<s>
In	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
OS/360	B-Application
,	O
MVS	B-Application
,	O
z/OS	B-Application
,	O
a	O
Data	B-Application
Control	I-Application
Block	I-Application
(	O
DCB	O
)	O
is	O
a	O
description	O
of	O
a	O
dataset	B-Application
in	O
a	O
program	O
.	O
</s>
<s>
A	O
DCB	O
is	O
coded	O
in	O
Assembler	B-Language
programs	I-Language
using	O
the	O
DCB	O
macro	O
instruction	O
(	O
which	O
expands	O
into	O
a	O
large	O
number	O
of	O
"	O
define	O
constant	O
"	O
instructions	O
)	O
.	O
</s>
<s>
High	O
level	O
language	O
programmers	O
use	O
library	B-Library
routines	I-Library
containing	O
DCBs	O
.	O
</s>
<s>
A	O
DCB	O
is	O
one	O
of	O
the	O
many	O
control	O
blocks	O
used	O
in	O
these	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
A	O
DCB	O
may	O
be	O
compared	O
to	O
a	O
FILE	O
structure	O
in	O
C	O
,	O
but	O
it	O
is	O
much	O
more	O
complex	O
,	O
offering	O
many	O
more	O
options	O
for	O
various	O
access	B-Application
methods	I-Application
.	O
</s>
<s>
The	O
control	O
block	O
acted	O
as	O
the	O
Application	B-Application
programming	I-Application
interface	I-Application
between	O
Logical	O
IOCS	B-Application
and	O
the	O
application	B-Application
program	I-Application
and	O
usually	O
was	O
defined	O
within	O
(	O
and	O
resided	O
within	O
)	O
the	O
application	B-Application
program	I-Application
itself	O
.	O
</s>
<s>
The	O
addresses	O
of	O
I/O	B-General_Concept
subroutines	O
would	O
be	O
resolved	O
during	O
a	O
linkedit	B-Application
phase	O
after	O
compilation	O
or	O
else	O
dynamically	O
inserted	O
at	O
OPEN	O
time	O
.	O
</s>
<s>
address	O
of	O
I/O	B-General_Concept
operating	B-General_Concept
system	I-General_Concept
library	B-Library
subroutines	I-Library
(	O
e.g.	O
</s>
<s>
The	O
OPEN	O
process	O
results	O
in	O
a	O
merge	O
of	O
the	O
constants	O
and	O
variables	O
specified	O
in	O
the	O
DD	O
JCL	B-Application
statement	O
,	O
and	O
the	O
dataset	B-Application
label	O
for	O
existing	O
magnetic	O
tape	O
and	O
direct-access	O
datasets	O
,	O
into	O
the	O
DCB	O
,	O
replacing	O
the	O
zero	O
values	O
with	O
actual	O
,	O
non-zero	O
values	O
.	O
</s>
<s>
A	O
control	O
block	O
called	O
the	O
JFCB	O
(	O
Job	O
File	O
Control	O
Block	O
)	O
initially	O
holds	O
the	O
information	O
extracted	O
from	O
the	O
DD	O
statement	O
for	O
the	O
dataset	B-Application
.	O
</s>
<s>
The	O
results	O
of	O
the	O
merge	O
are	O
stored	O
in	O
the	O
JFCB	O
which	O
may	O
also	O
be	O
written	O
into	O
the	O
DSCB	O
during	O
the	O
CLOSE	O
process	O
,	O
thereby	O
making	O
the	O
dataset	B-Application
definition	O
permanent	O
.	O
</s>
<s>
In	O
the	O
DD	O
statement	O
,	O
the	O
BLKSIZE	O
is	O
specified	O
as	O
a	O
non-zero	O
value	O
and	O
this	O
,	O
then	O
,	O
results	O
in	O
a	O
program-specified	O
LRECL	O
(	O
logical	O
record	O
length	O
)	O
and	O
a	O
JCL-specified	O
BLKSIZE	O
(	O
physical	O
block	O
size	O
)	O
,	O
with	O
the	O
merge	O
of	O
the	O
two	O
becoming	O
the	O
permanent	O
definition	O
of	O
the	O
dataset	B-Application
.	O
</s>
