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Socket-AM2, AM2+ and AM3: Backwards Compatibility

AMD fixed the cache size issue, it fixed the power consumption problem, and we even got higher clock speeds with Phenom II. What I didn’t expect was something more. AMD has always been a manufacturer for the customers. Over the past couple of years the problem has been that their processors haven’t really been desired by consumers, but prior to that the AMD that we know and love designed processors for today’s applications with a minimal number of platform changes between processors.

Phenom II carries AMD’s consumer focused nature to the next level. Today’s Phenom II parts are designed for Socket-AM2+ motherboards. AMD doesn’t qualify any of them for use on Socket-AM2 motherboards, but there’s nothing stopping a motherboard maker from enabling support on a standard AM2 motherboard. You will need a BIOS update.

Next month, AMD will launch the first Socket-AM3 Phenom II processors. The main difference here is that these parts will support DDR3 memory. Oh no, another socket, right? Wrong.

Socket-AM3 Phenom II parts will also work in Socket-AM2+ motherboards, the two are pin-compatible. When in an AM2+ board, these upcoming Phenom II processors will work in DDR2 mode, but when in an AM3 board they will work in DDR3 mode. How cool is that?

This unique flexibility is largely due to the work that was done on the DDR2 and DDR3 specs at JEDEC. The number of signaling pins and the signaling pins themselves between DDR2 and DDR3 don’t actually change on the memory controller side; the main differences are routing and termination at the memory socket side. AMD just needed a physical memory interface on Phenom II that could operate at both 1.8V (DDR2) and 1.5V (DDR3) as well as work with timings for either memory technology. The potential was there to do this on the first Phenom, it just wasn’t ready in time, but with the Socket-AM3 Phenom II processors you’ll be able to do it.

While I’m not sure how practically useful the AM3/AM2+ flexibility will be, I’d rather have it than not. Being able to take one CPU and stick it in two different sockets, each with a different memory technology, and have it just work is the most customer-centric move I’ve ever seen either company make. AMD told me that this plan was in the works before the original Phenom ever launched, somewhere in the 2004 timeframe. AMD was active in JEDEC on making the DDR2 and DDR3 specs similar enough that this one-CPU, two-sockets approach could work.

One of the biggest risks AMD faced when it chose to integrate the memory controller was what would happen if there was a sudden shift in memory technology. With the upcoming Socket-AM3 versions of Phenom II, that risk is completely mitigated by the fact that a single chip can work with either memory technology. It gives OEMs a tremendous amount of flexibility to ship systems with either DDR2 or DDR3 memory depending on which is more cost effective. It also ensures a much smoother transition to DDR3.

The downside for AMD is that because Socket-AM3 Phenom II chips are right around the corner, it makes little sense to buy one of these Socket-AM2+ Phenom II processors - at least not until we know the pricing and availability of the Socket-AM3 versions.

Slower North Bridge Frequency for AM2+, Faster when AM3 Arrives

An extra benefit of the Socket-AM3 Phenom II processors is that their uncore (memory controller + L3 cache) will be clocked at 2.0GHz instead of 1.8GHz like the two processors launching today. By comparison the Phenom 9850 and 9950 both have a 2.0GHz uncore clock; AMD had to go down to 1.8GHz to launch the Phenom II at 2.8GHz and 3.0GHz today.

As 45nm yields improve AMD will increase the uncore frequency, but today it's at 1.8GHz and the AM3 chips will have it at 2.0GHz. The Core i7 runs its uncore at 2.13GHz for the 920 and 940, and 2.66GHz for the 965.