AMD: Still in the Game

The AMD Memory Roadmap: DDR3, FBD and G3MX Examined

With today's announcement we can finally talk about AMD's memory roadmap; the two questions we often hear are: when is AMD planning on moving to DDR3 and what about Fully Buffered DIMM? Both are answered with today's disclosure.

AMD will begin the DDR3 transition on the desktop in the second half of 2008 with its Shanghai processor, but AMD won't fully move to DDR3 until 2009 with Bulldozer. The DDR3 transition beginning with Shanghai and completing with Bulldozer is very similar to the cautious approach AMD took to DDR2 adoption. By the time 2009 rolls around, DDR3 should be very cost competitive with DDR2 and the transition should be seamless.

Despite rumors to the contrary, Bulldozer won't support Intel's Fully Buffered DIMM standard, and instead will use what AMD is calling the "G3 Memory Extender" (G3MX).

FBD addresses the problem of not being able to maintain memory frequency while increasing the number of memory sockets on a motherboard, something that impacts the high end server market. The FBD solution is to serialize the memory bus by placing a buffer chip on each memory module that communicates with the memory controller and memory devices. The memory controller only needs to worry about driving data to these buffers, and the buffers deal with getting data in/out of the memory devices.

While FBD supports up to 8 DIMMs per memory channel, there are three major drawbacks: 1) higher cost per module, 2) higher latencies due to serialization and 3) higher power consumption. AMD has said that it evaluates new memory technologies at each generation, and although it won't rule out FBD for future products, it simply doesn't make sense today.

G3MX addresses the same issue of maintaining memory performance while driving up the number of slots per channel in a more economical manner. The technology simply calls for custom, ASIC-class, buffer logic to be placed on the motherboard itself between the memory controller (in this case the CPU) and the memory slots. There is no conversion of memory interface and thus performance/power shouldn't be impacted nearly as much as FBD, and the big upside is that you can use standard DDR3 memory in the sockets since G3MX is implemented at a motherboard level.

The downside is that with G3MX you are still dealing with a parallel memory interface, which becomes difficult to implement at higher speeds and loads. AMD insists that it can work around any issues related to motherboard design and trace routing, and that G3MX is presently a better solution than FBD for its needs.

The first G3MX implementation will arrive with Bulldozer in 2009; like FBD, it will be limited to high end server/workstation platforms.