<s>
In	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
,	O
Execute	B-Application
Direct	I-Application
Access	I-Application
Program	I-Application
(	O
XDAP	B-Application
)	O
is	O
a	O
pseudo	O
access	O
method	O
for	O
accessing	O
direct	O
access	O
datasets	O
on	O
a	O
block	O
by	O
block	O
basis	O
.	O
</s>
<s>
XDAP	B-Application
is	O
more	O
specifically	O
described	O
in	O
the	O
OS	O
System	O
Programmer	O
's	O
Guide	O
.	O
</s>
<s>
The	O
dataset	O
may	O
not	O
be	O
created	O
nor	O
extended	O
using	O
XDAP	B-Application
;	O
for	O
that	O
purpose	O
a	O
true	O
access	O
method	O
is	O
required	O
.	O
</s>
<s>
XDAP	B-Application
may	O
coexist	O
with	O
BSAM	O
or	O
BPAM	O
,	O
and	O
several	O
OS	O
components	O
are	O
implemented	O
in	O
this	O
way	O
.	O
</s>
<s>
List	O
and	O
Execute	O
forms	O
of	O
XDAP	B-Application
are	O
supported	O
,	O
whereby	O
a	O
prototype	O
form	O
,	O
called	O
the	O
List	O
Form	O
,	O
is	O
expanded	O
as	O
data	O
,	O
and	O
a	O
functional	O
form	O
,	O
called	O
the	O
Execute	O
Form	O
,	O
is	O
expanded	O
as	O
instructions	O
.	O
</s>
<s>
XDAP	B-Application
is	O
also	O
supported	O
conventionally	O
,	O
in	O
which	O
case	O
the	O
two	O
forms	O
are	O
combined	O
and	O
is	O
expanded	O
as	O
inline	O
data	O
and	O
instructions	O
.	O
</s>
<s>
XDAP	B-Application
results	O
in	O
the	O
EXCP	B-Application
supervisor	O
call	O
being	O
executed	O
,	O
hence	O
Execute	B-Application
Channel	I-Application
Program	I-Application
(	O
EXCP	B-Application
)	O
is	O
implicitly	O
embedded	O
within	O
XDAP	B-Application
.	O
</s>
<s>
The	O
IOB	O
,	O
which	O
is	O
the	O
sole	O
parameter	O
to	O
EXCP	B-Application
,	O
is	O
included	O
as	O
a	O
data	O
structure	O
within	O
the	O
XDAP	B-Application
macro	O
expansion	O
as	O
are	O
the	O
channel	O
command	O
words	O
.	O
</s>
<s>
XDAP	B-Application
must	O
reference	O
an	O
OPENed	O
DCB	O
and	O
a	O
full	O
direct	O
access	O
identifier	O
.	O
</s>
<s>
The	O
DCB	O
may	O
be	O
OPENed	O
for	O
input	O
,	O
for	O
output	O
or	O
for	O
input	O
and	O
output	O
(	O
"	O
INOUT	O
"	O
)	O
,	O
but	O
all	O
outputs	O
must	O
be	O
update	O
writes	O
of	O
existing	O
blocks	O
as	O
XDAP	B-Application
does	O
not	O
support	O
format	O
writes	O
.	O
</s>
<s>
This	O
enhancement	O
,	O
which	O
was	O
first	O
implemented	O
with	O
SVS	O
,	O
but	O
was	O
not	O
retrofitted	O
to	O
previous	O
versions	O
of	O
the	O
OS	O
,	O
significantly	O
facilitated	O
processing	O
a	O
dataset	O
already	O
OPENed	O
for	O
BSAM	O
or	O
BPAM	O
,	O
while	O
using	O
XDAP	B-Application
.	O
</s>
<s>
Otherwise	O
,	O
certain	O
pre-SVS	O
SAM	O
and	O
PAM	O
appendages	O
could	O
cause	O
random	O
supervisor	O
storage	O
overlays	O
when	O
presented	O
with	O
certain	O
XDAP	B-Application
channel	O
programs	O
.	O
</s>
<s>
As	O
IBM	O
did	O
not	O
guarantee	O
system	O
security	O
and	O
data	O
integrity	O
on	O
pre-SVS	O
systems	O
,	O
an	O
obvious	O
circumvention	O
was	O
to	O
utilize	O
only	O
those	O
XDAP	B-Application
channel	O
programs	O
which	O
did	O
not	O
also	O
utilize	O
RPS	O
as	O
it	O
was	O
the	O
RPS-type	O
XDAP	B-Application
channel	O
programs	O
which	O
were	O
confusing	O
the	O
SAM	O
and	O
PAM	O
appendages	O
,	O
thereby	O
leading	O
to	O
random	O
supervisor	O
storage	O
overlays	O
.	O
</s>
