<s>
Count	B-Application
key	I-Application
data	I-Application
(	O
CKD	O
)	O
is	O
a	O
direct-access	B-Application
storage	I-Application
device	I-Application
(	O
DASD	B-Application
)	O
data	O
recording	O
format	O
introduced	O
in	O
1964	O
,	O
by	O
IBM	O
with	O
its	O
IBM	B-Application
System/360	I-Application
and	O
still	O
being	O
emulated	O
on	O
IBM	O
mainframes	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
devices	O
using	O
fixed	B-Device
sector	I-Device
size	I-Device
or	O
a	O
separate	O
format	O
track	O
.	O
</s>
<s>
Count	B-Application
key	I-Application
data	I-Application
(	O
CKD	O
)	O
also	O
refers	O
to	O
the	O
set	O
of	O
channel	O
commands	O
(	O
collectively	O
Channel	O
Command	O
Words	O
,	O
CCWs	O
)	O
that	O
are	O
generated	O
by	O
an	O
IBM	O
mainframe	O
for	O
execution	O
by	O
a	O
DASD	B-Application
subsystem	O
employing	O
the	O
CKD	O
recording	O
format	O
.	O
</s>
<s>
IBM	O
provides	O
a	O
"	O
reference	O
card	O
"	O
for	O
each	O
device	O
,	O
which	O
can	O
be	O
used	O
to	O
compute	O
the	O
number	O
of	O
blocks	O
per	O
track	O
for	O
various	O
block	B-General_Concept
sizes	I-General_Concept
,	O
and	O
to	O
optimize	O
the	O
block	B-General_Concept
size	I-General_Concept
for	O
the	O
device	O
.	O
</s>
<s>
Because	O
blocks	O
are	O
normally	O
not	O
split	O
between	O
tracks	O
,	O
specification	O
of	O
an	O
incorrect	O
block	B-General_Concept
size	I-General_Concept
can	O
waste	O
up	O
to	O
half	O
of	O
each	O
track	O
.	O
</s>
<s>
Since	O
the	O
introduction	O
of	O
IBM	B-Application
's	I-Application
System/360	I-Application
in	O
1964	O
,	O
nearly	O
all	O
IBM	O
large	O
and	O
intermediate	O
system	O
DASDs	B-Application
have	O
used	O
the	O
count	B-Application
key	I-Application
data	I-Application
record	O
format	O
.	O
</s>
<s>
The	O
advantages	O
of	O
count	B-Application
key	I-Application
data	I-Application
record	O
format	O
are	O
:	O
</s>
<s>
Reduced	O
CPU	O
and	O
memory	O
prices	O
and	O
higher	O
device	O
and	O
interface	O
speeds	O
have	O
somewhat	O
nullified	O
the	O
advantages	O
of	O
CKD	O
,	O
and	O
it	O
is	O
retained	O
only	O
because	O
IBM	O
's	O
flagship	O
operating	O
system	O
z/OS	B-Application
does	O
not	O
support	O
sector-oriented	O
interfaces	O
.	O
</s>
<s>
Originally	O
CKD	O
records	O
had	O
a	O
one-to-one	O
correspondence	O
to	O
a	O
physical	O
track	O
of	O
a	O
DASD	B-Application
device	O
;	O
however	O
over	O
time	O
the	O
records	O
have	O
become	O
more	O
and	O
more	O
virtualized	O
such	O
that	O
in	O
modern	O
IBM	O
mainframes	O
there	O
is	O
no	O
longer	O
a	O
direct	O
correspondence	O
between	O
the	O
a	O
CKD	O
record	O
ID	O
and	O
a	O
physical	O
layout	O
of	O
a	O
track	O
.	O
</s>
<s>
The	O
group	O
of	O
CCWs	O
defined	O
by	O
IBM	O
for	O
DASD	B-Application
fall	O
into	O
five	O
broad	O
categories	O
:	O
</s>
<s>
Search	O
compare	O
information	O
from	O
the	O
CPU	O
with	O
information	O
