<s>
An	O
access	B-Application
method	I-Application
is	O
a	O
function	O
of	O
a	O
mainframe	B-Architecture
operating	B-General_Concept
system	I-General_Concept
that	O
enables	O
access	O
to	O
data	O
on	O
disk	O
,	O
tape	O
or	O
other	O
external	O
devices	O
.	O
</s>
<s>
Access	B-Application
methods	I-Application
were	O
present	O
in	O
several	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
since	O
the	O
late	O
1950s	O
,	O
under	O
a	O
variety	O
of	O
names	O
;	O
the	O
name	O
access	B-Application
method	I-Application
was	O
introduced	O
in	O
1963	O
in	O
the	O
IBM	O
OS/360	B-Application
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Access	B-Application
methods	I-Application
provide	O
an	O
application	O
programming	O
interface	O
(	O
API	B-General_Concept
)	O
for	O
programmers	O
to	O
transfer	O
data	O
to	O
or	O
from	O
device	O
,	O
and	O
could	O
be	O
compared	O
to	O
device	B-Application
drivers	I-Application
in	O
non-mainframe	O
operating	B-General_Concept
systems	I-General_Concept
,	O
but	O
typically	O
provide	O
a	O
greater	O
level	O
of	O
functionality	O
.	O
</s>
<s>
System/360	O
and	O
successor	O
systems	O
perform	O
input/output	O
using	O
a	O
special	O
program	O
for	O
an	O
I/O	B-Device
channel	I-Device
,	O
a	O
processor	O
dedicated	O
to	O
control	O
peripheral	O
storage	O
device	O
access	O
and	O
data	O
transfer	O
to	O
and	O
from	O
main	O
memory	O
.	O
</s>
<s>
Channel	O
programs	O
are	O
initiated	O
by	O
a	O
START	O
IO	O
instruction	O
issued	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
This	O
is	O
usually	O
front	O
ended	O
by	O
the	O
Execute	B-Application
Channel	I-Application
Program	I-Application
(	O
EXCP	B-Application
)	O
macro	O
for	O
application	O
programmer	O
convenience	O
.	O
</s>
<s>
EXCP	B-Application
issues	O
an	O
SVC	O
(	O
supervisor	B-Operating_System
call	I-Operating_System
instruction	I-Operating_System
)	O
that	O
directs	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
issue	O
the	O
START	O
IO	O
on	O
the	O
application	O
's	O
behalf	O
.	O
</s>
<s>
Access	B-Application
methods	I-Application
provide	O
:	O
</s>
<s>
A	O
program	O
designed	O
to	O
process	O
a	O
sequence	O
of	O
80-character	O
records	B-General_Concept
would	O
work	O
no	O
matter	O
where	O
the	O
data	O
are	O
stored	O
.	O
</s>
<s>
Ease	O
of	O
hardware	O
replacement	O
-	O
programmer	O
would	O
no	O
longer	O
alter	O
a	O
program	O
when	O
data	O
should	O
be	O
migrated	O
to	O
newer	O
model	O
of	O
storage	O
device	O
,	O
provided	O
it	O
supports	O
the	O
same	O
access	B-Application
methods	I-Application
.	O
</s>
<s>
Ease	O
shared	O
data	B-Application
set	I-Application
access	O
-	O
an	O
access	B-Application
method	I-Application
is	O
a	O
trusted	O
program	O
,	O
that	O
allows	O
multiple	O
programs	O
to	O
access	O
the	O
same	O
file	O
,	O
while	O
ensuring	O
the	O
basic	O
data	O
integrity	O
and	O
system	O
security	O
.	O
</s>
<s>
Read-ahead	O
-	O
Queued	O
access	B-Application
methods	I-Application
may	O
start	O
as	O
many	O
I/O	O
operations	O
as	O
there	O
are	O
buffers	B-General_Concept
available	O
,	O
anticipating	O
application	O
program	O
requirements	O
.	O
</s>
<s>
Unlike	O
systems	O
derived	O
from	O
Unix	B-Application
,	O
where	O
all	O
files	O
and	O
devices	O
are	O
considered	O
to	O
be	O
an	O
unformatted	O
stream	O
of	O
bytes	B-Application
,	O
mainframes	B-Architecture
offer	O
a	O
variety	O
of	O
data	O
options	O
and	O
formats	O
,	O
such	O
as	O
varying	O
types	O
and	O
sizes	O
of	O
records	B-General_Concept
,	O
and	O
different	O
ways	O
of	O
accessing	O
data	O
,	O
such	O
as	O
via	O
record	O
keys	O
.	O
</s>
<s>
Access	B-Application
methods	I-Application
provide	O
programs	O
a	O
way	O
of	O
dealing	O
with	O
this	O
complexity	O
.	O
</s>
<s>
Programs	O
can	O
read	O
or	O
write	O
a	O
record	O
or	O
block	O
of	O
data	O
and	O
wait	O
until	O
the	O
input/output	O
operation	O
is	O
complete	O
(	O
queued	O
access	B-Application
methods	I-Application
)	O
or	O
allow	O
the	O
operation	O
to	O
