Hardware Reference
In-Depth Information
Besides being a time-consuming and labor-intensive way to deal with memory, DIP chips
had one notorious problem—they crept out of their sockets over time as the system went
through thermal cycles. Every day, when you powered the system on and off, the system
heated and cooled, and the chips gradually walked their way out of the sockets—a phe-
nomenoncalled
chip creep
.Eventually,goodcontactwaslostandmemoryerrorsresulted.
Fortunately, reseating all the chips back in their sockets usually rectified the problem, but
that method was labor intensive if you had many systems to support.
Thealternativetothisatthetimewastohavethememorysolderedintoeitherthemother-
board or an expansion card. This prevented the chips from creeping and made the con-
nections 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.
Achipwasneededthatwasbothsolderedandremovable,whichwasmadepossiblebyus-
ing 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
hadtworowsandwerecalledDIMMs(dualinlinememorymodules)orRIMMs(Rambus
inline memory modules). These small boards plug into special connectors on a mother-
board or memory card. The individual memory chips are soldered to the module, so re-
moving 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 ofSIMMs,DIMMs,andRIMMs have been commonly used indesktop sys-
tems. The various types are often described by their pin count, memory row width, or
memory type.
SIMMs,forexample, areavailable intwomain physical types—30-pin (8bitsplusanop-
tion 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.
SIMMswere widely used fromthe late 1980stothe late 1990sbuthave 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
