<s>
Beginning	O
with	O
its	O
1964	O
System/360	B-Application
announcement	O
,	O
IBM	O
's	O
mainframes	O
initially	O
accessed	O
CKD	O
(	O
Count	O
key	O
data	O
)	O
subsystems	O
via	O
a	O
channel	O
connected	O
to	O
separate	O
Storage	O
Control	O
Units	O
(	O
SCUs	O
)	O
with	O
attached	O
Direct	B-Application
Access	I-Application
Storage	I-Application
Devices	I-Application
(	O
DASD	B-Application
)	O
,	O
typically	O
a	O
hard	O
disk	O
drive	O
.	O
</s>
<s>
The	O
System/360	B-Application
selector	O
channel	O
was	O
followed	O
by	O
the	O
System/370	O
block	O
multiplexor	O
channel	O
which	O
could	O
operate	O
as	O
a	O
selector	O
channel	O
to	O
allow	O
attachment	O
of	O
legacy	O
subsystems	O
.	O
</s>
<s>
The	O
SCU	O
evolved	O
into	O
a	O
Director	O
and	O
Controller	O
,	O
the	O
latter	O
typically	O
labelled	O
an	O
"	O
A-unit	O
"	O
(	O
or	O
A-Box	O
"	O
)	O
with	O
the	O
controller	O
and	O
at	O
least	O
one	O
DASD	B-Application
physically	O
in	O
an	O
A-unit	O
.	O
</s>
<s>
A	O
storage	O
control	O
,	O
frequently	O
called	O
a	O
Storage	O
Control	O
Unit	O
,	O
attaches	O
to	O
a	O
System/360	B-Application
(	O
or	O
System/370	O
)	O
channel	O
one	O
or	O
more	O
logically	O
separate	O
devices	O
.	O
</s>
<s>
The	O
control	O
unit	O
and	O
the	O
device	O
each	O
comporting	O
to	O
System/360	B-Application
and/or	O
System/370	O
Input/Output	O
architecture	O
.	O
</s>
<s>
Conventional	O
DASD	B-Application
storage	O
controls	O
listed	O
in	O
this	O
section	O
have	O
different	O
interfaces	O
to	O
the	O
attached	O
DASD	B-Application
,	O
unique	O
to	O
a	O
DASD	B-Application
or	O
DASD	B-Application
family	O
.	O
</s>
<s>
In	O
1972	O
,	O
IBM	O
introduced	O
a	O
standard	O
DASD	B-Application
to	O
control	O
interface	O
used	O
with	O
Director	O
type	O
storage	O
controls	O
.	O
</s>
<s>
The	O
IBM	O
2820	O
Storage	O
Control	O
Unit	O
(	O
SCU	O
)	O
connects	O
to	O
one	O
or	O
two	O
selector	O
channels	B-Device
and	O
can	O
attach	O
from	O
one	O
to	O
four	O
2301	O
Drum	O
Storage	O
units	O
.	O
</s>
<s>
The	O
IBM	O
2841	O
Storage	O
Control	O
Unit	O
was	O
a	O
Direct	B-Application
Access	I-Application
Storage	I-Application
Device	I-Application
(	O
DASD	B-Application
)	O
control	O
unit	O
introduced	O
with	O
System/360	B-Application
,	O
first	O
shipped	O
in	O
1965	O
and	O
offered	O
until	O
at	O
least	O
1975	O
.	O
</s>
<s>
The	O
2841	O
was	O
a	O
microprogrammed	B-Device
control	O
unit	O
"	O
intended	O
for	O
use	O
in	O
controlling	O
access	O
to	O
a	O
disk	O
or	O
strip	O
file	O
or	O
a	O
slow-speed	O
drum	O
storage	O
unit.	O
"	O
</s>
<s>
It	O
connected	O
to	O
one	O
or	O
two	O
standard	O
System/360	B-Application
