<s>
In	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
OS/360	B-Application
and	I-Application
its	I-Application
successors	I-Application
,	O
a	O
Unit	B-Application
Control	I-Application
Block	I-Application
(	O
UCB	O
)	O
is	O
a	O
memory	B-Language
structure	I-Language
,	O
or	O
a	O
control	O
block	O
,	O
that	O
describes	O
any	O
single	O
input/output	B-General_Concept
peripheral	B-Device
device	I-Device
(	O
unit	O
)	O
,	O
or	O
an	O
exposure	O
(	O
alias	O
)	O
,	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Certain	O
data	O
within	O
the	O
UCB	O
also	O
instructs	O
the	O
Input/Output	B-General_Concept
Supervisor	O
(	O
IOS	O
)	O
to	O
use	O
certain	O
closed	O
subroutines	O
in	O
addition	O
to	O
normal	O
IOS	O
processing	O
for	O
additional	O
physical	B-Device
device	I-Device
control	O
.	O
</s>
<s>
Some	O
other	O
operating	B-General_Concept
systems	I-General_Concept
have	O
similar	O
structures	O
.	O
</s>
<s>
During	O
initial	O
program	O
load	O
(	O
IPL	O
)	O
of	O
current	O
MVS	B-Application
systems	O
,	O
the	O
Nucleus	O
Initialization	O
Program	O
(	O
NIP	O
)	O
reads	O
necessary	O
information	O
from	O
the	O
I/O	B-General_Concept
Definition	O
File	O
(	O
IODF	O
)	O
and	O
uses	O
it	O
to	O
build	O
the	O
UCBs	O
.	O
</s>
<s>
The	O
UCBs	O
are	O
stored	O
in	O
system-owned	O
memory	O
,	O
in	O
the	O
Extended	B-Application
System	I-Application
Queue	I-Application
Area	I-Application
(	O
ESQA	O
)	O
.	O
</s>
<s>
After	O
IPL	O
completes	O
,	O
UCBs	O
are	O
owned	O
by	O
Input/Output	B-General_Concept
Support	O
.	O
</s>
<s>
disk	O
,	O
tape	O
,	O
printer	O
,	O
terminal	O
)	O
,	O
address	O
of	O
the	O
device	O
(	O
such	O
as	O
1002	O
)	O
,	O
subchannel	O
identifier	O
and	O
device	O
number	O
,	O
channel	O
path	O
ID	O
(	O
CHPID	O
)	O
which	O
defines	O
the	O
path	O
to	O
the	O
device	O
,	O
for	O
some	O
devices	O
the	O
volume	B-Application
serial	I-Application
number	I-Application
(	O
VOLSER	O
)	O
,	O
and	O
a	O
large	O
amount	O
of	O
other	O
information	O
,	O
including	O
OS	O
Job	O
Management	O
data	O
.	O
</s>
<s>
While	O
the	O
contents	O
of	O
the	O
UCB	O
has	O
changed	O
as	O
MVS	B-Application
evolved	O
,	O
the	O
concept	O
has	O
not	O
.	O
</s>
<s>
It	O
is	O
a	O
representation	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
an	O
external	B-Device
device	I-Device
.	O
</s>
<s>
In	O
the	O
earliest	O
implementations	O
of	O
OS/360	B-Application
,	O
the	O
UCBs	O
(	O
foundations	O
and	O
extensions	O
)	O
were	O
assembled	O
during	O
SYSGEN	O
,	O
and	O
were	O
located	O
within	O
the	O
first	O
64KB	O
of	O
the	O
system	O
area	O
,	O
as	O
the	O
I/O	B-General_Concept
device	I-General_Concept
lookup	O
table	O
consisted	O
of	O
16-bit	O
addresses	O
.	O
</s>
<s>
UCBs	O
were	O
introduced	O
in	O
the	O
1960s	O
with	O
OS/360	B-Application
.	O
</s>
<s>
Then	O
a	O
device	O
addressed	O
by	O
UCB	O
was	O
typically	O
a	O
moving	O
head	O
hard	B-Device
disk	I-Device
drive	I-Device
or	O
a	O
tape	O
drive	O
,	O
with	O
no	O
internal	O
cache	B-Device
.	O
</s>
<s>
Without	O
it	O
,	O
the	O
device	O
was	O
usually	O
grossly	O
outperformed	O
by	O
the	O
mainframe	O
's	O
channel	B-Device
processor	I-Device
.	O
</s>
<s>
Hence	O
,	O
there	O
was	O
no	O
reason	O
to	O
execute	O
multiple	O
input/output	B-General_Concept
operations	O
to	O
it	O
at	O
the	O
same	O
time	O
,	O
as	O
these	O
would	O
be	O
impossible	O
for	O
a	O
device	O
to	O
physically	O
handle	O
.	O
</s>
<s>
In	O
1968	O
,	O
IBM	O
introduced	O
the	O
2305-1	O
and	O
2305-2	O
fixed-head	O
disks	O
,	O
which	O
had	O
rotational	O
position	O
sensing	O
(	O
RPS	O
