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with some improvements in its electrical characteristics, enabling it to run even faster than DDR while using less power. The big speed increase from DDR2 comes by clock-doubling the input/output circuits on the chips. This does not speed up the core RAM (the part that holds the data), but speeding up the input/output and adding special buffers (sort of like a cache) makes DDR2 run much faster than regular DDR. DDR2 uses a 240-pin DIMM that’s not compatible with DDR (Figure 6-16). Likewise, the DDR2 200-pin SO-DIMM is incompatible with the DDR SO-DIMM. You’ll find motherboards running both single-channel and dual-channel DDR2.

Figure 6-16 240-pin DDR2 DIMM

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EXAM TIP DDR2 RAM sticks will not fit into DDR sockets, nor are they electronically compatible.

Table 6-2 shows some of the common DDR2 speeds.

Table 6-2 DDR2 speeds

DDR3

DDR2 has been the standard for several years, but now there’s a new kid on the block. DDR3 boasts higher speeds, more efficient architecture, and around 30 percent lower power consumption than DDR2 RAM, making it a compelling choice for system builders. Just like its predecessor, DDR3 uses a 240-pin DIMM, albeit one that is slotted differently to make it difficult for users to install the wrong RAM in their system without using a hammer (Figure 6-17). DDR3 SO-DIMMs for portable computers have 204 pins. Neither fits into a DDR2 socket.

Figure 6-17 DDR2 DIMM on top of a DDR3 DIMM

DDR3 doubles the buffer of DDR2 from 4 bits to 8 bits, giving it a huge boost in bandwidth over older RAM. Not only that, but some DDR3 modules also include a feature called XMP, or extended memory profile, that enables power users to overclock their RAM easily, boosting their already fast memory to speeds that would make Chuck Yeager nervous. DDR3 modules also use higher-density memory chips, which means we may eventually see 16 GB DDR3 modules.

Some chipsets that support DDR3 also support a feature called triple-channel memory, which works a lot like dual-channel before it, but with three sticks of RAM instead of two. You’ll need three of the same type of memory modules and a motherboard that supports it, but triple-channel memory can greatly increase performance for those who can afford it.

In keeping with established tradition, Table 6-3 shows the common DDR3 speeds. Note how DDR3 I/O speeds are quadruple the clock speeds, whereas DDR2 I/O speeds are only double the clock. This speed increase is due to the increased buffer size, which enables DDR3 to grab twice as much data every clock cycle as DDR2 can.

Table 6-3 DDR3 speeds

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NOTE Do not confuse DDR3 with GDDR3; the latter is a type of memory used solely in video cards. See Chapter 19, “Video,” for the scoop on video-specific types of memory.

RAM Variations

Within each class of RAM, you’ll find variations in packaging, speed, quality, and the capability to handle data with more or fewer errors. Higher-end systems often need higher-end RAM, so knowing these variations is of crucial importance to techs.

Double-Sided DIMMs

Every type of RAM stick, starting with the old FPM SIMMs and continuing through to 240-pin DDR3 SDRAM, comes in one of two types: single-sided and double-sided. As their name implies, single-sided sticks have chips on only one side of the stick. Double-sided sticks have chips on both sides (Figure 6-18). The vast majority of RAM sticks are single-sided, but plenty of double-sided sticks are out there. Double-sided sticks are basically two sticks of RAM soldered onto one board. There’s nothing wrong with double-sided RAM other than the fact that some motherboards either can’t use them or can only use them in certain ways—for example, only if you use a single stick and it goes into a certain slot.

Figure 6-18 Double-sided DDR SDRAM

Latency

If you’ve shopped for RAM lately, you may have noticed terms such as “CL2” or “low latency” as you tried to determine which RAM to purchase. You might find two otherwise identical RAM sticks with a 20 percent price difference and a salesperson pressuring