Hardware Reference
In-Depth Information
The alternative to this at the time was to have the memory soldered into either the motherboard or an
expansion card. This prevented the chips from creeping and made the connections more permanent,
but it caused another problem. If a chip did go bad, you had to attempt desoldering the old one and
resoldering a new one or resort to scrapping the motherboard or memory card on which the chip was
installed. This was expensive and made memory troubleshooting difficult.
A chip was needed that was both soldered and removable, which was made possible by using
memory modules instead of individual chips. Early modules had one row of electrical contacts and
were called SIMMs (single inline memory modules), whereas later modules had two rows and were
called DIMMs (dual inline memory modules) or RIMMs (Rambus inline memory modules). These
small boards plug into special connectors on a motherboard or memory card. The individual memory
chips are soldered to the module, so removing and replacing them is impossible. Instead, you must
replace the entire module if any part of it fails. The module is treated as though it were one large
memory chip.
Several types of SIMMs, DIMMs, and RIMMs have been commonly used in desktop systems. The
various types are often described by their pin count, memory row width, or memory type.
SIMMs, for example, are available in two main physical types—30-pin (8 bits plus an option for 1
additional parity bit) and 72-pin (32 bits plus an option for 4 additional parity bits)—with various
capacities and other specifications. The 30-pin SIMMs are physically smaller than the 72-pin
versions, and either version can have chips on one or both sides. SIMMs were widely used from the
late 1980s to the late 1990s but have become obsolete.
DIMMs are available in four main types. SDR (single data rate) DIMMs have 168 pins, one notch on
either side, and two notches along the contact area. DDR DIMMs, on the other hand, have 184 pins,
two notches on each side, and only one offset notch along the contact area. DDR2 and DDR3 DIMMs
have 240 pins, two notches on each side, and one near the center of the contact area. All DIMMs are
either 64 bits (non-ECC/parity) or 72 bits (data plus parity or error-correcting code [ECC]) wide.
The main physical difference between SIMMs and DIMMs is that DIMMs have different signal pins
on each side of the module, resulting in two rows of electrical contacts. That is why they are called
dual inline memory modules, and why with only 1 inch of additional length, they have many more pins
than a SIMM.
Note
There is confusion among users and even in the industry regarding the terms
single-sided
and
double-sided
with respect to memory modules. In truth, the single- or double-sided designation
actually has nothing to do with whether chips are physically located on one or both sides of the
module, and it has nothing to do with whether the module is a SIMM or DIMM (meaning
whether the connection pins are single- or double-inline). Instead, the terms single-sided and
double-sided indicate whether the module has one or two internal banks (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
