	O
developed	O
for	O
earlier	O
,	O
pre-MVS	O
instances	O
of	O
the	O
OS	O
to	O
function	O
largely	O
as	O
before	O
and	O
thereby	O
to	O
appear	O
to	O
be	O
in	O
TCB	O
mode	O
,	O
for	O
which	O
these	O
appendages	O
were	O
originally	O
designed	O
.	O
</s>