channels	B-Device
,	O
or	O
could	O
also	O
be	O
attached	O
to	O
an	O
IBM	B-Device
1130	I-Device
or	O
IBM	B-Device
1800	I-Device
Data	I-Device
Acquisition	I-Device
and	I-Device
Control	I-Device
System	I-Device
to	O
add	O
support	O
for	O
2311	O
disks	O
.	O
</s>
<s>
The	O
IBM	O
2841	O
Storage	O
Control	O
Unit	O
was	O
initially	O
capable	O
of	O
attaching	O
combinations	O
of	O
up	O
to	O
eight	O
of	O
following	O
DASD	B-Application
:	O
</s>
<s>
IBM	B-Device
2321	I-Device
Data	I-Device
Cell	I-Device
,	O
eight	O
units	O
maximum	O
.	O
</s>
<s>
The	O
standard	O
offering	O
was	O
a	O
2841	O
SCU	O
with	O
a	O
number	O
of	O
2311	O
DASD	B-Application
up	O
to	O
eight	O
.	O
</s>
<s>
With	O
less	O
than	O
eight	O
units	O
of	O
a	O
single	O
type	O
other	O
units	O
can	O
be	O
intermixed	O
,	O
for	O
example	O
a	O
2841	O
with	O
two	O
2303	O
Drum	O
Storages	O
supported	O
up	O
to	O
six	O
additional	O
2321	O
Data	B-Device
Cells	I-Device
.	O
</s>
<s>
DASD	B-Application
devices	O
introduced	O
after	O
the	O
2841	O
used	O
different	O
storage	O
controls	O
as	O
discussed	O
in	O
the	O
following	O
sections	O
.	O
</s>
<s>
In	O
IBM	O
2314	O
family	O
of	O
SCUs	O
and	O
DASDs	B-Application
,	O
the	O
SCUs	O
connect	O
to	O
one	O
or	O
two	O
selector	O
channels	B-Device
and	O
can	O
attach	O
from	O
one	O
to	O
nine	O
drives	O
,	O
at	O
most	O
eight	O
online	O
plus	O
one	O
optional	O
service	O
drive	O
which	O
is	O
accessible	O
from	O
the	O
SCU	O
for	O
maintenance	O
purposes	O
.	O
</s>
<s>
The	O
unbundled	O
DASD	B-Application
models	O
were	O
the	O
one	O
drive	O
2312	O
Disk	O
Storage	O
,	O
the	O
four	O
drive	O
2313	O
Disk	O
Storage	O
and	O
the	O
two	O
drive	O
2318	O
Disk	O
Storage	O
.	O
</s>
<s>
2844	O
Auxiliary	O
Storage	O
Control	O
-	O
This	O
SCU	O
is	O
a	O
second	O
2314	O
SCU	O
attached	O
to	O
one	O
set	O
of	O
2314	O
DASD	B-Application
allowing	O
simultaneous	O
access	O
to	O
any	O
two	O
of	O
the	O
attached	O
DASD	B-Application
.	O
</s>
<s>
The	O
2835	O
storage	O
Control	O
connects	O
to	O
one	O
or	O
two	O
block	O
multiplexer	O
channels	B-Device
and	O
attaches	O
one	O
or	O
two	O
2305	O
Fixed	O
Head	O
storage	O
Modules	O
.	O
</s>
<s>
The	O
3830	O
Model	O
1	O
Storage	O
Control	O
Unit	O
connects	O
to	O
one	O
,	O
two	O
or	O
four	O
channels	B-Device
and	O
can	O
attach	O
one	O
to	O
eight	O
3330	O
DASD	B-Application
spindles	O
.	O
</s>
<s>
The	O
3380	O
Model	O
CJ2	O
provides	O
both	O
the	O
storage	O
functions	O
of	O
one	O
3380	O
DASD	B-Application
(	O
two	O
devices	O
)	O
and	O
the	O
storage	O
control	O
functions	O
of	O
a	O
3990	O
model	O
I	O
(	O
two	O
paths	O
to	O
A-units	O
)	O
in	O
a	O
single	O
unit	O
