<s>
A	O
direct-access	B-Application
storage	I-Application
device	I-Application
(	O
DASD	B-Application
)	O
(	O
pronounced	O
)	O
is	O
a	O
secondary	O
storage	O
device	O
in	O
which	O
"	O
each	O
physical	O
record	O
has	O
a	O
discrete	O
location	O
and	O
a	O
unique	O
address	O
"	O
.	O
</s>
<s>
The	O
term	O
was	O
coined	O
by	O
IBM	O
to	O
describe	O
devices	O
that	O
allowed	O
random	B-General_Concept
access	I-General_Concept
to	O
data	O
,	O
the	O
main	O
examples	O
being	O
drum	B-General_Concept
memory	I-General_Concept
and	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
Later	O
,	O
optical	O
disc	O
drives	O
and	O
flash	B-Device
memory	I-Device
units	O
are	O
also	O
classified	O
as	O
DASD	B-Application
.	O
</s>
<s>
The	O
term	O
DASD	B-Application
contrasts	O
with	O
sequential	B-General_Concept
access	I-General_Concept
storage	O
device	O
such	O
as	O
a	O
magnetic	O
tape	O
drive	O
,	O
and	O
unit	O
record	O
equipment	O
such	O
as	O
a	O
punched	B-Architecture
card	I-Architecture
device	O
.	O
</s>
<s>
A	O
record	O
on	O
a	O
DASD	B-Application
can	O
be	O
accessed	O
without	O
having	O
to	O
read	O
through	O
intervening	O
records	O
from	O
the	O
current	O
location	O
,	O
whereas	O
reading	O
anything	O
other	O
than	O
the	O
"	O
next	O
"	O
record	O
on	O
tape	O
or	O
deck	O
of	O
cards	O
requires	O
skipping	O
over	O
intervening	O
records	O
,	O
and	O
requires	O
a	O
proportionally	O
long	O
time	O
to	O
access	O
a	O
distant	O
point	O
in	O
a	O
medium	O
.	O
</s>
<s>
Access	B-Application
methods	I-Application
for	O
DASD	B-Application
include	O
sequential	O
,	O
partitioned	O
,	O
indexed	B-Application
,	O
and	O
direct	B-Application
.	O
</s>
<s>
The	O
DASD	B-Application
storage	O
class	O
includes	O
both	O
fixed	O
and	O
removable	O
media	O
.	O
</s>
<s>
IBM	O
mainframes	O
access	O
I/O	O
devices	O
including	O
DASD	B-Application
through	O
channels	B-Device
,	O
a	O
type	O
of	O
subordinate	O
mini-processor	O
.	O
</s>
<s>
The	O
operating	O
system	O
uses	O
a	O
four	O
byte	O
relative	O
track	O
and	O
record	O
(	O
TTR	O
)	O
for	O
some	O
access	B-Application
methods	I-Application
and	O
for	O
others	O
an	O
eight-byte	O
extent-bin-cylinder-track-record	O
block	O
address	O
,	O
or	O
MBBCCHHR	O
,	O
Channel	O
programs	O
address	O
DASD	B-Application
using	O
a	O
six	O
byte	O
seek	O
address	O
(	O
BBCCHH	O
)	O
and	O
a	O
five	O
byte	O
record	O
identifier	O
(	O
CCHHR	O
)	O
.	O
</s>
<s>
BB	O
representing	O
the	O
Bin	O
(	O
from	O
2321	B-Device
data	I-Device
cells	I-Device
)	O
,	O
</s>
<s>
When	O
the	O
2321	O
data	B-Device
cell	I-Device
was	O
discontinued	O
in	O
January	O
1975	O
,	O
the	O
addressing	O
scheme	O
and	O
the	O
device	O
itself	O
was	O
referred	O
to	O
as	O
CHR	O
or	O
CTR	O
for	O
cylinder-track-record	O
,	O
as	O
the	O
bin	O
number	O
was	O
always	O
0	O
.	O
</s>
<s>
IBM	O
refers	O
to	O
the	O
data	O
records	O
programmers	O
work	O
with	O
as	O
logical	O
records	O
,	O
and	O
the	O
format	O
on	O
DASD	B-Application
as	O
blocks	B-General_Concept
or	O
physical	O
records	O
.	O
</s>
<s>
The	O
queued	O
access	B-Application
methods	I-Application
,	O
such	O
as	O
QSAM	B-Application
,	O
are	O
responsible	O
for	O
blocking	O
and	O
deblocking	O
logical	O
records	O
as	O
they	O
are	O
written	O
to	O
or	O
read	O
from	O
external	O
media	O
.	O
</s>
<s>
The	O
basic	O
access	B-Application
methods	I-Application
,	O
such	O
as	O
BSAM	O
,	O
require	O
the	O
user	O
program	O
to	O
do	O
it	O
.	O
</s>
<s>
CKD	O
is	O
an	O
acronym	O
for	O