)	O
and	O
eight	O
exposures	O
(	O
alias	O
addresses	O
)	O
per	O
disk	O
;	O
the	O
OS/360	B-Application
support	O
provided	O
a	O
UCB	O
per	O
exposure	O
in	O
order	O
to	O
permit	O
multiple	O
concurrent	O
channel	O
programs	O
.	O
</s>
<s>
Similarly	O
,	O
later	O
systems	O
derived	O
from	O
OS/360	B-Application
required	O
an	O
additional	O
UCB	O
for	O
each	O
allocated	O
virtual	O
volume	O
in	O
a	O
IBM	B-Device
3850	I-Device
Mass	I-Device
Storage	I-Device
System	I-Device
(	O
MSS	O
)	O
,	O
and	O
for	O
each	O
exposure	O
on	O
a	O
3880-11	O
and	O
its	O
successors	O
.	O
</s>
<s>
Since	O
only	O
one	O
set	O
of	O
channel	O
commands	O
or	O
I/O	B-General_Concept
could	O
be	O
run	O
at	O
one	O
time	O
.	O
</s>
<s>
This	O
was	O
fine	O
in	O
the	O
1960s	O
when	O
CPUs	O
were	O
slow	O
and	O
I/O	B-General_Concept
could	O
only	O
be	O
processed	O
as	O
fast	O
as	O
CPUs	O
could	O
process	O
it	O
.	O
</s>
<s>
As	O
systems	O
matured	O
and	O
CPU	O
speed	O
greatly	O
surpassed	O
I/O	B-General_Concept
input	O
capacity	O
,	O
access	O
to	O
the	O
device	O
that	O
was	O
serialized	O
at	O
the	O
UCB	O
level	O
became	O
a	O
serious	O
bottleneck	O
.	O
</s>
<s>
Parallel	O
Access	O
Volume	O
(	O
PAV	O
)	O
allow	O
UCBs	O
to	O
clone	O
themselves	O
to	O
allow	O
multiple	O
I/O	B-General_Concept
to	O
run	O
simultaneously	O
.	O
</s>
<s>
With	O
appropriate	O
support	O
by	O
the	O
DASD	O
hardware	O
,	O
PAV	O
provides	O
support	O
for	O
more	O
than	O
one	O
I/O	B-General_Concept
to	O
a	O
single	O
device	O
at	O
a	O
time	O
.	O
</s>
<s>
To	O
maintain	O
backward	B-General_Concept
compatibility	I-General_Concept
,	O
operations	O
are	O
still	O
serialized	O
below	O
the	O
UCB	O
level	O
.	O
</s>
<s>
In	O
the	O
second	O
version	O
of	O
PAV	O
processing	O
,	O
Workload	B-Application
Manager	I-Application
(	O
WLM	O
)	O
reassigns	O
a	O
PAV	O
to	O
new	O
UCBs	O
from	O
time	O
to	O
time	O
.	O
</s>
<s>
In	O
the	O
third	O
version	O
of	O
PAV	O
processing	O
,	O
with	O
the	O
IBM	B-Device
DS8000	I-Device
series	I-Device
,	O
each	O
I/O	B-General_Concept
uses	O
any	O
available	O
PAV	O
with	O
the	O
UCB	O
it	O
needs	O
.	O
</s>
<s>
A	O
static	O
alias	O
address	O
is	O
defined	O
,	O
in	O
both	O
DASD	O
hardware	O
and	O
z/OS	B-Application
,	O
to	O
refer	O
to	O
a	O
specific	O
single	O
base	O
address	O
.	O
</s>
<s>
The	O
management	O
of	O
these	O
dynamic	O
aliases	O
is	O
left	O
to	O
WLM	O
,	O
running	O
in	O
goal	O
mode	O
(	O
which	O
is	O
always	O
the	O
case	O
with	O
supported	O
levels	O
of	O
z/OS	B-Application
)	O
.	O
</s>
<s>
As	O
WLM	O
watches	O
over	O
the	O
I/O	B-General_Concept
activity	O
in	O
the	O
system	O
,	O
WLM	O
determines	O
if	O
there	O
a	O
high-importance	O
workload	O
is	O
delayed	O
due	O
to	O
high	O
contention	O
for	O
a	O
specific	O
PAV-enabled	O
device	O
.	O
</s>
<s>
HyperPAVs	O
are	O
significantly	O
more	O
responsive	O
because	O
they	O
acquire	O
a	O
UCB	O
from	O
a	O
pool	O
for	O
the	O
duration	O
of	O
a	O
single	O
I/O	B-General_Concept
operation	I-General_Concept
,	O
before	O
returning	O
it	O
to	O
the	O
pool	O
.	O
</s>
<s>
Digital	O
's	O
VMS	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
uses	O
an	O
identically	O
named	O
structure	O
,	O
the	O
UCB	O
,	O
for	O
similar	O
purposes	O
.	O
</s>
<s>
A	O
UCB	O
is	O
created	O
for	O
each	O
I/O	B-General_Concept
device	I-General_Concept
.	O
</s>
<s>
The	O
data	O
in	O
the	O
UCB	O
includes	O
the	O
device	O
's	O
unit	O
number	O
(	O
a	O
part	O
of	O
the	O
device	O
name	O
)	O
and	O
a	O
listhead	O
to	O
which	O
pending	O
I/O	B-General_Concept
requests	O
may	O
be	O
queued	O
.	O
</s>
