<s>
Inter	B-Application
User	I-Application
Communication	I-Application
Vehicle	I-Application
(	O
IUCV	B-Application
)	O
is	O
a	O
data	O
transfer	O
mechanism	O
in	O
IBM	O
VM	B-Application
line	O
of	O
operating	O
systems	O
.	O
</s>
<s>
It	O
was	O
introduced	O
with	O
VM/SP	B-Application
Release	O
1	O
in	O
1980	O
.	O
</s>
<s>
In	O
effect	O
,	O
IUCV	B-Application
provides	O
a	O
form	O
of	O
message-based	O
interaction	O
between	O
virtual	O
machines	O
that	O
anticipated	O
the	O
client/server	O
interaction	O
between	O
network	O
connected	O
physical	O
machines	O
that	O
emerged	O
later	O
on	O
distributed	O
systems	O
.	O
</s>
<s>
IUCV	B-Application
is	O
implemented	O
by	O
CP	O
(	O
the	O
VM	B-Application
hypervisor	O
)	O
and	O
controls	O
all	O
aspects	O
of	O
session	O
establishments	O
,	O
message	O
passing	O
and	O
flow	O
control	O
.	O
</s>
<s>
Before	O
a	O
virtual	O
machine	O
can	O
use	O
the	O
IUCV	B-Application
service	O
,	O
it	O
must	O
first	O
indicate	O
the	O
address	O
of	O
an	O
area	O
within	O
its	O
address	O
space	O
where	O
CP	O
will	O
be	O
able	O
to	O
store	O
information	O
regarding	O
pending	O
information	O
or	O
status	O
.	O
</s>
<s>
In	O
IUCV	B-Application
terminology	O
,	O
the	O
session	O
between	O
two	O
end	O
points	O
is	O
called	O
a	O
PATH	O
.	O
</s>
<s>
Provided	O
the	O
target	O
has	O
initialized	O
IUCV	B-Application
itself	O
,	O
the	O
target	O
will	O
be	O
notified	O
of	O
the	O
pending	O
incoming	O
path	O
connection	O
and	O
may	O
then	O
either	O
use	O
the	O
ACCEPT	O
method	O
-	O
to	O
complete	O
path	O
establishment	O
-	O
or	O
the	O
SEVER	O
method	O
-	O
which	O
effectively	O
closes	O
the	O
pending	O
path	O
.	O
</s>
<s>
IUCV	B-Application
Messages	O
are	O
bounded	O
,	O
that	O
is	O
,	O
they	O
have	O
a	O
beginning	O
and	O
an	O
end	O
.	O
</s>
<s>
If	O
more	O
than	O
one	O
message	O
is	O
pending	O
on	O
a	O
path	O
for	O
an	O
endpoint	O
,	O
IUCV	B-Application
will	O
not	O
merge	O
the	O
messages	O
.	O
</s>
<s>
Data	O
transfer	O
may	O
be	O
temporarily	O
suspended	O
by	O
using	O
the	O
QUIESCE	B-Device
method	O
.	O
</s>
<s>
A	O
virtual	O
machine	O
may	O
poll	O
for	O
IUCV	B-Application
notifications	O
using	O
the	O
TEST	O
MESSAGE	O
and	O
TEST	O
COMPLETION	O
methods	O
.	O
</s>
<s>
IUCV	B-Application
methods	O
are	O
invoked	O
by	O
using	O
the	O
'	O
B2F0'x	O
instruction	O
.	O
</s>
<s>
The	O
instruction	O
is	O
then	O
interpreted	O
by	O
CP	O
as	O
an	O
IUCV	B-Application
request	O
.	O
</s>
<s>
IPARML	O
is	O
the	O
Iucv	B-Application
PARaMeter	O
List	O
.	O
</s>
<s>
CP	O
has	O
a	O
specific	O
macro	O
(	O
IUCV	B-Application
)	O
which	O
generates	O
the	O
appropriate	O
code	O
-	O
including	O
the	O
instruction	O
and	O
filling	O
in	O
the	O
IPARML	O
-	O
so	O
that	O
the	O
details	O
regarding	O
the	O
parameters	O
about	O
such	O
and	O
such	O
method	O
can	O
be	O
defaulted	O
or	O
checked	O
for	O
conflict	O
.	O
</s>
<s>
CMS	O
can	O
be	O
made	O
to	O
handle	O
IUCV	B-Application
application	O
requests	O
.	O
</s>
<s>
CMS	O
has	O
its	O
own	O
set	O
of	O
IUCV	B-Application
macros	O
(	O
CMSIUCV	O
)	O
which	O
allow	O
multiple	O
applications	O
to	O
share	O
the	O
IUCV	B-Application
facility	O
