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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darkdemyze - Tuesday, May 23, 2006 - link
z-ram isn't due for AMD procs for quite some time, I doubt this is their plan for June..mlittl3 - Tuesday, May 23, 2006 - link
Basically this is what I said above for my guess of the "trick" AMD will use. Anand said it will only affect some high-end users, read FX series so it can't be price cuts as some have suggested (that would effect everyone). Adding L3 cache is the only performance improvement I can think of that doesn't require changing the microarchitecture of the cores (well at least not a big change).However, TDP is still an issue here as someone above suggested. I don't know how much more power it takes to run L3 cache. Last time AMD did it was on K6 and power wasn't really measured back then.
By the way, please ignore Questar's comment below about z-ram being pig slow. I really don't think he knows what he is talking about. /shields eyes from incoming Questar flame
johnsonx - Wednesday, May 24, 2006 - link
K6-III did not have L3 cache. It had L2 cache, making the cache that all socket-7 boards had then an L3 cache.So, let's stop saying things like 'AMD hasn't done L3 cache since K6-III', etc.
mino - Wednesday, May 24, 2006 - link
Well, IMHO the point is AMD has used exclusive 3-level cache structure in the past so they have som experience with thi arrangement.Questar - Tuesday, May 23, 2006 - link
No flame here, look it up for yourself.Z-RAM has high capacitive loading, which results in slow speed.
At 4MB it'll run half the speed of SRAM.
Questar - Tuesday, May 23, 2006 - link
Large amounts of Z-RAM are pig slow.Ecmaster76 - Tuesday, May 23, 2006 - link
Seriously, the one area an Athlon X2 would be bandwidth starved and does it get tested in the preview? NOIn the review? NO
How long ago did we know that the K8 was not bandwidth limited in single application usage? YEARS
So yeah, DDR2 din't increase the 3dMark, big surprise
mlittl3 - Tuesday, May 23, 2006 - link
I think 3dMark06 is multithreaded now so all available cores and bandwidth should be used within the limits of the program. I could be wrong about this however.Ecmaster76 - Tuesday, May 23, 2006 - link
3Dmark06 is almost completely GPU limited. The 3Dmark CPU score did increase a bit, but I really was referring to graphics benchmarks in general.cscpianoman - Tuesday, May 23, 2006 - link
I was just noticing the performance differences between the FX and the EE. In some cases the FX tromps the EE by "gasp" 30%! In other cases the EE makes it's mark. This is part of the reason I am skeptic on Conroe. Yeah it's good. But I always take what Intel, or AMD for that matter, with a grain of salt. Just today we saw the 30% advantage translate down to about 15%. This seems just like any other generation change where 15% is to be expected. The current hype for the Conroe is a product of Intel's excellent marketing dept.