Trying to Understand RAM and FSB interaction

[TheSaxon]

I'm coming from knowing a lot about computers maybe 4-5 years ago, and not knowing anything now.

What I think I know:

Manufacturers in their infinite wisdom, do not tell you the actual clock speed of their FSB/RAM, but the "effective" clock speed.

So old school PC-100 RAM, had a clock speed of 100 and that was that. if a mobo had a higher or lower FSB speed, it would flip to the lower of the 2 and operate at that speed.

Then the wonderous days of DDR come and suddenly, data is delivered on the rise and fall of the clock cycle, so manufacturers now say: sweet we can market this 100 mhz RAM at 200 mhz. So if my understanding is correct, DDR-200 RAM is technically only operating at 100 mhz, but since its delivering data on the rise and fall of the clock, its effectively 200 mhz. Manufacturers also started advertising mobos with FSB speeds to match the new DDR. Rather than advertise a 100 mhz FSB, they say oh we have a 200 mhz FSB, even though it is in fact only 100 mhz due to the fact that it can spit back 2 pieces of data in the SAME CLOCK CYCLE. They market this way for whatever reason, but aside from the fact that bigger = better, they probably did it so that consumers can understand that "hey the number next to my RAM = the number on my FSB. Sounds like a good plan"

Now come the days of DDR2 RAM. DDR2 ram doubles the "prefetch buffer?" and so now every time a request is made of the RAM, it turns out 2 pieces of data per clock edge rather than one, right (so now we can deliver 4 pieces of data in one clock cycle - 2 per edge)? However, this is only beneficial if the FSB can deliver (the 2 units of) those 2 pieces of data fast enough to where when it comes back to get more, the RAM will be ready with another buffer of data. Is this how we arrive at the present situation of the "mhz" of your RAM (which really isnt the actual mhz), needing to be 1/2 of the rated FSB speed of your mobo (or better - to leave room for over clocking)?

This leads me to another question: Why does anyone buy anything above DDR2 800 RAM? seeing as how the highest supported FSB on a motherboard is 1333, to take full advantage of this, it sounds like you'd need DDR2 667 RAM, and if you wanted to have room to overclock, DDR2 800 RAM. in other words, isnt DDR2-1066 way too much overkill? especially since, the transition to DDR3 will probably be complete by the time DDR-1066 could be taken full advantage of.

Please correct any assumptions I've made that are wrong because everything I read on the internet seems to conflict with each other.



Thanks

 

[Comdrpopnfresh]

some boards (asus is one) natively support 1066 regardless of fsb (I'd say the mass native support is for 800). There is an invisible multiplier going on so the stock fsb speeds will run your 1066 at it's stock speeds.
Even if you have a board that supports only 800mhz, if you put in 1066 and overclock, you know the ram has enough headroom to stay at a 1:1 ratio with the fsb.
Another thing to add- intel fsb speeds are quad-pumped. So the baseline fsb on 800mhz is 200, 1333 is 333- just more tricks going on...

 

[n7]

More bandwidth = better performance.

It's not entirely quite that simple, but that's the basics of it.

It doesn't make a huge difference, but if you have two identically clocked CPUs, but one with DDR2-667 4-4-4, & one with DDR2-1066 5-5-5, the system with the DDR2-1066 (yes, it's actually 533 MHz) will indeed be faster overall.
Heck, it might take synthetic benchmarks to even show the difference, but it's there.

Some people like that small difference.

 

[TheSaxon]

will it have better performance though? because isnt the speed of the RAM bottlenecked by the 1333 FSB? i.e. DDR2 800 or DDR2 1066 wont buy you any additionaly performance, if the FSB can't go any higher than 1333, which happens to be the same effective spped as DDR2 667 RAM?

i.e. the FSB is running at 1333, the DDR2 RAM is running at 333 Mhz, but since its DDR they called it 667 (333x2) and since its DDR2, its effective speed is actually 1333 (667x2). so as I understand it, (but im guessing im wrong), buying 800 or 1066 doesnt gain you any benefit because your motherboards FSB, is only 1333 and therefore can only fully utilize DDR2667 RAM unless you overclock, in which case DDR2 800 ram would help you becasue it could handle the overclock. So why buy DDR2-1066? 800 leaves plenty of room for an overclock makign 1066 seem like overkill.

plz help me understand

 

[BonzaiDuck]

I made that mistake, paying for Crucial Bal'x DDR2-1000's when the DDR2-800 version or the Tracers at 800 Mhz would've given me the same. It's confirmed from another party running DDR2-800 Crucial Ballistx and we're getting the same latency-setting performance at the same bus-speed.

But if you only buy DDR2-667's, and you want to over-clock, you may be limited. Getting DDR2-800's may give you some extra room.

I don't regret buying the DDR2-1000's, but I won't be running them between 800 and 1000, either. Another member's experience with the 800s broke the "rules" as I thought I knew them -- with a tRAS timing of 3 @ DDR2-667. If he can get that at 667 Mhz, either one of us might be able to get it at DDr2-700.

I'm not all goo-gah about the high-speed modules and running them on a <> to 1:1 divider, since my benchies are keeping up with them. But the comparison benches most likely use stock latencies, and I'll need to see if I can set to tighter values than 5,5,5,15 @ 1000 Mhz.