<s>
A	O
memory	B-General_Concept
rank	I-General_Concept
is	O
a	O
set	O
of	O
DRAM	O
chips	O
connected	O
to	O
the	O
same	O
chip	B-Architecture
select	I-Architecture
,	O
which	O
are	O
therefore	O
accessed	O
simultaneously	O
.	O
</s>
<s>
In	O
practice	O
all	O
DRAM	O
chips	O
share	O
all	O
of	O
the	O
other	O
command	O
and	O
control	O
signals	O
,	O
and	O
only	O
the	O
chip	B-Architecture
select	I-Architecture
pins	O
for	O
each	O
rank	O
are	O
separate	O
(	O
the	O
data	O
pins	O
are	O
shared	O
across	O
ranks	O
)	O
.	O
</s>
<s>
On	O
a	O
DDR	O
,	O
DDR2	O
,	O
or	O
DDR3	O
memory	B-General_Concept
module	I-General_Concept
,	O
each	O
rank	O
has	O
a	O
64-bit-wide	O
data	O
bus	O
(	O
72	O
bits	O
wide	O
on	O
DIMMs	B-General_Concept
that	O
support	O
ECC	B-General_Concept
)	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
rank	O
of	O
×8	O
(	O
8-bit	O
wide	O
)	O
DRAMs	O
would	O
consist	O
of	O
eight	O
physical	O
chips	O
(	O
nine	O
if	O
ECC	B-General_Concept
is	O
supported	O
)	O
,	O
but	O
a	O
rank	O
of	O
×4	O
(	O
4-bit	O
wide	O
)	O
DRAMs	O
would	O
consist	O
of	O
16	O
physical	O
chips	O
(	O
18	O
,	O
if	O
ECC	B-General_Concept
is	O
supported	O
)	O
.	O
</s>
<s>
Multiple	O
ranks	O
can	O
coexist	O
on	O
a	O
single	O
DIMM	B-General_Concept
,	O
and	O
modern	O
DIMMs	B-General_Concept
can	O
consist	O
of	O
one	O
rank	O
(	O
single	B-General_Concept
rank	I-General_Concept
)	O
,	O
two	O
ranks	O
(	O
dual	B-General_Concept
rank	I-General_Concept
)	O
,	O
four	O
ranks	O
(	O
quad	B-General_Concept
rank	I-General_Concept
)	O
,	O
or	O
eight	O
ranks	O
(	O
octal	B-General_Concept
rank	I-General_Concept
)	O
.	O
</s>
<s>
There	O
is	O
only	O
a	O
little	O
difference	O
between	O
a	O
dual	B-General_Concept
rank	I-General_Concept
UDIMM	B-General_Concept
and	O
two	O
single-rank	O
UDIMMs	B-General_Concept
in	O
the	O
same	O
memory	O
channel	O
,	O
other	O
than	O
that	O
the	O
DRAMs	O
reside	O
on	O
different	O
PCBs	O
.	O
</s>
<s>
The	O
electrical	O
connections	O
between	O
the	O
memory	B-General_Concept
controller	I-General_Concept
and	O
the	O
DRAMs	O
are	O
almost	O
identical	O
(	O
with	O
the	O
possible	O
exception	O
of	O
which	O
chip	B-Architecture
selects	I-Architecture
go	O
to	O
which	O
ranks	O
)	O
.	O
</s>
<s>
Increasing	O
the	O
number	O
of	O
ranks	O
per	O
DIMM	B-General_Concept
is	O
mainly	O
intended	O
to	O
increase	O
the	O
memory	O
density	O
per	O
channel	O
.	O
</s>
<s>
Also	O
some	O
memory	B-General_Concept
controllers	I-General_Concept
have	O
a	O
maximum	O
supported	O
number	O
of	O
ranks	O
.	O
</s>
<s>
DRAM	O
load	O
on	O
the	O
command/address	O
(	O
CA	O
)	O
bus	O
can	O
be	O
reduced	O
by	O
using	O
registered	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Predating	O
the	O
term	O
rank	O
(	O
sometimes	O
also	O
called	O
row	O
)	O
is	O
the	O
use	O
of	O
single-sided	O
and	O
double-sided	O
modules	B-General_Concept
,	O
especially	O
with	O
SIMMs	B-General_Concept
.	O
</s>
<s>
Multi-rank	O
modules	B-General_Concept
allow	O
several	O
open	O
DRAM	O
pages	O
(	O
row	O
)	O
in	O
each	O
rank	O
(	O
typically	O
eight	O
pages	O
per	O
rank	O
)	O
.	O
</s>
<s>
The	O
performance	O
gain	O
that	O
can	O
be	O
achieved	O
is	O
highly	O
dependent	O
on	O
the	O
application	O
and	O
the	O
memory	B-General_Concept
controller	I-General_Concept
's	O
ability	O
to	O
take	O
advantage	O
of	O
open	O
pages	O
.	O
</s>
<s>
Multi-rank	O
modules	B-General_Concept
have	O
higher	O
loading	O
on	O
the	O
data	O
bus	O
(	O
and	O
on	O
unbuffered	O
DIMMs	B-General_Concept
the	O
CA	O
bus	O
as	O
well	O
)	O
.	O
</s>
<s>
Therefore	O
if	O
more	O
than	O
dual	B-General_Concept
rank	I-General_Concept
DIMMS	B-General_Concept
are	O
connected	O
in	O
one	O
channel	O
,	O
the	O
speed	O
might	O
be	O
reduced	O
.	O
</s>
<s>
Controlling	O
interleaved	O
accesses	O
like	O
so	O
is	O
done	O
by	O
the	O
memory	B-General_Concept
controller	I-General_Concept
.	O
</s>
<s>
For	O
two	O
ranks	O
on	O
a	O
single	O
DIMM	B-General_Concept
it	O
might	O
not	O
even	O
be	O
required	O
,	O
but	O
this	O
parameter	O
is	O
often	O
programmed	O
independently	O
of	O
the	O
rank	O
location	O
in	O
the	O
system	O
(	O
if	O
on	O
the	O
same	O
DIMM	B-General_Concept
or	O
different	O
DIMMs	B-General_Concept
)	O
.	O
</s>