stored	O
in	O
the	O
DASD	B-Application
;	O
the	O
Channel	O
operates	O
in	O
the	O
Write	O
mode	O
while	O
the	O
storage	O
unit	O
operates	O
in	O
the	O
Read	O
mode	O
.	O
</s>
<s>
CKD	O
CCWs	O
are	O
the	O
specific	O
set	O
of	O
CCWs	O
used	O
to	O
access	O
CKD	O
DASD	B-Application
subsystems	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
fixed	B-Device
block	I-Device
architecture	I-Device
(	O
FBA	O
)	O
CCWs	O
which	O
are	O
used	O
to	O
access	O
FBA	O
DASD	B-Application
subsystems	O
.	O
</s>
<s>
CKD	O
DASD	B-Application
are	O
addressed	O
like	O
other	O
Input/Output	O
devices	O
;	O
for	O
System/360	B-Application
and	O
System/370	O
DASD	B-Application
are	O
addressed	O
directly	O
,	O
through	O
channels	O
and	O
the	O
associated	O
control	O
units	O
(	O
SCU	O
or	O
Storage	O
Control	O
Unit	O
)	O
,	O
initially	O
using	O
three	O
hexadecimal	O
digits	O
,	O
one	O
for	O
channel	O
and	O
two	O
for	O
control	O
unit	O
and	O
device	O
,	O
providing	O
addressing	O
for	O
up	O
to	O
16	O
channels	O
,	O
for	O
up	O
to	O
256	O
DASD	B-Application
access	O
mechanisms/channel	O
and	O
4,096	O
DASD	B-Application
addresses	O
total	O
.	O
</s>
<s>
Modern	O
IBM	O
mainframes	O
use	O
four	O
hexadecimal	O
digits	O
as	O
an	O
arbitrary	O
subchannel	O
number	O
within	O
a	O
channel	O
subsystem	O
subset	O
,	O
whose	O
definition	O
includes	O
the	O
actual	O
channels	O
,	O
control	O
units	O
and	O
device	O
,	O
providing	O
addressing	O
for	O
up	O
to	O
65,536	O
DASD	B-Application
per	O
channel	O
subsystem	O
subset	O
.	O
</s>
<s>
In	O
practice	O
,	O
physical	O
and	O
design	O
constraints	O
of	O
the	O
channel	O
and	O
of	O
the	O
controllers	O
limited	O
the	O
maximum	O
number	O
of	O
attached	O
DASD	B-Application
attachable	O
to	O
a	O
system	O
to	O
a	O
smaller	O
amount	O
than	O
the	O
number	O
that	O
could	O
be	O
addressed	O
.	O
</s>
<s>
Initially	O
there	O
was	O
a	O
high	O
degree	O
of	O
correspondence	O
between	O
the	O
logical	O
view	O
of	O
DASD	B-Application
accesses	O
and	O
the	O
actual	O
hardware	O
,	O
as	O
shown	O
in	O
the	O
illustration	O
above	O
.	O
</s>
<s>
The	O
controller	O
handles	O
the	O
CKD	O
track	O
formatting	O
and	O
is	O
packaged	O
with	O
the	O
first	O
drive	O
or	O
drives	O
in	O
a	O
string	O
of	O
drives	O
and	O
having	O
a	O
model	O
number	O
with	O
the	O
letter	O
"	O
A	O
"	O
as	O
a	O
prefix	O
,	O
an	O
"	O
A-Unit	O
"	O
(	O
or	O
"	O
A-Box	O
"	O
)	O
as	O
in	O
3350	O
Model	O
A2	O
containing	O
a	O
controller	O
and	O
two	O
DASDs	B-Application
.	O
</s>
<s>
DASD	B-Application
without	O
a	O
controller	O
,	O
that	O
is	O
B-Units	O
,	O
have	O
a	O
"	O
B	O
"	O
prefix	O
in	O
their	O
model	O
number	O
.	O
</s>
<s>
CKD	O
subsystems	O
and	O
directors	O
were	O
offered	O
by	O
IBM	O
and	O
plug	O
compatible	O
competitors	O
until	O
at	O
least	O
1996	O
(	O
2301	O
to	O
3390	O
Model	O
9	O
)	O
;	O
in	O
total	O
22	O
unique	O
