DDR2 and what is Prefetch

Hi all I have a few questions to ask about DDR2:

Firstly, in regards to DDR, what is prefetch all about, DDr has 2-bit and DDR2 has 4-bit, DDR2 is better than DDR1 and a lot of this is suposedly because of the prefetch, anyone able to explain what prefetch is and why it makes DDR2 better?

Why is it said that DDR2 is only faster when it is over 600MHz Frequency? I know the latencies are higher on DDR2, but frequencies make this better, an dI am guessing Prefetch also has a role in it?

I have this, but I feel I am missing something:

DDR2 or Double Data Rate 2 is the second version of DDR RAM.



DDR2 is faster than that of DDR while using less power than that of DDR, DDR acquires 2.5v of power whereas DDR2 requires 1.8v. DDR2 takes off where DDR stops, by this I mean, DDR2 starts at frequencies where DDR had to stop.



DDR2 has higher frequencies, increasing bandwidth (The amount of data that can be received and sent a second) which makes DDR2 increasingly faster that that of DDR, while maintaining lower Voltage rates, however DDR2 is not backwards compatible with DDR sockets, which means new Motherboards need to be purchased.



Though DDR2 is said to be faster, it generally does not outperform DDR until it reaches higher frequencies above 600 this is because DDR2 has higher latencies than that of DDR, however the prefetch of DDR2 is 4bits while DDR is 2bits.

Thanks for your time and effort

Kind Regards

DislexiK

i don't know much better than that of DDR...

Well as far as I know you want a lower prefetch. It seems to me that 1 is the best, and 2 is normal. However, maybe Im thinking of something else. But I do know that DDR2 566 is the same performance as DDR400, simply because of latencies. DDR400 can get to about 2-2-2-6 these days, but DDR2 is still like 4-4-4-8 or something. But thats why they perform the same (for now)...

Prefetch is gathering information before it's needed. Generally, when you randomly grab something from memory, chances are you are likely to need the block right beside it as well sooner or later.

How a memory module works, is you have banks of memory chips. You can see these if you look at a normal stick of RAM. Those are the storage cells. However, when the controller asks for information, you have to light up the right area, and transfer the data over a bus. In between the bus and the storage area is something called a "buffer". It's a collection/staging area for data on it's way in or out.

In DDR:

Memory -------> Buffer ----------> Bus
200MHz -------> 200MHz ............ 400MHz

The -------> indicate bit lines. In normal DDR, two bits are prefetched to the buffer at 200MHz, then sent out on the bus to the memory controller at an effective 400MHz. What I mean by "effective" is that on each edge of the clock, one bit is sent at 200MHz speed. So in one clock cycle 2x200MHz = 400MHz.

In DDR2

Memory -------> Buffer ----------> Bus
100MHz -------> 200MHz ............ 400MHz
.......... ------->
.......... ------->


As you can see, there's now a 4 bit prefetch, but at half the speed of before. The math all works out. Where the advantage comes, is the big memory storage areas now work at slower speed. Little buffers are easy to get to run quick. Large, fast cells are expensive. So, you make the big ones run slower, but send bigger chunks of data each time. That's why DDR2 will be able to scale further in external clock speed than DDR.

Some of the reason why, is that for random transactions where you only want 1 bit... well, you are getting 4, and have to wait for them. And you have 3 to throw away.

I don't know if the higher latency ratings will be as bad as the numbers suggest. One reason is that now there will be less interference, so you'll have fewer garbled transactions where you have to throw data away and ask for it again. DDR2 has a few different technologies to aid in keeping errors down over DDR. We'll need to get our hands on some of this stuff and test the snot out of it first to see just what the differences are, and how current programs are affected by this.

[QUOTE=DMOS]Prefetch is gathering information before it's needed. Generally, when you randomly grab something from memory, chances are you are likely to need the block right beside it as well sooner or later.

Feel free to clear out your prefetch file anytime. If you start a program up again, it's prefetch will just reload. Plus, clearing it out saves memory.

so basically whats the imporvement of this DDR2 or dual channle ddr??

has ddr2 made an apperance anywhere? does anyone no where to buy it for australians

plus when u delete the prefetch files, windows loads faster

Oh really?

Necro.

I guess that means that you called it

As soon as you see DMOS, check the dates! lol

yup, when u delete the prefetch files, windows xp loads faster, when ur xp is loading, count the blue bars which go across the screen, after that, delete the prefetch files, and restart the pc, the number of blues bars which go across the screen should be less now.

my teacher told me of this and when i checked and counted the blue bars when xp was loading, they were a total of 15.after i deleted the prefetch files and restarted the pc, i then only counted a total of 5.

i had a lot of files in the prefetch folder, about 100 and something.

That's pretty interesting since the "bars" that you see are a premade bitmap. They should never change.

Deleting your prefetch can improve startup times, however, it will decrease overall performance, especially if you multitask or frequently change programs.