.	O
</s>
<s>
It	O
connects	O
to	O
two	O
,	O
four	O
or	O
eight	O
block	O
multiplexor	O
channels	B-Device
.	O
</s>
<s>
The	O
IBM	O
Storage	O
Control	O
Unit	O
evolved	O
into	O
a	O
Director	O
and	O
Controller	O
,	O
the	O
latter	O
typically	O
packaged	O
and	O
labelled	O
as	O
an	O
"	O
A-unit	O
"	O
(	O
or	O
"	O
A-Box	O
"	O
)	O
with	O
the	O
controller	O
and	O
at	O
least	O
one	O
DASD	B-Application
physically	O
in	O
an	O
A-unit	O
.	O
</s>
<s>
A	O
DASD	B-Application
unit	O
without	O
controller	O
then	O
became	O
known	O
as	O
a	O
"	O
B-unit	O
"	O
(	O
or	O
"	O
B-Box	O
"	O
)	O
.	O
</s>
<s>
The	O
terms	O
"	O
A-unit	O
"	O
(	O
or	O
"	O
A-Box	O
"	O
)	O
and	O
"	O
B-unit	O
"	O
(	O
or	O
"	O
B-Box	O
"	O
)	O
were	O
not	O
initially	O
used	O
,	O
instead	O
units	O
containing	O
the	O
controller	O
and	O
DASDs	B-Application
were	O
given	O
product	O
type	O
numbers	O
,	O
specifically	O
3333	O
for	O
the	O
A-unit	O
containing	O
3330	O
DASD	B-Application
and	O
3333-11	O
for	O
the	O
A-unit	O
containing	O
two	O
double	O
capacity	O
3330-11	O
DASDs	B-Application
.	O
</s>
<s>
The	O
use	O
of	O
the	O
suffixes	O
"	O
A	O
"	O
and	O
"	O
B	O
"	O
in	O
the	O
model	O
number	O
to	O
designate	O
an	O
A-unit	O
and	O
B-units	O
began	O
with	O
the	O
3340	O
DASD	B-Application
with	O
the	O
3340	O
Model	O
A2	O
and	O
3340	O
B2	O
,	O
respectively	O
.	O
</s>
<s>
This	O
practice	O
of	O
using	O
the	O
digits	O
of	O
a	O
suffix	O
to	O
a	O
DASD	B-Application
Model	O
number	O
to	O
represent	O
the	O
type	O
of	O
unit	O
and	O
the	O
number	O
of	O
DASD	B-Application
in	O
the	O
unit	O
continued	O
for	O
all	O
subsequent	O
CKD	O
DASD	B-Application
,	O
for	O
example	O
,	O
the	O
3380	O
Model	O
A4	O
has	O
one	O
controller	O
and	O
four	O
access	O
mechanisms	O
in	O
one	O
unit	O
,	O
the	O
3380	O
Model	O
AA4	O
has	O
two	O
controllers	O
and	O
four	O
access	O
mechanisms	O
in	O
one	O
unit	O
while	O
the	O
3380	O
Model	O
B4	O
has	O
four	O
access	O
mechanisms	O
.	O
</s>
<s>
"	O
String	O
switch	O
"	O
is	O
an	O
optional	O
feature	O
on	O
most	O
A-units	O
that	O
allows	O
the	O
controller	O
in	O
the	O
A-unit	O
to	O
be	O
switched	O
between	O
two	O
directors	O
,	O
thereby	O
providing	O
additional	O
paths	O
into	O
a	O
string	O
of	O
DASD	B-Application
.	O
</s>
<s>
The	O
3830-2	O
connects	O
to	O
one	O
,	O
two	O
or	O
four	O
block	O
multiplexer	O
channels	B-Device
and	O
can	O
attach	O
any	O
combination	O
of	O
up	O
to	O
four	O
A-units	O
,	O
i.e.	O
,	O
any	O
combination	O
of	O
3333s	O
,	O
3340-A2s	O
,	O
or	O
3350-A2/A2Fs	O
up	O
to	O
a	O
maximum	O
of	O
32	O
physical	O
drives	O
(	O
up	O
to	O
6	O
additional	O
drives	O
attached	O
to	O
each	O
A-unit	O