within	O
a	O
virtual	O
machine	O
.	O
</s>
<s>
Access	O
to	O
some	O
IUCV	B-Application
functions	O
is	O
controlled	O
by	O
statements	O
in	O
the	O
CP	O
Directory	O
(	O
the	O
list	O
of	O
virtual	O
machines	O
and	O
their	O
specifications	O
)	O
.	O
</s>
<s>
A	O
virtual	O
machine	O
can	O
be	O
permitted	O
to	O
accept	O
communications	O
from	O
all	O
other	O
virtual	O
machines	O
via	O
the	O
IUCV	B-Application
ALLOW	O
directory	O
statement	O
,	O
or	O
establish	O
a	O
communication	O
path	O
with	O
any	O
other	O
virtual	O
machine	O
via	O
the	O
IUCV	B-Application
ANY	O
statement	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
allow	O
a	O
virtual	O
machine	O
to	O
issue	O
path	O
connection	O
requests	O
to	O
other	O
specific	O
virtual	O
machines	O
by	O
specifying	O
the	O
virtual	O
machine	O
name	O
in	O
an	O
IUCV	B-Application
statement	O
,	O
for	O
example	O
:	O
IUCV	B-Application
TARGETVM	O
.	O
</s>
<s>
The	O
IUCV	B-Application
statement	O
controls	O
CP-imposed	O
access	O
control	O
for	O
IUCV	B-Application
connections	O
.	O
</s>
<s>
The	O
CP	O
system	O
services	O
are	O
IUCV	B-Application
end	O
points	O
which	O
are	O
not	O
virtual	O
machines	O
by	O
themselves	O
,	O
but	O
allow	O
a	O
virtual	O
machine	O
to	O
perform	O
hypervisor	O
functions	O
asynchronously	O
or	O
to	O
access	O
specific	O
hypervisor	O
facilities	O
.	O
</s>
<s>
Allows	O
a	O
virtual	O
machine	O
to	O
receive	O
through	O
IUCV	B-Application
specific	O
virtual	O
machine	O
console	O
outputs	O
such	O
as	O
the	O
results	O
of	O
the	O
'	O
CP	O
MESSAGE	O
 '	O
or	O
Console	O
I/O	O
.	O
</s>
<s>
This	O
is	O
used	O
by	O
VM	B-Application
subsystems	O
such	O
as	O
PROP	O
(	O
The	O
PRogrammable	O
OPerator	O
)	O
or	O
Fullscreen	O
CMS	O
.	O
</s>
<s>
RSCS	B-Application
(	O
The	O
Remote	O
Spool	O
Communication	O
Subsystem	O
)	O
is	O
an	O
example	O
of	O
an	O
application	O
that	O
uses	O
this	O
system	O
service	O
.	O
</s>
<s>
GCS	B-Application
(	O
The	O
Group	B-Application
Control	I-Application
System	I-Application
)	O
of	O
VM	B-Application
uses	O
IUCV	B-Application
to	O
perform	O
maintenance	O
of	O
shared	O
memory	O
areas	O
between	O
virtual	O
machines	O
.	O
</s>
<s>
By	O
using	O
implicit	O
path	O
termination	O
,	O
the	O
GCS	B-Application
recovery	O
virtual	O
machine	O
can	O
ensure	O
that	O
any	O
locks	O
held	O
on	O
the	O
shared	O
area	O
by	O
a	O
virtual	O
machine	O
that	O
entered	O
the	O
group	O
but	O
left	O
unexpectedly	O
is	O
properly	O
released	O
.	O
</s>
<s>
VM	B-Application
TCP/IP	B-Protocol
-	O
the	O
TCP/IP	B-Protocol
stack	I-Protocol
for	O
VM	B-Application
-	O
uses	O
IUCV	B-Application
to	O
either	O
allow	O
a	O
virtual	O
machine	O
to	O
perform	O
socket	O
operations	O
or	O
to	O
allow	O
a	O
virtual	O
machine	O
to	O
act	O
as	O
a	O
network	O
interface	O
to	O
pass	O
whole	O
frames	O
or	O
datagrams	O
between	O
itself	O
and	O
the	O
TCP/IP	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
The	O
S/390	B-Device
and	O
z/Architecture	B-Device
implementation	O
of	O
Linux	B-Application
uses	O
this	O
facility	O
to	O
implement	O
a	O
network	O
interface	O
to	O
the	O
VM	B-Application
TCP/IP	B-Protocol
stack	I-Protocol
.	O
</s>
