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DRAM Technologies and Relative Performance

DRAM technology has unfortunately become an exercise in buzzword juggling. It can be difficult to determine what kind you need to use in your system. And trying to understand what the "next big thing" will be in the industry is getting to be virtually impossible.

The most important factor that controls the type of memory that can be used in a motherboard is the system chipset. In fact, chipset support can be the difference between a technology being rapidly and universally accepted (i.e., EDO) and one lingering with little support and never becoming a popular standard (i.e., BEDO). The documentation for the motherboard will normally indicate what types of memory are supported by the board.

In general, you want to use the fastest type of memory in the board that it supports, provided that it is not prohibitively expensive. Some people waste money buying memory that is a few percentage points faster than another type, when they could be improving performance a more efficient way (such as buying more memory). Some boards support different types of memory and you can usually use any kind that is compatible. Of course you will want to make your decision to buy a newer or faster technology type based on its incremental cost as well. Certain newer technologies can cost significantly more than older ones and offer only marginal performance improvements.

There are two reasons to be wary of claims that new memory technology will greatly improve performance. The first is that the use of a secondary cache will hide much of the benefit of faster memory, since only 5-10% of the data requirements of the processor are typically supplied from the memory itself. The second is that with most PCs now running in burst mode, you must look at the total number of clock cycles required for the full 4-cycle burst, not just the ideal maximum transfer rate.

Here's an example of a commonly-made comparison from the Pentium world. For all of 1996, the two most popular Pentium chipsets were the 430HX and 430VX chipsets from Intel (confusingly, both sometimes called "Triton II"). The 430VX supports SDRAM while the 430HX does not. Many people said that the VX was superior for this reason, since SDRAM can transfer memory in 1 clock cycle ideally, while the 430HX must use EDO, with a 2 clock cycle ideal transfer rate. SDRAM provides an improvement of 50%! Well, not really. Because when you look at the full burst timing, you see the VX's timing for SDRAM is 7-1-1-1, and the HX's for EDO is 5-2-2-2. When you add up the clock cycles, the VX comes to 10 and the HX 11. When you add in the HX's other improved features compared to the VX, the HX is actually considered by many to be faster than the VX even using slower EDO memory. (Intel's 430TX changed this whole equation when it was introduced since it supports SDRAM at 5-1-1-1).

Finally, using a newer technology type will often give you more flexibility for future upgrades, since you may be able to transfer the memory more easily to a newer motherboard. However, this is not always the case, since often, new motherboards require a new type anyway. Also, sometimes new technologies don't catch on (for example, BEDO). The "latest and greatest" is of course often a lot more money.

Next: Conventional DRAM

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