)	O
.	O
</s>
<s>
The	O
3830-3	O
only	O
attaches	O
up	O
to	O
four	O
3333s	O
and	O
3350-A2/A2F/C2/C2Fs	O
but	O
has	O
additional	O
functions	O
to	O
support	O
the	O
3850	B-Device
Mass	I-Device
Storage	I-Device
System	I-Device
.	O
</s>
<s>
Each	O
director	O
can	O
attach	O
to	O
one	O
,	O
two	O
,	O
four	O
or	O
eight	O
block	O
multiplexor	O
channels	B-Device
.	O
</s>
<s>
Each	O
3380	O
Storage	O
Control	O
,	O
Model	O
11	O
,	O
Model	O
13	O
,	O
Model	O
21	O
and	O
Model	O
23	O
has	O
two	O
directors	O
;	O
each	O
director	O
can	O
attach	O
to	O
one	O
,	O
two	O
or	O
four	O
block	O
multiplexor	O
channels	B-Device
.	O
</s>
<s>
Large	O
semiconductor	O
buffers	O
,	O
called	O
subsystem	O
storage	O
were	O
added	O
to	O
the	O
3380	O
Storage	O
Controls	O
in	O
the	O
Models	O
13	O
and	O
23	O
for	O
caching	B-General_Concept
and	O
the	O
Models	O
11	O
and	O
21	O
for	O
paging	B-Architecture
.	O
</s>
<s>
Caching	B-General_Concept
in	O
semiconductor	O
buffers	O
was	O
first	O
introduced	O
in	O
DASD	B-Application
CKD	O
subsystems	O
by	O
Memorex	O
(	O
1978	O
)	O
and	O
StorageTek	O
(	O
1981	O
)	O
.	O
</s>
<s>
The	O
3880	O
Model	O
13	O
has	O
two	O
caching	B-General_Concept
storage	O
directors	O
that	O
access	O
subsystem	O
storage	O
;	O
a	O
larger	O
portion	O
of	O
subsystem	O
storage	O
is	O
the	O
cache	B-General_Concept
which	O
is	O
used	O
to	O
store	O
active	O
data	O
for	O
quick	O
access	O
;	O
a	O
smaller	O
portion	O
of	O
the	O
storage	O
is	O
the	O
directory	O
which	O
is	O
used	O
to	O
locate	O
the	O
data	O
stored	O
in	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
The	O
cache	B-General_Concept
storage	O
director	O
attaches	O
only	O
one	O
or	O
two	O
3380	O
A-units	O
each	O
of	O
which	O
can	O
in	O
turn	O
attach	O
up	O
to	O
three	O
3380	O
B-units	O
for	O
a	O
total	O
of	O
16	O
devices	O
.	O
</s>
<s>
Because	O
each	O
3380	O
DASD	B-Application
has	O
two	O
actuators	O
the	O
3880	O
Model	O
13	O
can	O
have	O
up	O
to	O
32	O
device	O
addresses	O
.	O
</s>
<s>
The	O
Model	O
23	O
increased	O
the	O
cache	B-General_Concept
size	O
and	O
somewhat	O
improved	O
performance	O
but	O
otherwise	O
performed	O
the	O
same	O
functions	O
.	O
</s>
<s>
Paging	B-Architecture
is	O
well	O
established	O
in	O
computer	O
with	O
dedicated	O
CKD	O
paging	B-Architecture
devices	O
going	O
back	O
to	O
the	O
drums	O
included	O
in	O
the	O
S/360	B-Application
announcement	O
.	O
</s>
<s>
The	O
1978	O
StorageTek	O
4305	O
was	O
the	O
first	O
CKD	O
device	O
using	O
semiconductor	O
memory	O
for	O
paging	B-Architecture
.	O
</s>
<s>
The	O
3880	O
Model	O
11	O
has	O
two	O
storage	O
directors	O
,	O
director	O
1	O
can	O
access	O
