Packaging

Most memory comes in the form of chips , individual integrated circuits meant for permanent installation on a printed circuit board. The capacity of each chip is measured in bits ”or, as is more likely in the modern world, megabits. The first chips had narrow, one-bit-wide data buses, so they moved information one bit at a time. To achieve a byte-wide data bus, eight chips had to be used together (under the coordination of the memory controller). Most modern chips use wider data buses ”four or eight or more bits ”but none comes close to matching the 64-bit data buses of modern computers.

To make memory more convenient to install and upgrade in practical computers, memory- makers package several memory chips on a small circuit board to make a memory module . Despite the added circuit board, modules provide a more compact memory package because the chips are soldered to the modules, thus eliminating space-wasting sockets. Soldering the chips down also allows them to be installed closer together (because no individual access is required). Moreover, because the discrete chips that are installed in the memory module never need to be individually manipulated except by a machine, they can use more compact surface-mount packages. A single memory module just a few inches long can thus accommodate a full bank of hundreds of megabytes of memory.

Memory modules are the large economy size ”RAM in a bigger package to better suit the dietary needs of today's computers. Besides the more convenient package that allows you to deftly install a number of chips in one operation, the memory module also better matches the way your computer uses memory. Unlike most chips, which are addressed at the bit level, memory modules usually operate in bytes. Whereas chip capacities are measured in kilobits and megabits, memory modules are measured in megabytes.

The construction of a memory module is straightforward; it's simply a second level of integration above the basic memory chip. Several chips are brought together on a small glass- epoxy circuit board with their leads soldered down to the circuit traces and the entire assembly terminated in an external connector suitable for plugging into a socket or soldering to another circuit board.

Memory modules come in a variety of types, many of which are no longer common. Single Inline Memory Modules , which are commonly called SIMMs , connect the contacts on the opposite sides of the board together so that each pair provides a single signal contact. Single Inline Pin Package modules, or SIPPs , have a row of pins projecting from the bottom of the module instead of edge connectors. Both of these packages are no longer used in new computers.

Several designs are in current use. Dual Inline Memory Modules , called DIMMs , put separate signals on each contact so that the contacts on each side of the board serve different purposes. Small Outline DIMMs , termed SoDIMMs , shrink the package to fit more compact computers. Rambus Inline Memory Modules , or RIMMs , basically follow the DIMM design but use Rambus instead of SDRAM memory.

Dual Inline Memory Modules

Engineers created the DIMMs to accommodate the needs of Pentium computers. Earlier module designs ”in particular SIMMs ”had 8- or 32-bit data buses. As a consequence, a single bank of 64-bit Pentium memory required several modules. Not only were multiple modules a pain when you wanted to upgrade, they took up valuable motherboard space. Moreover, the many sockets they required were incompatible with the high-speed needs of modern memory systems. With more sockets and longer circuit traces, the more difficult it is to achieve high-frequency operation. Figure 16.2 shows a typical DIMM.

Figure 16.2. A 168-pin DIMM module showing components .

All DIMMs provide a 64-bit data bus that exactly matches the needs of Pentium-class computers. Practical module capacity now ranges from 64MB to 512MB. Larger capacity modules often put memory chips on both sides of the circuit board. To fit even more memory on each module, one manufacturer (Kingston Technology) has developed a stacking scheme that piggybacks a physically larger chip over a smaller package, thus doubling the holding capacity of a single module.

DIMMs come in three types: Those using old-technology (FPM or EDO memory) chips, those for SDR DRAM memory, and those for DDR memory. In size, the three types of modules are identical. They are 5.25 inches wide and (usually) 1 or 1.5 inches tall, although modules with unusually large memory endowments often are taller. Figure 16.3 shows the dimensions of a typical DIMM. To prevent you from sliding a DDR module into a slot meant for a SDR module, the two designs use different connector designs that are keyed to be incompatible.

Figure 16.3. Dimensions of a typical 168-pin DIMM.

DIMMs can be equipped with non-parity, parity, or error-correction code (ECC) memory. The packages look identical and slide into the same sockets. Old-technology memory modules use signals on several connector contacts called presence detect pins to indicate these parameters. SDR and DDR modules use a more sophisticated design that is sometimes called serial presence detect . Each module is equipped with 2048 bytes of onboard nonvolatile memory that your computer can read and use to identify the module memory type. After reading this information, your computer can then configure itself appropriately.