Count	B-Application
Key	I-Application
Data	I-Application
,	O
the	O
physical	O
layout	O
of	O
a	O
block	O
on	O
a	O
DASD	B-Application
device	O
,	O
and	O
should	O
not	O
be	O
confused	O
with	O
BBCCH	O
and	O
CCHHR	O
,	O
which	O
are	O
the	O
addresses	O
used	O
by	O
the	O
channel	O
program	O
.	O
</s>
<s>
In	O
1979	O
IBM	O
introduced	O
fixed	B-Device
block	I-Device
architecture	I-Device
(	O
FBA	O
)	O
for	O
mainframes	O
.	O
</s>
<s>
At	O
the	O
programming	O
level	O
,	O
these	O
devices	O
do	O
not	O
use	O
the	O
traditional	O
CHR	O
addressing	O
,	O
but	O
reference	O
fixed-length	O
blocks	B-General_Concept
by	O
number	O
,	O
much	O
like	O
sectors	O
in	O
mini-computers	O
.	O
</s>
<s>
FBA	O
is	O
supported	O
by	O
VM/370	B-Application
and	O
DOS/VSE	B-Application
,	O
but	O
not	O
MVS	B-Application
or	O
successor	O
operating	O
systems	O
in	O
the	O
OS/360	O
line	O
.	O
</s>
<s>
Processors	O
with	O
FICON	B-Architecture
channels	B-Device
can	O
access	O
SCSI	O
drives	O
using	O
Fibre	O
Channel	O
Protocol	O
(	O
FCP	O
)	O
.	O
</s>
<s>
While	O
z/VM	O
and	O
z/VSE	B-Application
fully	O
support	O
FCP	O
,	O
z/OS	B-Application
provides	O
only	O
limited	O
support	O
through	O
IOSFBA	O
.	O
</s>
<s>
Some	O
programming	O
interface	O
macros	O
and	O
routines	O
are	O
collectively	O
referred	O
to	O
as	O
access	B-Application
methods	I-Application
with	O
names	O
ending	O
in	O
Access	B-Application
Method	I-Application
.	O
</s>
<s>
DOS/360	O
through	O
z/VSE	B-Application
support	O
datasets	O
on	O
DASD	B-Application
with	O
the	O
following	O
access	B-Application
methods	I-Application
:	O
</s>
<s>
OS/360	O
through	O
z/OS	B-Application
support	O
datasets	O
on	O
DASD	B-Application
with	O
the	O
following	O
access	B-Application
methods	I-Application
:	O
</s>
<s>
In	O
MVS	B-Application
,	O
starting	O
with	O
OS/VS2	B-Application
Release	O
2	O
and	O
continuing	O
through	O
z/OS	B-Application
,	O
all	O
of	O
the	O
access	B-Application
methods	I-Application
including	O
EXCP[VR],	O
use	O
the	O
privileged	O
STARTIO	B-Application
macro	O
.	O
</s>
<s>
IBM	O
in	O
its	O
1964	O
first	O
version	O
of	O
the	O
"	O
IBM	O
System/360	O
System	O
Summary	O
"	O
used	O
the	O
term	O
File	O
to	O
collectively	O
described	O
devices	O
now	O
called	O
DASD	B-Application
.	O
</s>
<s>
Files	O
provided	O
"	O
random	B-General_Concept
access	I-General_Concept
storage	O
 '	O
"	O
At	O
the	O
same	O
time	O
IBM	O
's	O
product	O
reference	O
manual	O
described	O
such	O
devices	O
as	O
"	O
direct	B-Application
access	I-Application
storage	I-Application
devices	I-Application
"	O
without	O
any	O
acronym	O
.	O
</s>
<s>
An	O
early	O
public	O
use	O
of	O
the	O
acronym	O
DASD	B-Application
is	O
in	O
IBM	O
's	O
March	O
1966	O
manual	O
,	O
"	O
Data	O
File	O
Handbook.	O
"	O
</s>
<s>
The	O
earliest	O
non-IBM	O
use	O
of	O
the	O
acronym	O
DASD	B-Application
found	O
by	O
the	O
"	O
Google	O
ngram	O
viewer	O
"	O
to	O
refer	O
to	O
storage	O
devices	O
dates	O
from	O
1968	O
.	O
</s>
<s>
Both	O
drums	O
and	O
data	B-Device
cells	I-Device
have	O
disappeared	O
as	O
products	O
,	O
so	O
DASD	B-Application
remains	O
as	O
a	O
synonym	O
of	O
disk	O
,	O
flash	O
and	O
optical	O
devices	O
.	O
</s>
<s>
Modern	O
DASD	B-Application
used	O
in	O
mainframes	O
only	O
very	O
rarely	O
consist	O
of	O
single	O
disk-drives	O
.	O
</s>
<s>
Most	O
commonly	O
"	O
DASD	B-Application
"	O
means	O
large	O
disk	B-General_Concept
arrays	I-General_Concept
utilizing	O
RAID	B-Architecture
schemes	O
.	O
</s>
