AMD AM2: More than just a Memory Change

The new AMD technology refresh, now called AM2, will bring DDR2 memory to the Athlon64 on-processor memory controller. Many in the industry have speculated about the impact of this low latency memory controller on DDR2 performance, which to this point has suffered under the impact of the higher latency Intel Netburst architecture. We are looking forward to the opportunity to take a closer look at DDR2 performance on AM2 - which is everyone's big question.

There will be more than just new memory with AM2, however. We now have details on the new Socket 940 for AM2. It has been widely reported that the new Socket 940 will not be compatible with the existing Socket 940 used for Opteron and early Athlon 64.

Photos of the back of the new AM2 processor show how very close the new design is to the current Socket 940.

AM2 Processor

Current Socket 940 (Reversed)

By flipping the current 940 socket photo and comparing pin-outs to the back of the AM2, you can clearly see the top 2 "lands", areas without pins, are identical in the current Socket 940 and the AM2 Socket 940. However, the bottom two lands are in different locations. The bottom right land is the same 2 pins up, but 7pins form the edge instead of the 5 in the current 940 design. The left land is also in the same row position as the current 940, but it is 6 pins from the left edge instead of 8 as in the current 940 design.

The AM2 940 and current Opteron 940 are so close in design; you have to conclude that AMD could have used the same socket design. But there are numerous electrical differences and different memory is required. AMD apparently used a revised Socket 940 to prevent any possibility that AM2 chips might get accidentally mounted in current Socket 940 motherboards. That is probably a very wise decision.

New AM2 Cage

With the two 940 sockets so similar you would reasonably expect existing heatsink/fans to work on the new AM2.

After all AMD has maintained the same "cage" design though 754, 940, and 939 sockets. The same heatsink/fan can be mounted on any of these sockets without concern about compatibility.. Those with large and exotic cooling solutions have other concerns about whether motherboard layout may block their cooling device, but overall there has been one HSF design through all the Athlon 64 socket designs.

Click to enlarge.

It appears AM2 will change this ongoing HSF compatibility with a brand new, and mostly incompatible, heatsink cage design. The new AM2 design, if it makes it to final production, is a nice upgrade to the existing Athlon 64 HSF mounting design, but there are almost no current Athlon 64 HSFs that will fit the new design.

If you look at the designs side-by-side you can see the differences more clearly.

Socket 939/940/754 HSF Cage

AM2 HSF Cage

While the spacing width of the center mounting lug is the same on both the current cage and the AM2 version, that's about all that is the same. Most current basic designs connect with the center lug and have push down cam locks that lock onto the extra lug near the cage corners. There are no extra lugs on the new AM2 cage, so designs that require cam locking won't work. Neither will most of the "step-up" 3 lug mounting clips. AM2 only has one lug per side, and the 3-lug clip is normally intruding on the new screw locations (just a little too wide) to mount on the new AM2 cage.

In addition the current high-end designs, which replace the existing cage with a new mounting system just simply won't work. The current Athlon 64 design uses one screw on each side in the center of the mounting lug. The new AM2 design uses four corner mounted screws - more secure but not useful for mounting current 2 screw mounting schemes.

Current Athlon 64 heatsinks that use a single lug for mounting and no cam for locking will likely work on the new AM2 cage. Recently shipped AMD retail heatsinks have a single lug connection and a lever lock without the locking cam. Those should work fine on the new AM2. However, not a single third party HSF in our lab - the kinds our readers most likley use - would mount properly on the new AM2 cage.

Bottom Line

The upcoming move to AMD Athlon64 AM2 will introduce a new Socket 940 design. The new Socket is very close in design to the current 940 design used on Opteron motherboards. It appears the pin-outs have been changed just enough to prevent the new AM2 processors from accidentally being plugged into older Socket 940 boards.

It was already clear the new CPU would require DDR2 memory, but it appears AMD is also using this architecture change to upgrade the current mounting scheme for Athlon 64 heatsink/fans. The new design is certainly very secure - more stable than the current design - but there are very few existing heatsink/fans that will work on the new cage. From Socket 754, 940, and 939 the same heatsinks have all worked on all Athlon 64 sockets. With AM2 that compatibility basically ends.

For those buying a retail CPU with the HSF this news won't matter at all. For enthusiasts that have invested in more exotic air, water or phase-change cooling solutions for Athlon 64 this could be important news.

Heatsink, water-cooling, and phase-change designers can make a few changes to existing designs that will allow their products to work with both the new AM2 design and the existing A64 754/940/939 design. Keep in mind, though, that it is not likely your current favorite cooling solution for Athlon 64 can go with you to the new AM2 platform.

Speculation is still circulating on exactly when the new AM2 will be launched. Several weeks ago, it appeared likely an AM2 launch would happen in the Cebit (early March) to end-of-April time frame. Announcements of delays the last few weeks now point to a July release date. Whenever the launch happens, those moving to AM2 will at a minimum need a new processor for a new Socket 940, new DDR2 memory to replace existing DDR memory for Athlon 64, and a new or revised heatsink/fan cooling solution.