be	O
started	O
and	O
the	O
program	O
to	O
continue	O
to	O
run	O
,	O
waiting	O
for	O
the	O
completion	O
at	O
a	O
later	O
time	O
(	O
basic	O
access	B-Application
methods	I-Application
)	O
.	O
</s>
<s>
Programs	O
can	O
specify	O
the	O
size	O
and	O
number	O
of	O
buffers	B-General_Concept
for	O
a	O
file	O
.	O
</s>
<s>
The	O
same	O
buffer	B-General_Concept
or	O
pool	O
can	O
be	O
used	O
for	O
multiple	O
files	O
,	O
allowing	O
blocks	O
of	O
data	O
to	O
be	O
read	O
from	O
one	O
file	O
and	O
written	O
to	O
another	O
without	O
requiring	O
data	O
movement	O
in	O
memory	O
.	O
</s>
<s>
Storage-oriented	O
access	B-Application
methods	I-Application
in	O
approximate	O
chronological	O
order	O
:	O
</s>
<s>
Distributed	B-Operating_System
Data	I-Operating_System
Management	I-Operating_System
Architecture	I-Operating_System
-	O
access	B-Application
methods	I-Application
for	O
distributed	O
file	O
access	O
.	O
</s>
<s>
Both	O
types	O
of	O
access	O
deal	O
with	O
records	B-General_Concept
of	O
a	O
data	B-Application
set	I-Application
.	O
</s>
<s>
Basic	O
access	B-Application
methods	I-Application
read	O
or	O
write	O
one	O
physical	O
record	O
block	O
at	O
a	O
time	O
.	O
</s>
<s>
Queued	O
methods	O
support	O
internal	O
blocking	B-General_Concept
of	O
data	O
and	O
also	O
often	O
read-ahead	O
scheme	O
.	O
</s>
<s>
Queued	O
access	B-Application
methods	I-Application
generally	O
provide	O
better	O
performance	O
,	O
while	O
basic	O
methods	O
provide	O
more	O
flexibility	O
.	O
</s>
<s>
Sequential	O
access	O
assumes	O
that	O
records	B-General_Concept
can	O
be	O
processed	O
only	O
sequentially	O
,	O
as	O
opposed	O
to	O
direct	O
(	O
or	O
random	O
)	O
access	O
.	O
</s>
<s>
Some	O
devices	O
,	O
such	O
as	O
magnetic	B-Architecture
tape	I-Architecture
,	O
naturally	O
enforce	O
sequential	O
access	O
,	O
but	O
it	O
can	O
be	O
used	O
as	O
well	O
on	O
direct	B-Application
access	I-Application
storage	I-Application
devices	I-Application
(	O
DASD	B-Application
)	O
,	O
such	O
as	O
disk	O
drives	O
.	O
</s>
<s>
In	O
the	O
latter	O
case	O
,	O
a	O
data	B-Application
set	I-Application
written	O
with	O
sequential	O
access	O
can	O
be	O
later	O
processed	O
in	O
a	O
direct	O
manner	O
.	O
</s>
<s>
Network-oriented	O
access	B-Application
methods	I-Application
in	O
approximate	O
chronological	O
order	O
:	O
</s>
<s>
The	O
IBM	B-Application
Information	I-Application
Management	I-Application
System	I-Application
(	O
IMS	O
)	O
uses	O
the	O
term	O
"	O
access	B-Application
method	I-Application
"	O
to	O
refer	O
to	O
its	O
methods	O
for	O
manipulating	O
"	O
segments	O
in	O
a	O
database	O
record	O
"	O
.	O
</s>
<s>
Generalized	O
Sequential	O
Access	B-Application
Method	I-Application
(	O
GSAM	O
)	O
,	O
</s>
<s>
Hierarchical	O
Direct	O
Access	B-Application
Method	I-Application
(	O
HDAM	O
)	O
,	O
</s>
<s>
Hierarchical	O
Indexed	O
Direct	O
Access	B-Application
Method	I-Application
(	O
HIDAM	O
)	O
,	O
</s>
<s>
Hierarchical	O
Indexed	B-Application
Sequential	I-Application
Access	I-Application
Method	I-Application
(	O
HISAM	O
)	O
,	O
</s>
<s>
Hierarchical	O
Sequential	O
Access	B-Application
Method	I-Application
(	O
HSAM	O
)	O
,	O
</s>
<s>
Overflow	O
sequential	O
access	B-Application
method	I-Application
(	O
OSAM	O
)	O
,	O
</s>
<s>
Partitioned	O
Hierarchical	O
Direct	O
Access	B-Application
Method	I-Application
(	O
PHDAM	O
)	O
,	O
</s>
<s>
Partitioned	O
Hierarchical	O
Indexed	O
Direct	O
Access	B-Application
Method	I-Application
(	O
PHIDAM	O
)	O
,	O
</s>
<s>
Simple	O
Hierarchical	O
Indexed	B-Application
Sequential	I-Application
Access	I-Application
Method	I-Application
(	O
SHISAM	O
)	O
.	O
</s>
<s>
This	O
is	O
a	O
different	O
use	O
of	O
the	O
term	O
from	O
the	O
other	O
access	B-Application
methods	I-Application
mentioned	O
in	O
this	O
article	O
.	O
</s>
<s>
In	O
the	O
z/OS	B-Application
operating	B-General_Concept
system	I-General_Concept
,	O
two	O
elements	O
provide	O
access	B-Application
methods	I-Application
:	O
</s>
