Hardware

Use DDR SDRAM to double the speed of your RAM

RAM is RAM, right? Well, not necessarily. With the new DDR SDRAM standard hitting the market, there is a lot to learn about how this new chip works. In this Daily Feature, Steven Pittsley provides an overview.


Would you like to double the speed at which your computer accesses the information stored in the system's RAM? Sure you would. Double Data Rate (DDR) SDRAM will technically double the speed of the system's RAM, boosting the performance of today's memory-intensive applications such as those used for video editing, photo publishing, databases, and games. In this Daily Feature, I will provide you with a brief overview of SDRAM and then show you how DDR SDRAM will make your system run faster by accessing memory at twice the speed of the memory bus.

A brief review of SDRAM
Random access memory (RAM) suffers from two problems that are familiar to those in the computer industry: limited bandwidth and performance latency. When discussing RAM, bandwidth refers to the amount of information that can be written to or read from the memory. The faster that you can perform these functions, the faster your system will run. RAM also suffers from latency, which is the amount of time that it takes for a memory request to be fulfilled. As you might surmise, increasing bandwidth will lower latency, making your system run faster.

The current industry standard for RAM that is in use is called synchronous dynamic random access memory, or SDRAM. The term synchronous is used because SDRAM is tied, or synchronized, to the system clock. For example, if the CPU wants a piece of data that is stored in memory, it sends a request to the SDRAM, waits for a certain number of clock ticks, and then retrieves the data when it has been located. Because SDRAM is tied to the system clock, the CPU will not waste time trying to access the data before it has been found.

To combat the bandwidth and latency issues, SDRAM uses the divide-and-conquer theory, breaking up the RAM into two banks of memory. While one bank is servicing a request, the other bank will receive another request. Because this process is synchronized to the system clock, each action is performed on the tick of the system clock.

Before we move on to the newest memory technology, let’s take a moment to discuss how RAM is packaged. SDRAM modules for today's computer systems are commonly found in a 168-pin dual inline memory module, or DIMM package. However, DIMMs are not the only format that SDRAM can be packaged in. Laptop computers use the small outline DIMM, or SO-DIMM. In addition, other peripheral devices, such as printers, can use SDRAM modules that come in different formats.

DDR changes the face of SDRAM
DDR SDRAM is somewhat similar to regular SDRAM. Both will break the RAM into smaller chunks for simultaneous, synchronized request-and-reply access. In addition, both types of memory can be packaged in DIMM modules. However, DDR SDRAM will perform the alternating request-and-reply rhythm on both the rise and fall of the clock cycle. This method effectively doubles the bandwidth available and increases the speed the system can access data in memory.

DDR SDRAM is currently offered in two speeds: PC1600, which is used with a 100-MHz memory bus and PC2100, which will work with a 133-MHz memory bus. Because DDR SDRAM effectively doubles the speed of memory access, the stated memory bus speeds will be doubled, making the 100-MHz bus a 200-MHz bus, and the 133-MHz bus a 266-MHz bus.

Although DDR SDRAM modules will be called DIMMs, they will use a new 184-pin interface. Thus, to use DDR SDRAM, you must have a motherboard that has a DDR SDRAM DIMM interface. If you've recently purchased a new system, this is not good news because you won't be able to take advantage of DDR SDRAM until you upgrade both the motherboard and memory. The good news, however, is DDR SDRAM is very comparable in price with standard SDRAM. Should you decide to bite the bullet and upgrade your motherboard, you won’t go broke buying the latest and greatest SDRAM for the new system. In addition, as DDR SDRAM becomes more widely used, the prices should continue to fall.

A word of caution about DDR SDRAM
Before you rush out and buy a new DDR SDRAM-compatible motherboard loaded with 256 MB of memory, you should realize that although the SDRAM will be faster, today's current processors were not designed with DDR in mind. Because memory access has long been a limiting factor in system performance due to low bandwidth and high latency, CPU manufacturers designed their processors to help compensate for the bottleneck. The result is that the effects of DDR SDRAM will not be dramatic until it is coupled with a CPU that will take advantage of the new memory access technology. Luckily for us, that will not take very long to happen.

Conclusion
The many positive features of DDR SDRAM far outweigh the negatives, even after you get past the fact you will need a new motherboard for them to work. However, the processor manufacturers need to begin making CPUs that take advantage of DDR SDRAM capabilities before you see a significant increase in your system’s performance. When that day arrives, the increased system performance will certainly have a major impact on your future hardware purchases: You really will get double the speed without doubling the price.

Editorial disclaimer
The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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