System RAM Comparison

– Jon Bach –

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Purpose of RAM

Even though many of you undoubtedly have a good understanding on the subject, this article has a natural beginning in a conceptual explanation of the function of RAM in your computer system. RAM acts as a buffer to speed up access to your programs and files that are in use. Though all of your data is stored on your hard drive, RAM is much much faster. It is capable of a much higher bandwidth, meaning that it can transfer more data at a faster rate, and of a much lower latency, meaning that the delay between the time the data is requested and when it is sent is much shorter. Whenever your computer pulls up a file off the hard drive, it loads it into your system RAM so that it can continue to access it faster. The end result is a much faster interface with your computer. The more RAM you have, the more programs and files your computer can keep stored in your RAM. That is why people who run multiple programs at one time often need more RAM — since more programs are being run at once, the computer needs more space in order to have them all ‘on hand’ at the same time.

SDRAM and a Problem

A very important aspect in the performance of a computer is the elimination of any ‘bottlenecks.’ A bottleneck is a component of your computer that is comparatively slow. For example, if you have a nice new computer system, but insist on using the hard drive you bought back in 1992, then your hard drive is most definitely a bottleneck. Your computer can work as hard and as fast as it wants, but any time it uses the hard drive, it has to just sit and wait as the comparatively slow hard drive does its job. For the last six years, SDRAM has been the standard type of RAM for consumer level computers. In this time, the RAM has not changed, while the speed of other components has been steadily rising. It should not be surprising, then, that SDRAM now presents a bottleneck to the rest of the computer system. The bandwidth of the ram is simply not large enough to accommodate the abilities of the rest of the system.

The Solution

While SDRAM has made one more kick with in increase from PC100 to PC133 (and even the rare PC150), this kick is most likely its final one. The obvious solution is to find an entirely new RAM design. The problem is bandwidth — we need more. In order to keep the RAM from being saturated and becoming a bottleneck, the RAM must be able to deliver to the rest of the system without causing the system to have to wait. Currently, there are two major designs competing for the new standard, and they are very different in their approach.

On one hand, we have Intel with RDRAM, an implementation that has been designed and marketed to go hand-in-hand with the Pentium 4 processor. On the other hand, we have AMD and VIA with DDR technology, which is most commonly seen with AMD Athlon processors.

Both implementations solve the bandwidth problem, and have competitively co-existed for almost a year now. However, both are aiming to become the new standard in consumer level computer systems, and it is expected that only one will stand the test of time.

The New Standard of Performance

Let’s begin with Intel and RDRAM. RDRAM is a totally new design, and its approach is to simply operate at a much higher frequency. While PC100 SDRAM runs at 100MHz, RDRAM runs at a much faster 400MHz. Adding to this impressive operating rate, RDRAM also reads and writes at both the rising and falling edge of a clock cycle, while SDRAM writes only once per clock cycle. Now, this does not mean that it is capable of eight times the bandwidth. To achieve the higher frequency, RDRAM runs on a 16-bit bus (while SDRAM is 64-bit). After all the dust clears and we look at raw performance, we see that RDRAM is capable of roughly twice the bandwidth as SDRAM.

AMD and VIA have taken a very different approach with DDR technology. While RDRAM represents an entirely new memory technology, DDR-SDRAM is nothing more than an evolution of the existing SDRAM technology. Like RDRAM, DDR transfers data on both the rising and falling edges of a clock cycle. Its name is very representative of what it is — Double Data Rate SDRAM. Since it is otherwise nearly the same as SDRAM (same frequency, same bus width), it is easy math — DDR-SDRAM is capable of twice the bandwidth as the old SDRAM.


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