AMD's Phenom II AM3 has memory problems

No more than two DDR3-1333 Modules allowed.

Published Thu, Feb 12 2009 6:44 PM CST   |   Updated Tue, Nov 3 2020 12:37 PM CST
*** Edit 12 February***

It seems that AMD's AM3 Phenom II has a memory problem. The problem rears its ugly head when you have more than one DDR3-1333 (or greater) module per channel. If you have all four slots on your new AM3 board filled you could encounter "unreliable operation".

The Phenom II's memory controller has two 64-bit wide memory channels that when combined (in ganged mode) offer an effective 128-bit wide controller. On most mainboards this means you have four memory slots (2 slots per channel). For the time being AMD is not recommending using more than 2 modules (one per channel) for any DDR3 clocked at 1333 or above. If you have more than 2 modules the AMD quick fix is to change the speed to 1066 and adjust the timings to help overcome the speed loss.

It seems that the root cause of the issue may be linked to DRAM voltage.
It is possible that by slightly overvolting the RAM you may be able to overcome this issue. AMD is working on a more permanent fix already.


***I spoke with Damon Muzny at AMD about this and it is not actually a problem. This behavior is actually by design. The reason is that AMD could not predict the behavior of all DDR3-1333 memory. They found that with certain lower quality memory modules and all four slots populated (2 modules per channel) they encountered instability. They were faced with a choice; either they could drop listing support for DDR3 - 1333 or they could design the system to down clock the memory to 1066 and recommend it to everyone using 2 modules per channel.

You have probably guessed what they chose.

In all honesty you can usually get more performance from a lower clock and tighter latency than a high clock and loose latency. Or you could always bump up the memory voltage and bring stability back that way.

AMD's Phenom II AM3 has memory problems

The AMD K10 memory controllers on AM3-socket processor provide a 128-bit wide memory interface (with DRAM Ganged mode enabled), which amount to two 64-bit wide memory channels. On most motherboards, four DIMM slots with two slots sharing a memory channel are present. With this issue, one is not recommended to use more than one DDR3-1333 (or beyond) memory module per channel. This issue however, does not affect systems with a module per channel (two modules in all). AMD recommends a quick fix for the issue for systems using more than one DDR3-1333 module per memory channel: to manually specify the memory to run at 533 MHz (1066 MHz DDR), and accordingly set DRAM timings. As a little compensation, one can tighten DRAM timings with the drop in frequency. AMD will fix this issue in the next stepping (sub-version) of the CPUs.

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