Basic HTT/ RAM O/C question for A64

[abyssalloris]

My understanding of these things is limited. Besides, since what I'm asking about is old news, it is hard to find concise information.

So why do A64 boards (s754 and 939) support only upto DDR400 DIMMs? As far as I can decipher, this is because the bus (HTT or whatever) only runs at a particular maximum speed (200MHz x some multiplier, 16? = 3200MB/s) so having DDR600 would achieve nothing extra. Is this correct?

If it is, then what happens when you O/C your DDR400? I guess you are actually O/C'ing your bus and your RAM along with it (1:1). So all is fine. In which case, is there theoretically any reason why you can't O/C your bus to 300MHz and use DDR600 on a 2/3 ratio? So you don't O/C your RAM. Forget about practicality. Can someone just tell me if this makes sense in theory? If it does, wouldn't the board need to support DDR600? Or does that mean anything at all? Because, when you are O/C'ing your DDR400 to 300MHz, aren't you in effect running the same as a stock DDR600 module?

Insight will be appreciated. Thanks.

 

[fishmahn]

So why do A64 boards (s754 and 939) support only upto DDR400 DIMMs?

Some new 939 boards have support for faster memory (939 A64's started supporting it last year) ratios. DDR400 is the fastest official specification for DDR, so the official word is DDR400. DDR600 would improve performance some - it gives 50% more memory bandwidth if your system can handle running the RAM at 300...

If it is, then what happens when you O/C your DDR400? I guess you are actually O/C'ing your bus and your RAM along with it (1:1).

Exactly.

In which case, is there theoretically any reason why you can't O/C your bus to 300MHz and use DDR600 on a 2/3 ratio?

If you mean use DDR400 on a 2/3 ratio, then yes. Umm, oh, do you mean a 3(memory) to 2('fsb') ratio? If you do then that works - in fact, see above about support for faster ratios.

when you are O/C'ing your DDR400 to 300MHz, aren't you in effect running the same as a stock DDR600 module?

Yep, in a simple way (things like timings, voltage, etc. may need to be different, depending on lots of variables).

Mike.

 

[abyssalloris]

Hey thanks for the reply. I guess I was just thinking a few things out loud. People like me have not followed the whole thing since its beginning and sometimes approach it from the wrong end. So I was wondering why people O/C RAM (apart from the fun of it).

[Note: There should have been no 2/3 divider when I mentioned DDR600. I was talking about sticking in a stock DDR600, O/C'ing HTT to 300MHz and running 1:1]

Which brings me to my conclusions. We try to O/C RAM because:

1. It is cheaper to buy DDR400 modules and O/C them to DDR600 while running 300MHz HTT than it is to buy DDR600 modules and not O/C them.

and/or

2. Stock DDR600 modules have looser timings than we believe we can achieve by O/C'ing a DDR400 module. Especially so with 2GB kits.

Are these correct? Can anyone confirm my thoughts? Please don't take the specific numbers (DDR600 may be a stretch) too seriously.

 

[fishmahn]

I think originally it was: There was no such thing as DDR600, so DDR400 is all there was - if you wanted to go faster, you had to overclock. (that actually started happening before the DDR era, I believe).

Then #2 is probably next most common, with #1 last - at least, that would be my reasoning. Others may differ.

Mike.

 

[ZOldDude]

Hey thanks for the reply. I guess I was just thinking a few things out loud. People like me have not followed the whole thing since its beginning and sometimes approach it from the wrong end. So I was wondering why people O/C RAM (apart from the fun of it).

[Note: There should have been no 2/3 divider when I mentioned DDR600. I was talking about sticking in a stock DDR600, O/C'ing HTT to 300MHz and running 1:1]

Which brings me to my conclusions. We try to O/C RAM because:

1. It is cheaper to buy DDR400 modules and O/C them to DDR600 while running 300MHz HTT than it is to buy DDR600 modules and not O/C them.

and/or

2. Stock DDR600 modules have looser timings than we believe we can achieve by O/C'ing a DDR400 module. Especially so with 2GB kits.

Are these correct? Can anyone confirm my thoughts? Please don't take the specific numbers (DDR600 may be a stretch) too seriously.

Well you might get some ddr500 ram to reach ddr600....but I dont think you will find any ddr400 doing it!

If you OC the ram and gain 500Mhz @ 1:1 do you really care that the ram takes 1-2 extra clocks when you gain 500 Million clock each sec on the CPU ?

Befor you start testing you had better lower your LDT ratio to about half of it stock stting of the HT will be way over it's specs.

 

[abyssalloris]

I was only using DDR600 as a hypothetical example. It could be any number. Thanks for the warning on the LDT, I'm sure I'll remember.

Something about your logic on the CPU/RAM thing doesn't figure, however. If what you say were true RAM timings would be absolutely inconsequential (by orders of magnitude). I guess you meant the CPU would gain 50 million clocks/s , not 500 (going from DDR400 to DDR500 on 1:1).

As far as I can see (which is not very far mind you), the comparison is not correct. The RAM takes 1-2 extra clocks per bit accessed or whatever, not per second. There are many such accesses per second, so that 1-2 would be multiplied by a huge factor. Once you take that into account, the numbers would be comparable, by my estimation. But I would be glad to hear a better explanation.