X-bit labs

DDR vs. DDRII: Fight!

Category: Memory

by Victor Kartunov

[ 05/19/2004 | 11:30 AM ]

If you’re watching closely the events of the PC market, you should have noticed that the term “DDR2” has been used more frequently. So, I think it will be really interesting to know how this second generation differs from the previous one, what its advantages and shortcomings are. In other words, let’s try to figure out why the industry needs to take this memory type up.

Table of contents:

• SDRAM (Synchronous Dynamic Random-Access Memory)
  
• DDR (Double Data Rate SDRAM)
  
• DDR2
  
• Besides the Speed
  
• Conclusion
  

If you’re watching closely the events of the PC market, you should have noticed that the term “DDR2” has been used more frequently. As you know, it stands for the second generation of DDR SDRAM (Double Data Rate Synchronous Dynamic Random-Access memory, if you’re not in the know). Platforms with support of the new memory type are starting out in this year. In the next year DDR2 is going to become widespread (or even predominant) memory type on the PC.

So it makes sense to know beforehand how this second generation differs from the previous one, what its advantages and shortcomings, if any, are. In other words, let’s try to figure out why the industry needs to take this memory type up.

Current processors are just gobbling down each data chunk the main memory can supply them. There’s no end to this process as the performance of the CPU is only growing with time. Accordingly, the memory should provide more data per second for the processor to be well-fed. No achievable memory speed seems to be absolutely enough, but the transition to DDR2 is a way to higher memory bandwidths, which removes the problem for a while.

As you know, performance of any memory is calculated by the formula:

Speed = Width x Frequency,

where Speed is the memory performance (Mb/s), Width is the width of the memory bus (in bits), and Frequency is the frequency at which the data are being transferred (in megahertz).

Thus, to improve performance, we need to increase either the memory bus or its operational frequency. Or both. Let’s see what the industry can do today.

Although there are many variations of RAM, they are all based around the DRAM cell, which is in fact a combination of a transistor and a capacitor. There have been numerous attempts to discard this elementary cell as obsolete, offering other data storage technologies like MRAM (Magnetoresistive RAM), FRAM (Ferroelectric RAM) and others, but without much success. No other memory type can provide a similar combination of capacity, cost and speed as the good old DRAM does.

There are faster elementary cells (like Static RAM – SRAM), but they are much costlier and larger, so the memory chip cannot reach the same capacity. There are cheap alternatives, which are noticeably slower and thus cannot be used as the computer’s main memory. There are cheap and fast alternatives, but they serve for a very short period of time.