DASD	B-Application
offered	O
by	O
IBM	O
configured	O
in	O
at	O
least	O
35	O
different	O
subsystem	B-Application
configurations	I-Application
.	O
</s>
<s>
Plug-compatible	O
offered	O
many	O
of	O
the	O
same	O
DASD	B-Application
including	O
4	O
CKD	O
subsystems	O
featuring	O
unique	O
DASD	B-Application
.	O
</s>
<s>
Such	O
programs	O
can	O
search	O
a	O
large	O
amount	O
of	O
information	O
stored	O
on	O
a	O
DASD	B-Application
,	O
upon	O
successful	O
completion	O
returning	O
only	O
the	O
desired	O
data	O
and	O
thereby	O
freeing	O
CPU	O
resources	O
for	O
other	O
activity	O
.	O
</s>
<s>
+IBM	O
S/360	O
DASD	B-Application
Channel	O
Commands	O
Command	O
Class	O
Command‡	O
2301	O
2302	O
23037320	O
2311	B-Device
2321	O
23142319	O
MTOff	O
MTOn	O
†	O
Count	O
LengthControlNo	O
OpSSSSSS03	O
SeekSSSSSS076	O
Seek	O
CylinderSSSSSS0B6	O
Seek	O
HeadSSSSSS1B6	O
Set	O
File	O
MaskSSSSSS1F1	O
Space	O
CountSSSSSS0F3	O
RecalibrateSS13Not	O
zero	O
RestoreS17Not	O
zeroSenseSense	O
I/OSSSSSS046	O
Release	O
DeviceOOOOOO946	O
Reserve	O
DeviceOOOOOOB46SearchHome	O
Address	O
EQSSSSSS39B94	O
(	O
usually	O
)	O
Identifier	O
EQSSSSSS31B15	O
(	O
usually	O
)	O
Identifier	O
HISSSSSS51D15	O
(	O
usually	O
)	O
Identifier	O
EQ	O
or	O
HISSSSSS71FI5	O
(	O
usually	O
)	O
Key	O
EQSSSSSS29A91	O
to	O
255	O
Key	O
HISSSSSS49C91	O
to	O
255	O
Key	O
EQ	O
or	O
HISSSSSS69E91	O
to	O
255	O
Key	O
&	O
Data	O
EQO	O
OOS2DADSee	O
Note	O
2	O
Key	O
&	O
Data	O
HIO	O
OOS4DCDSee	O
Note	O
2	O
Key	O
&	O
Data	O
EQ	O
or	O
HIO	O
OOS6DEDSee	O
Note	O
2Continue	O
Scan(see Note 1 )	O
Search	O
EQO	O
OOS25A5See	O
Note	O
2	O
Search	O
HIO	O
OOS45C5See	O
Note	O
2	O
Search	O
HI	O
or	O
EQO	O
OOS65E5See	O
Note	O
2	O
Set	O
CompareO	O
OOS35B5See	O
Note	O
2	O
Set	O
CompareO	O
OOS75F5See	O
Note	O
2	O
No	O
CompareO	O
OOS55D5See	O
Note	O
2ReadHome	O
AddressSSSSSS1A9A5	O
CountSSSSSS12928	O
Record	O
0SSSSSS1696Number	O
of	O
bytes	O
transferred	O
DataSSSSSS0686	O
Key	O
&	O
DataSSSSSS0E8E	O
Count	O
.	O
</s>
<s>
File	O
Scan	O
Feature	O
(	O
9	O
CCWs	O
)	O
only	O
available	O
on	O
2841	O
for	O
2302	O
,	O
2311	B-Device
and	O
2321	O
;	O
they	O
were	O
not	O
available	O
on	O
subsequent	O
DASD	B-Application
controllers	O
for	O
DASD	B-Application
later	O
than	O
2314	O
.	O
</s>
<s>
The	O
2820	O
SCU	O
controlled	O
the	O
2301	O
Drum	O
while	O
the	O
2841	O
SCU	O
controlled	O
combinations	O
of	O
the	O
2302	B-Device
Disk	I-Device
Storage	I-Device
,	O
2311	B-Device
Disk	O
Drive	O
,	O
2321	B-Device
Data	I-Device
Cell	I-Device
and/or	O
7320	O
Drum	O
Storage	O
.	O
</s>
<s>
The	O
block	O
multiplexor	O
channel	O
was	O
introduced	O
beginning	O
in	O
1971	O
on	O
some	O
high	O
end	O
System/360	B-Application
systems	O
along	O
with	O
the	O
2835	O
Control	O
Unit	O
and	O
associated	O
2305	O
DASD	B-Application
,	O
This	O
channel	O
was	O
then	O
standard	O
on	O
IBM	O
System/370	O
and	O
subsequent	O
