X-bit labs

AMD: Per Aspera Ad Astra (page 5)

Category: CPU

by Victor Kartunov

[ 03/31/2004 | 06:54 AM ]

Memories of the Future

To my opinion, Intel will do its utmost to make DDR2 memory popular. Intel’s efforts usually materialize (the old story with Rambus is an exception that only confirms the rule) and AMD must have some plan of actions in case this memory does become widespread.

Let’s first examine what DDR2 memory is. As you know, each memory type can be characterized by a couple of parameters: access latencies and bandwidth. It’s all right with bandwidth – DDR2 has the same bandwidth as DDR (per megahertz) while DDR2 will gain frequency easier. Moreover, DDR2 brings some purely technological advantages like on-chip termination, which help with the PCB wiring.

The latencies are another matter. DDR2 has high latencies. It seems like this memory type was specifically designed to negate the basic advantage of the Athlon 64 architecture, its low latencies. Well, there’s no conspiracy against AMD, of course. There’re no other ways to increase the performance of dynamic memory left. It’s only possible to slowly increase its frequency as the constantly improving technological process allows. But as the elementary memory cell hasn’t principally changed, we can’t hope for any breakthroughs in this front. Alas, the laws of physics are inexorable. Until the structure of the elementary memory cell changes, we’ll have no revolution in the PC memory field. Thus, the main direction to improving the memory performance is in speeding up the channel between the memory controller and the memory chip. In other words, the data-transfer rate and the peak bandwidth of a memory module grow, while the relatively low speed of the memory cell itself is compensated by various techniques like phase shift when accessing different cells.

Let’s view this in numbers. The typical timings combination for DDR400 memory of average quality is now 2.5-3-3 or 3-3-3. In other words, the access latency is 12.5 or 15 nanoseconds. Typical DDR2-400 available at the moment works according to the 4-4-4 scheme (20 nanoseconds for access). That’s the progress (with a minus sign) they are promising to us!

Once again, DDR2 has no advantages over DDR in bandwidth if their frequencies are equal. DDR2 has higher latencies than the previous standard. DDR2 has on-chip termination and heats up more than DDR (you may recall DDR2 chips on graphics cards: unlike DDR chips of the same frequency, they have to be covered with heatsinks). It’s no wonder then that Intel got busy with a new system case form-factor, BTX, as we’ll have another heat source in the system. The only advantage of DDR2 over DDR is its being able to reach higher frequencies, although DDR has already notched 550-560MHz (so far, only in overclocker modules). Well, DDR2 will grow to 800MHz in the future, and DDR won’t get that high, that’s sure. On the other hand, we won’t see those 800MHz soon.

As for the price factor, DDR2 memory will be expensive. Right now, the manufacturers promise a price proportion of 2:1. In other words, the user who supports “progressive technologies” will have to pay twice the money he would spend for the same amount of DDR SDRAM. I think there’ll be only one winning side – the memory makers who are welcoming and supporting this transition.