subsystem	O
storage	O
for	O
paging	B-Architecture
and	O
director	O
2	O
is	O
for	O
nonpaging	O
.	O
</s>
<s>
Director	O
1	O
attaches	O
one	O
string	O
of	O
up	O
to	O
eight	O
3350	O
DASDs	B-Application
.	O
</s>
<s>
Director	O
2	O
attaches	O
any	O
combination	O
of	O
from	O
one	O
to	O
four	O
3330	O
A-units	O
or	O
3350	O
A-units	O
,	O
each	O
A-unit	O
attaching	O
up	O
to	O
three	O
additional	O
B-units	O
for	O
up	O
to	O
32	O
DASDs	B-Application
.	O
</s>
<s>
All	O
3990	O
's	O
are	O
dual	O
directors	O
and	O
each	O
director	O
can	O
attach	O
to	O
four	O
or	O
eight	O
block	O
multiplexor	O
channels	B-Device
.	O
</s>
<s>
Attached	O
DASD	B-Application
A-units	O
:	O
The	O
3990	O
Model	O
1	O
can	O
attach	O
up	O
to	O
four	O
3380	O
A-units	O
.	O
</s>
<s>
On	O
January	O
4	O
,	O
1968	O
,	O
IBM	O
announced	O
the	O
System	B-Application
360	I-Application
Model	O
25	O
which	O
included	O
the	O
"	O
Disk	O
Attachment	O
Control	O
"	O
(	O
DAC	O
)	O
attaching	O
up	O
to	O
four	O
2311	O
DASD	B-Application
.	O
</s>
<s>
On	O
September	O
23	O
,	O
1970	O
,	O
IBM	O
announced	O
as	O
part	O
of	O
its	O
System/370	O
Model	O
145	O
a	O
new	O
"	O
Integrated	O
File	O
Adapter	O
"	O
along	O
with	O
a	O
new	O
2319-A1	O
DASD	B-Application
.	O
</s>
<s>
Conventional	O
2314	O
type	O
DASD	B-Application
,	O
e.g.	O
,	O
2312	O
,	O
2313	O
and/or	O
2318	O
,	O
could	O
be	O
attached	O
to	O
the	O
2319-A1	O
.	O
</s>
<s>
Direct	O
attachment	O
of	O
the	O
DASD	B-Application
Subsystem	O
or	O
the	O
3340	O
DASF	O
Subsystem	O
or	O
the	O
DAS	O
Subsystem	O
(	O
only	O
)	O
is	O
provided	O
.	O
</s>
<s>
One	O
or	O
two	O
DASD	B-Application
adapters	O
are	O
available	O
(	O
depending	O
upon	O
model	O
)	O
for	O
attachment	O
of	O
one	O
3340	O
A-unit	O
and	O
with	O
up	O
to	O
three	O
additional	O
3340	O
or	O
3344	O
B-units	O
providing	O
up	O
to	O
eight	O
devices	O
.	O
</s>
<s>
String	O
Switch	O
feature	O
in	O
the	O
3340	O
A-unit	O
allows	O
sharing	O
of	O
DASD	B-Application
with	O
another	O
IBM	O
processor	O
or	O
director	O
that	O
supports	O
the	O
A-units	O
and	O
string	O
switching	O
.	O
</s>
<s>
Up	O
to	O
four	O
3340	O
A-units	O
can	O
be	O
attached	O
to	O
an	O
optional	O
DASD	B-Application
Adapter	O
;	O
to	O
each	O
3340	O
A-unit	O
can	O
be	O
attached	O
from	O
one	O
to	O
three	O
3340	O
B-units	O
or	O
3344	O
B-units	O
for	O
a	O
total	O
of	O
32	O
DASD	B-Application
in	O
four	O
strings	O
of	O
eight	O
.	O
</s>
<s>
String	O
Switch	O
feature	O
in	O
the	O
3340	O
A-unit	O
allows	O
sharing	O
of	O
DASD	B-Application
with	O
another	O
IBM	O
processor	O
or	O
director	O
that	O
supports	O
the	O
A-units	O
and	O
string	O
switching	O
.	O
</s>