mainframes	O
;	O
when	O
contrasted	O
to	O
the	O
prior	O
Selector	O
channel	O
it	O
offered	O
performance	O
improvements	O
for	O
high	O
speed	O
devices	O
such	O
as	O
DASD	B-Application
,	O
including	O
:	O
</s>
<s>
With	O
a	O
block	O
multiplexer	O
channel	O
the	O
channel	O
is	O
free	O
during	O
the	O
time	O
the	O
DASD	B-Application
is	O
seeking	O
and	O
again	O
while	O
the	O
disk	O
rotates	O
to	O
beginning	O
of	O
the	O
track	O
.	O
</s>
<s>
First	O
introduced	O
with	O
the	O
3380	O
DASD	B-Application
on	O
the	O
3880	O
Storage	O
Control	O
Unit	O
in	O
1981	O
the	O
feature	O
was	O
included	O
with	O
the	O
later	O
CKD	O
DASD	B-Application
subsystems	O
.	O
</s>
<s>
Caching	B-General_Concept
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
was	O
subsequently	O
introduced	O
in	O
late	O
1981	O
by	O
IBM	O
on	O
the	O
3880	O
Model	O
13	O
for	O
models	O
of	O
the	O
3380	O
with	O
dynamic	O
pathing	O
.	O
</s>
<s>
The	O
cache	B-General_Concept
is	O
dynamically	O
managed	O
by	O
an	O
algorithm	O
;	O
high	O
activity	O
data	O
is	O
accessed	O
from	O
the	O
high-performance	O
cache	B-General_Concept
and	O
low	O
activity	O
data	O
is	O
accessed	O
from	O
less-expensive	O
DASD	B-Application
storage	O
.	O
</s>
<s>
A	O
large	O
memory	O
in	O
the	O
Director	O
,	O
the	O
cache	B-General_Concept
,	O
is	O
divided	O
into	O
track	O
slots	O
that	O
store	O
data	O
from	O
the	O
3380	O
tracks	O
.	O
</s>
<s>
A	O
smaller	O
area	O
is	O
a	O
directory	O
that	O
contains	O
entries	O
that	O
allow	O
data	O
to	O
be	O
located	O
in	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
Caches	B-General_Concept
were	O
also	O
provided	O
on	O
subsequently	O
introduced	O
storage	O
controls	O
.	O
</s>
<s>
Read	O
Multiple	O
Count	B-Application
Key	I-Application
Data	I-Application
could	O
more	O
efficiently	O
read	O
full	O
tracks	O
allowing	O
for	O
more	O
efficient	O
backups	O
.	O
</s>
<s>
Reduced	O
CPU	O
and	O
memory	O
prices	O
and	O
higher	O
device	O
and	O
interface	O
speeds	O
have	O
somewhat	O
nullified	O
the	O
advantages	O
of	O
CKD	O
,	O
and	O
support	O
continues	O
by	O
IBM	O
to	O
this	O
date	O
because	O
its	O
flagship	O
operating	O
system	O
z/OS	B-Application
continues	O
to	O
use	O
CKD	O
CCWs	O
for	O
many	O
functions	O
.	O
</s>
<s>
Originally	O
CKD	O
records	O
had	O
a	O
one-to-one	O
correspondence	O
to	O
a	O
physical	O
track	O
of	O
a	O
DASD	B-Application
device	O
;	O
however	O
over	O
time	O
the	O
records	O
have	O
become	O
more	O
and	O
more	O
virtualized	O
such	O
that	O
in	O
a	O
modern	O
IBM	O
mainframe	O
there	O
is	O
no	O
longer	O
a	O
direct	O
correspondence	O
between	O
the	O
a	O
CKD	O
record	O
ID	O
and	O
a	O
physical	O
layout	O
of	O
a	O
track	O
.	O
</s>
<s>
Of	O
the	O
83	O
CKD	O
CCWs	O
implemented	O
for	O
System/360	B-Application
and	O
System/370	O
channels	O
56	O
are	O
emulated	O
on	O
System/390	O
and	O
later	O
systems	O
.	O
</s>
