Memory Divider Questions

[jph1589]

I really got interested in looking at the memory divider table and it prompted me to start trying out some different dividers to see what affect they would have (or not have).

While I was testing I found out that my motherboard or CPU may not like certain dividers. The results went from instability to complete refusal to post. I also found that as I increased the FSB the system seemed to tolerate more dividers. What controls that? Is it the cpu or the motherboard or both?

Is there any hard and fast rule about what memory divider goes with which cpu / fsb speed? It is just a curiousity more than anything. I am of course referring to an Intel setup with DDR2. I guess that probably limits the complexity of the answers?

[gvblake22]

Well, the dividers are really just there to give the user the ability to adjust the speed of the memory and the FSB of the processor independently.
What specific combinations of things were you trying in response to your question(s)?

There's not really any overriding rules about using dividers, just that you don't put the divider to extremes and make your RAM run out of control fast to the point where it is unstable. Just like everything else in overclocking, it is mostly a matter of doing some math to figure out exactly what speeds your system's components are ACTUALLY running at (since dividers make your memory run a different speed than the FSB).

[jph1589]

Well let me go through it as it happened and see if I remember it all.

When I first put the system together at stock the FSB was at 200 and the memory at automatic, which put the memory at 667. That is the rated speed of this memory so that made complete sense. That is also a multiplier of 3.33 in my bios. As I raised the FSB up to 300 that would have put the memory close to 1000 so I lowered the multiplier to 2.66 to put the memory at 798. This was all fine for memtest86 and superpi etc but there was one problem with cold booting so I lowered the multiplier to 2.5 putting the memory at 750 with the FSB still at 300. This was where I found out that something doesn't like that multiplier. The system wouldn't even post, even though it is a slightly slower memory speed.

Next I decided to try a 1:1 ratio on the memory whigh is a 2 multiplier in my bios. Finally as I am typing this I now understand where they get the 2.5 at 2.66 etc multipliers. The FSB times the multiplier = memory speed....duh!!... Anyway, the point I was about to make is that the system wont even post at 1:1 at 200 fsb. It doesnt' like 1:1 at all until I get up to 300 fsb.

So based on all of this I assumed there was some internal table somewhere that either the cpu or motherboard are guided by. I guess that isn't the case?

Dividers.....multipliers.........arghhhhhhhhhhhhhh h LOL!

[gvblake22]

Not sure if this is tripping you up, but 200 Mhz FSB and 667 Mhz memory speed is not 1:1 with your processor.

Intel processors make things very confusing with their whole "Quad-Pumped FSB" crap.
SO... The Pentium-D 920 has an 800 Mhz "quad-pumped" FSB, which means that the actual frequency of the memory at 1:1 with the FSB is 200 Mhz (but 400 Mhz effective speed because it is "Double Data Rate" (DDR2) memory)
BECAUSE:

800 mhz / 4 = 200 mhz FSB (divided by four because it is quad-pumped)

So in order to get your DDR2-667 memory running at it's rated speed on an Intel 800 mhz Quad-Pumped FSB processor, you would need to run a 1:1.667 FSB:Memory divider (which will effectively multiply the FSB by 1.667) which would yield a memory speed of 333 Mhz (667 Mhz effective DDR2 memory speed).

This might explain why you get better stability with your processor overclocked to 300 Mhz FSB 1:1 because that puts your memory closer to it's rated 333 Mhz (DDR2-667) speed.

As you can see (and have probably experienced), it is really hard to put this stuff into understandable terms, so please tell me if something doesn't make any sense...

[jph1589]

Originally Posted by gvblake22

Not sure if this is tripping you up, but 200 Mhz FSB and 667 Mhz memory speed is not 1:1 with your processor.

Intel processors make things very confusing with their whole "Quad-Pumped FSB" crap.
SO... The Pentium-D 920 has an 800 Mhz "quad-pumped" FSB, which means that the actual frequency of the memory at 1:1 with the FSB is 200 Mhz (but 400 Mhz effective speed because it is "Double Data Rate" (DDR2) memory)

BECAUSE:

800 mhz / 4 = 200 mhz FSB (divided by four because it is quad-pumped)

So in order to get your DDR2-667 memory running at it's rated speed on an Intel 800 mhz Quad-Pumped FSB processor, you would need to run a 1:1.667 FSB:Memory divider (which will effectively multiply the FSB by 1.667) which would yield a memory speed of 333 Mhz (667 Mhz effective DDR2 memory speed).

This might explain why you get better stability with your processor overclocked to 300 Mhz FSB 1:1 because that puts your memory closer to it's rated 333 Mhz (DDR2-667) speed.

As you can see (and have probably experienced), it is really hard to put this stuff into understandable terms, so please tell me if something doesn't make any sense...

I must not have explained that very well. Sorry. I didn't mean to say the 200 fsb and 667 was 1:1 . That is just where the stock setting/automatic put everything in the beginning.

[gvblake22]

Originally Posted by jph1589

I must not have explained that very well. Sorry. I didn't mean to say the 200 fsb and 667 was 1:1 . That is just where the stock setting/automatic put everything in the beginning.

OH, ok, cool.

Soo.... What was your question again

[jph1589]

Originally Posted by gvblake22

OH, ok, cool.

Soo.... What was your question again

Laughing on the floor here LOL. Never mind!! My brain hurts!

[gvblake22]

Originally Posted by jph1589

Laughing on the floor here LOL. Never mind!! My brain hurts!

DITTO!

[jph1589]

Originally Posted by gvblake22

DITTO!

The most baffling one was why it won't post with the FSB at 300 and the memory at 750. It is fine with the FSB at 300 and the memory at 798.

You probably are asking.....why bother with it. Well I was trying to slow the memory down just a bit to get the FSB higher. So that was where I had the bright idea that I needed to understand all this hooha!!

[jph1589]

Well I just got a note that gigabyte will be looking into some P4 memory issues with the DS3. So what I was seeing probaly wasn't normal anyway.