AM3 CPU question

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AM3 CPU question

Postposted on Wed Mar 31, 2010 4:08 pm

I'm doing a bit of reading up, mostly as a result of my last thread about 'e' AM3 processors. On wikipedia and bit-tech.net, it says that the memory controller can handle a maximum memory speed of DDR3-1333, is this correct? If so, what's the point in, for example with the board I typically use for system builds, (ASUS M4A78T-E) saying it can do DDR3-1600? Can the memory controller be overclocked? Is Asus just doing a bit of potential future-proofing, in case AMD produces an AM3 chip with a better memory controller?
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Re: AM3 CPU question

Postposted on Wed Mar 31, 2010 4:34 pm

Officially it's DDR3-1333, yes, but in practice it can handle 1600 just fine. The spec is more to do with what JEDEC had standardised on at the time of design than an electrical one.

Some have had issues using four slots all running at 1600 without relaxing timings, but I've never heard any issues running 1600 with 2 slots.

AMD's DDR3 platforms for next year are meant to be DDR3-1600, but I don't know if that's official.
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Re: AM3 CPU question

Postposted on Wed Mar 31, 2010 5:39 pm

It is still running the processor memory controller out of spec, so it is a form of overclocking. The performance benefits (or lack thereof) aside, it goes to the usual "YMMV" (your mileage may vary) pile.
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Re: AM3 CPU question

Postposted on Wed Mar 31, 2010 10:04 pm

It is really no different than what happened with DDR2. Official spec maxed out at 1066, but there were motherboards and DIMMs which advertised and/or allowed you to clock the memory bus higher than that.

IIRC (I could be mistaken) there were also motherboards advertising DDR2-1066 support when the official spec still topped out at 800.

This stuff isn't nearly as cut-and-dried as you might think. There's "what is the official spec today", there's "what is the official spec going to be a few months from now", there's "what are enthusiast-oriented vendors going to claim in their marketing literature", and there's "what are overclockers going to be able to achieve by tweaking their system". Depending on which one of these questions you're actually asking, you may come up with 4 different answers. :wink:
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Re: AM3 CPU question

Postposted on Thu Apr 01, 2010 3:52 am

mikeymike wrote:I'm doing a bit of reading up, mostly as a result of my last thread about 'e' AM3 processors. On wikipedia and bit-tech.net, it says that the memory controller can handle a maximum memory speed of DDR3-1333, is this correct? If so, what's the point in, for example with the board I typically use for system builds, (ASUS M4A78T-E) saying it can do DDR3-1600? Can the memory controller be overclocked? Is Asus just doing a bit of potential future-proofing, in case AMD produces an AM3 chip with a better memory controller?


That's a really good question Mikey... 8)

To be honest most good boards have problems with the 1600 Mem divider, it doesn't seem to be so much of a problem with the chips IMC, but more of a problem with Mobo Bios...
Any decent 790/890 board will do 1600 Mem clocks with tight timings, but alot of them don't seem to like the 1600 divider (unless you want to run them @ CL8 ) :-?

Set the Mem divider to 1333 and raise the HtRef clock to 240 (resulting in 1600 clocks) and most boards won't have a problem....

There does seem to be a limitation as to what the IMC can handle, but with a good board & Memory that limit is closer to 1800.

Just for reference, here's what my board does with a 1333 divider and and an overclocked HTRef (Stable):
Image

Phenom II's actually seem to like tighter Mem timings, I'm still working on that... 8)
My sticks will do CL6-7-6-18-24 @ 1400 and it seems to be better than 1600 7-8-7-24-33.
But like I said, it's still a work in progress... :wink:
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Re: AM3 CPU question

Postposted on Tue Apr 13, 2010 2:46 pm

My buddy's got his Asus 790FX Chipset with a 965BE (140w variant) which are supposed to cap out at 1333 when running all 4 slots filled. However, his hits 1600 no problems (Corsair Ram, Corsair PSU, Asus Motherboard). You'll notice that a lot of boards will list higher speeds for its ram, usually this denotes that you can merely overclock the memory controller on the CPU and that the motherboard can electrically handle those speeds. Also if you were to buy a compatible CPU later (that the BIOS recognizes) and its stock memory controller is 1600 then you should be able to run straight 1600's without much worry... again... as long as the motherboard's BIOS recognizes the DIMMS.
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