Old Technology DIMMs

Memory modules that use old memory technology have edge connectors with 168 separate contacts arrayed on both sides of their small circuit boards . Eight of these contacts are reserved for presence-detect indications . In particular, these presence-detect signals indicate the speed rating of the module. Four of these contacts indicate the organization and addressing characteristics of the memory on the module. One pin, PD5, indicates whether the module contains FPM or EDO memory. Two, PD6 and PD7, indicate the speed rating of the chip, as indicated by the code listed in Table 16.4. (Note the rating in nanoseconds does not correspond to today's PC66/100/133 nomenclature because these modules are not used in modern computers.) The eighth contact indicates whether the module uses parity or ECC technology. In addition to the eight presence-detect indicators, two other contacts carry module ID signals that describe the module type and refresh mode used by the chip.

Table 16.4. Old-Technology (FPM and EDO) 168-Pin DIMM Speed Code

Signal	Pin Number	60 ns	70 ns	80 ns
PD6	165	H	L	H
PD7	82	H	H	L

SDR DIMMs

SDR modules have 168 pins on their bottom edge. They are keyed with two notches, dividing the contacts into three groups with short gaps between. The first group runs from pin 1 to pin 10; the second group from pin 11 to pin 40; and the third group from pin 41 to pin 84. Pin 85 is opposite pin 1. The notches are designed to prevent you from sliding a smaller SIMM with fewer connections into a DIMM socket ”the longest space between two notches is shorter than the shortest SIMM. The asymmetrical arrangement of the notches on the DIMM prevent its inadvertent improper insertion into its socket ” turn a DIMM end-for-end and it won't fit into a socket. Some DIMMs have holes to allow you to latch the modules into their sockets, although some DIMMs lack these holes.

DDR DIMMs

The standard DIMM for DDR memory has a connector with 184 pins. It bears a single notch near the center of the connector. Otherwise, DDR modules look much the same as SDR modules and can be handled similarly.

Rambus Inline Memory Modules

Because of the radical interface required by Rambus memory, memory-makers redesigned individual modules to create a special Rambus Inline Memory Module (RIMM) to accommodate the technology. The basic module appears similar to a standard DIMM, with individual memory chips soldered to a printed circuit board substrate that links to a socket using a conventional edge connector. Despite the relatively narrow bus interface used by the Rambus systems (16 bits), the RIMM package pushes its pin count to 184, partly because alternate pins are held to ground potential as a form of shielding to improve stability and decrease interference at the high clock frequency for which the modules are designed (800MHz).

RIMMs also differ in their operating voltage. The Rambus standard calls for 2.5-volt operation, as opposed to the 3.3 volts standard with DIMMs. Future RIMMs may operate at 1.8 volts .

The standard 184-pin design supports both non-parity and error-correction RIMMs, with bus widths of 16 and 18 bits, respectively. Figure 16.4 shows a RIMM package.

Figure 16.4. A RIMM package.

Small Outline DIMMs

Although convenient for desktop computers, full-size memory modules are unnecessarily large for notebook machines. Consequently, the makers of miniaturized computers trimmed the size of ordinary memory modules about in half to create what they called the Small Outline DIMM (SoDIMM).

The first SoDIMMs were offshoots of the 72-pin SIMMs used for FPM and EDO memory. They had the same 72 contacts as full-size SIMMs but arrayed them on both sides of the module (making it "dual inline"). In a SIMM, the contacts on opposite sides of the module's circuit board are electrically connected together. On a SoDIMM, each has a different function, putting the connection space to more efficient use and allowing the smaller size. Figure 16.5 illustrates a Small Outline Dual Inline Memory Module of this design.

Figure 16.5. A 72-pin Small Outline Dual Inline Memory Module package.

As you would expect, a 72-pin SoDIMM is about half the length of a 72-pin SIMM, measuring about 2.35 inches long. As with other module styles, a notch at one end of a 72-pin SoDIMM prevents you from latching it into its socket with the wrong orientation. The notch is on your left when you look at the chip side of the SoDIMM. Figure 16.6 shows the dimensions of a typical SoDIMM.

Figure 16.6. Dimensions of a typical 72-pin Small Outline Dual Inline Memory Module.

The SoDIMM package proved so compelling it has been adapted for SDRAM modules. The SDRAM SoDIMMs are slightly longer than those used by older technologies, measuring 2.67 inches (67.6 millimeters) long.

As with full-size DIMMs, SDRAM SoDIMMs come in two physically incompatible styles for SDR and DDR.

SDR SoDIMMs have 144 contacts on their edge connectors. DDR modules have 200. Each style of SoDIMM has a single notch, but the position of the notch is different for SDR and DDR modules. With SDR modules, the notch is only slightly offset from the center; with DDR modules, the notch is near one edge (the pin 1 side) of the module. Figure 16.7 shows the difference in the notches used by the two varieties of SDRAM modules.

Figure 16.7. The difference between the SDR and DDR SoDIMM notches.