Hardware Reference
In-Depth Information
inline memory modules, and why with only 1 inch of additional length, they have many
more pins than a SIMM.
Note
Thereisconfusionamongusersandevenintheindustryregardingtheterms
single-sided
and
double-sided
with respect to memory modules. In truth, the single- or double-sided designa-
tionactuallyhasnothingtodowithwhetherchipsarephysicallylocatedononeorbothsides
ofthemodule,andithasnothingtodowithwhetherthemoduleisaSIMMorDIMM(mean-
ing whether the connection pins are single- or double-inline). Instead, the terms single-sided
anddouble-sidedindicatewhetherthemodulehasoneortwointernalbanks(called
ranks
)of
memory chips installed. A dual-rank DIMM module has two complete 64-bit wide banks of
chips logically stacked so that the module is twice as deep (has twice as many 64-bit rows).
In most (but not all) cases, this requires chips to be on both sides of the module; therefore,
the term
double-sided
often indicates that a module has two ranks, even though the term is
technically incorrect. Single-rank modules (incorrectly referred to as single-sided) can also
have chips physically mounted on both sides of the module, and dual-rank modules can have
chips physically mounted on only one side. I recommend using the terms
single rank
or
dual
rank
instead because they are much more accurate and easily understood.
Figures 6.3
through
6.9
show a typical 30-pin (8-bit) SIMM, 72-pin (32-bit) SIMM,
168-pinSDRAMDIMM,184-pinDDRSDRAM(64-bit)DIMM,240-pinDDR2DIMM,
240-pinDDR3DIMM,and184-pinRIMM,respectively.Thepinsarenumberedfromleft
to right and are connected to both sides of the module on the SIMMs. The pins on the
DIMM are different on each side, but on a SIMM, each side is the same as the other and
theconnections carrythrough.Notethatalldimensions areinbothinches andmillimeters
(in parentheses), and modules are generally available in ECC versions with 1 extra ECC
(or parity) bit for every 8 data bits (multiples of 9 in data width) or versions that do not
include ECC support (multiples of 8 in data width).
Figure 6.3
A typical 30-pin SIMM.
