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for every clock cycle. This synchronized (pardon the pun) nicely with the Athlon and later AMD processors’ double-pumped frontside bus. DDR SDRAM could not run as fast as RDRAM—although relatively low frontside bus speeds made that a moot point—but cost only slightly more than regular SDRAM.

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NOTE Most techs drop some or all of the SDRAM part of DDR SDRAM when engaged in normal geekspeak. You’ll hear the memory referred to as DDR, DDR RAM, and the weird hybrid, DDRAM.

DDR SDRAM for desktops comes in 184-pin DIMMs. These DIMMs match 168-pin DIMMs in physical size, but not in pin compatibility (Figure 6-13). The slots for the two types of RAM appear similar as well, but have different guide notches, so you can’t insert either type of RAM into the other’s slot. DDR SDRAM for laptops comes in either 200-pin SO-DIMMs or 172-pin micro-DIMMs (Figure 6-14).

Figure 6-13 DDR SDRAM

Figure 6-14 172-pin DDR SDRAM micro-DIMM (photo courtesy of Kingston/Joint Harvest)

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NOTE RAM makers use the term single data rate SDRAM (SDR SDRAM) for the original SDRAM to differentiate it from DDR SDRAM.

DDR sticks use a rather interesting naming convention actually started by the Rambus folks based on the number of bytes per second of data throughput the RAM can handle. To determine the bytes per second, take the MHz speed and multiply by 8 bytes (the width of all DDR SDRAM sticks). So 400 MHz multiplied by 8 is 3200 megabytes per second. Put the abbreviation “PC” in the front to make the new term: PC3200. Many techs also use the naming convention used for the individual DDR chips; for example, DDR400 refers to a 400-MHz DDR SDRAM chip running on a 200-MHz clock.

Even though the term DDRxxx is really just for individual DDR chips and the term PCxxxx is for DDR sticks, this tradition of two names for every speed of RAM is a bit of a challenge because you’ll often hear both terms used interchangeably. Table 6-1 shows all the speeds for DDR; not all of these are commonly used.

Following the lead of AMD, VIA, and other manufacturers, the PC industry adopted DDR SDRAM as the standard system RAM. In the summer of 2003, Intel relented and stopped producing motherboards and memory controllers that required RDRAM.

Table 6-1 DDR speeds

One thing is sure about PC technologies: Any good idea that can be copied will be copied. One of Rambus’ best concepts was the dual-channel architecture—using two sticks of RDRAM together to increase throughput. Manufacturers have released motherboards with MCCs that support dual-channel architecture using DDR SDRAM. Dual-channel DDR motherboards use regular DDR sticks, although manufacturers often sell RAM in matched pairs, branding them as dual-channel RAM.

Dual-channel DDR works like RDRAM in that you must have two identical sticks of DDR and they must snap into two paired slots. Unlike RDRAM, dual-channel DDR doesn’t have anything like CRIMMs; you don’t need to put anything into unused slot pairs. Dual-channel DDR technology is very flexible, but also has a few quirks that vary with each system. Some motherboards have three DDR SDRAM slots, but the dual-channel DDR works only if you install DDR SDRAM in two of the slots. Other boards have four slots and you must install matching pairs in the same colored slots to run in dual-channel mode (Figure 6-15). If you populate a third slot, the system uses the full capacity of RAM installed, but turns off the dual-channel feature.

Figure 6-15 A motherboard showing the four RAM slots. By populating the same color slots with identical RAM, you can run in dual-channel mode.

DDR2

The fastest versions of DDR RAM run at a blistering PC4800. That’s 4.8 gigabytes per second (GBps) of data throughput! You’d think that kind of speed would satisfy most users, and to be honest, DRAM running at approximately 5 GBps really is plenty fast for yesterday. However, the ongoing speed increases ensure that even these speeds won’t be good enough in the future. Knowing this, the RAM industry came out with DDR2, the successor to DDR. DDR2 is DDR RAM