Thanks for the memory

The impact of memory capacity on PC performance is well known. But what's not so well known is how current memory technologies are coping with ever-faster processors. Will increasing the memory in higher-clocked PCs solve everything, or is there something wrong with our Ram?

In terms of memory bandwidth, processors currently don't come any hungrier than the Pentium 4. The buses on the P4A and P4B models are capable of handling 3.2GB and 4.2GB of data every second, and the memory must keep up.

Check the specifications of most PCs and you'll see that the dominant memory technology is DDR, running at either 266 or 333MHz. These are also known as PC2100 and PC2700, as they deliver 2.1GB and 2.7GB respectively.

While sufficient for the buses of current Athlon XP processors, they're well below what the P4 is capable of.

When Intel first launched the Pentium 4 there was only one chipset available, designed exclusively for RD-Ram memory. Intel's theory certainly made sense, as dual-channel PC800 RD-Ram was the only memory that could match the 3.2GB bus of the original P4.

Sadly at the time RD-Ram was expensive, and the first P4s weren't delivering decent scores, so it's no wonder many people went for Athlon systems instead.

A few months ago Intel released an improved Pentium 4 core which, coupled with fast clock speeds, revealed what the platform was capable of. Better still, when accompanied by new chipsets supporting cheap DDR memory, the P4 became affordable.

The only problem was that people had forgotten why Intel had recommended RD-Ram. DDR may have given the P4 a new lease of life, but popular memory technology was holding it back.

While the P4 was making friends with DDR though, a faster type of RD-Ram running at 1,066MHz was announced. With Intel's 850E chipset, this so-called PC1066 RD-Ram matched the P4B's increased bus bandwidth of 4.2GB.

At June's Computex show in Taiwan, a multitude of 850E motherboards running 1,066MHz RD-Ram were demonstrated. The one which caught my eye was at the Asus stand. It was showing support for another new type of RD-Ram memory called PC4200, which also ran at 1,066MHz.

PC1066 RD-Ram, like its RD-Ram predecessors, is a 16bit technology where the memory cards need to be installed in pairs. PC4200 is also RD-Ram running at 1,066MHz, but it's a 32bit technology where cards can be installed singly.

A single PC4200 Rimm is just a pair of PC1066 Rimms back to back, but this doesn't give you any greater flexibility as most PC1066 boards have four memory slots, while the only PC4200 board so far has just two.

As far as I'm concerned, releasing these two types of RD-Ram so close to each other will do Rambus no favours. No memory supplier I spoke to could say which was going to be cheaper or outlive the other - an important question as both types of memory employ physically incompatible interfaces.

So why would anyone take the risk of RD-Ram over DDR for their new P4 system? How about because it significantly outperforms DDR, and could be more affordable than you think. At the time of writing, you could buy 256MB of PC2700 DDR for £60 or 256MB of PC1066 RD-Ram for £105.

I built two systems using a 2.4GHz P4B. The one with PC1066 RD-Ram scored 10 per cent higher in Sysmark 2002 than the PC2700 system. The differences became more pronounced as clock speeds increased, proving that RD-Ram really is the only technology to make the most out of the fastest Pentium 4s.

A Sysmark increase of 10 per cent may not sound like much, but it's as significant as selecting a CPU clocked one or two notches higher. The difference between the top two and three P4s can run into hundreds of pounds, which is more than selecting RD-Ram and a compatible motherboard.

So clearly RD-Ram is the most sensible choice for high-end P4 systems, but does it have a future on desktop PCs?

Intel never comments on unannounced products, but it's a safe bet that the 850E will be its last RD-Ram chipset; even the forthcoming alternative from Sis doesn't offer anything new.

Leaked chipset roadmaps suggest that next year Intel will probably ditch RD-Ram in favour of dual-channel DDR solutions, doubling the bandwidth of existing memory.

Memory lapse
Dual-channel PC2100 would deliver 4.2GB, while dual-channel PC3200 (400MHz DDR) could handle 6.4GB. Even single-channel 400MHz DDR systems could be a good match for P4A processors, and this memory is due imminently.

I'd say RD-Ram will be almost forgotten in a year for desktop PC environments. In the long term, DDR looks unbeatable, especially in dual-channel configurations.

That said, if I were prepared to pay the premium for the fastest P4B processor now, I would go for 1,066MHz RD-Ram.

It's a false economy to spend so much on a CPU and to then skimp on memory. RD-Ram may become rare or expensive but, if you go for 512MB today, you'll be covered. As to PC1066 versus PC4200, I'd go for the cheapest on the day, as they appear to perform identically.

The bottom line is that, until faster DDR solutions arrive, 1,066MHz RD-Ram is the only memory that can do justice to the P4B processors. And, since it's affordable, it gets my vote.