AMD Socket-AM2: Same Performance, Faster Memory, Lower Power
by Anand Lal Shimpi on May 23, 2006 12:14 PM EST- Posted in
- CPUs
The Odd Multiplier Issue
Another item that was working against the Athlon 64 X2 5000+ on the previous page was the fact that it used an odd clock multiplier, in this case 13.0, in order to achieve its 2.6GHz clock speed. The problem with odd clock multipliers on AM2 CPUs is that the memory controller actually runs at DDR2-742 instead of DDR2-800. AM2 CPUs with even clock multipliers can run at DDR2-800 without any problems, and the reason why is pretty simple.
Below is the equation for calculating the memory speed of any Athlon 64 processor:
Reference Clock * Clock Multiplier = CPU Frequency
CPU Frequency / Memory Divisor = Memory Frequency
AMD only supports integer memory divisors, but let's start out by looking at how an AM2 CPU with an even clock multiplier fits the equation. For example, an Athlon 64 X2 4800+ runs at 2.4GHz and supports DDR2-800.
200MHz Reference Clock * 12x Clock Multiplier = 2400MHz CPU Frequency
2400MHz CPU Frequency / 6 = 400MHz DDR2-800 Memory Frequency
No problems, right? Now let's see how an odd clock multiplier changes things:
200MHz Reference Clock * 13x Clock Muliplier = 2600MHz CPU Frequency
2600MHz CPU Frequency / 6 = 433MHz DDR2-866 Memory Frequency
2600MHz CPU Frequency / 7 = 371MHz DDR2-742 Memory Frequency
See a problem? Because we can only use integer memory dividers, the only options for memory speed on a CPU with an odd clock multiplier are DDR2-866 or DDR2-742. Since AMD can't run above DDR2-800 spec, the only option is to underclock the memory to DDR2-742. This wasn't a problem on Socket-939 CPUs because DDR-400 ran at a 200MHz frequency, which you could always obtain by dividing the CPU clock frequency by an integer (since AMD never supported half multipliers). In fact, you simply used the same integer as the CPU multiplier. With DDR2-800, you need a 400MHz clock frequency, which you can only generate if you have an even CPU clock multiplier.
The problem gets even more complicated when you take into account the fact that Semprons and single-core Athlon 64s only support DDR2-667, which also has a similar issue.
While we haven't seen any significant downside to only running at DDR2-742 vs. DDR2-800, it is something to keep in mind when deciding what CPU to purchase. If you want your memory controller running at DDR2-800, you may want to stay away from the odd clock multiplier CPUs (X2 5000+, 4400+ and 4200+).
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mlittl3 - Tuesday, May 23, 2006 - link
Until K8L (which will have microarchitectural improvements), there are a few things that could allow AMD to look good against Core 2.1) Price drops so that comparable K8 and Core 2 processors are the same price giving the same performance/$ ratio (this metric is important for the budget constrained)
2) Nov. '06 release of 65 nm AM2 processors so that K8 and Core 2 processors will have the same performance/watt ratio (65 nm could give K8 a 20% drop in power and with Core 2 being 20% faster, they will have the same performance/watt ratio which is popular now). Also, everytime AMD transitions to a new die process they add some minor bug fixes and minor memory/microarchitectural enhancements which could also boost performance by a few percent.
3) Continual improvements to DDR2 latency might yield a 2-2-2 DDR2 800 memory module which will probably benefit K8 more (maybe ~5% improvement) than Core 2 but this is a wild guess here and I don't know if it is even possible. However, DDR400 latency started around 4-4-4 and dropped to 2-2-2 so it could happen.
With the same performance/price and performance/watt as Core 2, K8 could stay competitive and OEMS and users decide on which company (if not both) they would like to do business with. This is all speculation and of course everyone is more than welcome to rip my reasoning to death.
JarredWalton - Tuesday, May 23, 2006 - link
I seriously doubt we will ever see DDR2-800 running at 2-2-2 timings. (Feel free to quote me on this in the future and make fun of me if I'm proven wrong. :-)) Just think how long we had DDR memory around, and no one ever managed to create 1-1-1 DDR-400 memory. I do think we will see 3-3-3 DDR2-800, and possibly even higher bandwidth with those timings. In fact, we almost have that already judging from my experiences so far with socket AM2. (I can post and run benchmarks, but I wouldn't call the system 100% stable.)mlittl3 - Tuesday, May 23, 2006 - link
LOL! I will!MacGuffin - Tuesday, May 23, 2006 - link
From Patriot's PDC22G8000+XBLK Rev. 2 review on PCSTATS.Rated for DDR2-667 @ 3-3-3-9 (Maintains those timings through DDR2-940!)
Rated for DDR2-1000 @ 4-4-4-12 (Goes Up to DDR2-1020!)
Completely stable on the Intel platform they used. It's extremely expensive (saw it for $400+ at NewEgg). But yes, it is possible to run 2GB at these timings already. Its just extremely expensive.
EdisonStarfire - Tuesday, May 23, 2006 - link
any opinions on AMD offering a Clearspeed solution as stop-gap in the high end desktop arena ?Griswold - Tuesday, May 23, 2006 - link
The bottom line is, we now know what we knew last fall, or rather (rightfully) assumed.Now you made me curious. Could that be the "noise in june" which Henri Richards mentioned in a Register interview earlier this month?
smn198 - Wednesday, May 24, 2006 - link
It is called quad-core.temp2 - Tuesday, May 23, 2006 - link
The extremetech.com article has a similar teaser at the end, but it is slightly more specific:
"And given recent discussions with AMD, we can safely say that the company hasn't launched its last FX series CPU for the year quite yet."
mino - Wednesday, May 24, 2006 - link
This provided, 3.2 or even 3.4 FX's on 65nm are on the way...Scrogneugneu - Tuesday, May 23, 2006 - link
Beware the mighty Sempron FX 32 !