Why 5:4 ratio? How does that work?

[VIAN]

 

[EarthwormJim]

Pretty vague there eh? If you are refering to the fsb to memory ratio all it is is a ratio of fsb speed to memory speed. So say you have your fsb at 250mhz and your memory can't handle 250mhz, so in order to have a stable system yet have a high overclock you use a ratio setting. A 5:4 ratio at 250mhz is 200mhz, so your fsb will be at 250mhz and your memory will be at 200mhz. Other ratios are 1:1, same fsb speed and memory speed; 4:3, even greater difference than 5:4 (i.e. 250mhz fsb means 187.5mhz memory).

 

[VIAN]

But why the ratio, why not just overclock to the max both FSB and RAM?

In other words, are we all struggling to get to 1:1?

Why not just keep the fsb low? I'm sure the multiplier is high enough for that.

 

[Soulkeeper]

i use 5:4 because my memory don't like to run at 230mhz or higher

 

[VIAN]

but why do you have your fsb up for? Why not run it 1:1?

 

[Peter D]

You use RAM dividers when your processor can run at higher FSB's compared to your RAM so you can overclock and the memory won't be much of an issue. i.e. You have a P4C chip and want to overclock past 200 and you only have lets say PC2100/2700 RAM, then dividers can come in handy.

 

[VIAN]

I have a lot of trouble understanding what you just said. Can you separate them into sentences.

 

[Jeff7181]

Since Pentium 4's have locked multipliers, the only way to overclock them is by increasing the FSB. When you increase the FSB of a 2.4C to 250 Mhz to get a decent overclock, there's not many types of RAM that can run at 250 Mhz, and even the ones that can don't benefit very much from the increased bandwidth ever since the P4 was given dual channel memory. It's no longer starved for bandwidth, in fact, with dual channel DDR400 RAM, it actually has more bandwidth than it actually needs. This is proven by the fact that performance while running 5:4 seems to pick up where the maximum 1:1 overclock leaves off, and still scales as it did while running 1:1... if that makes any sense 😀

 

[VIAN]

This is proven by the fact that performance while running 5:4 seems to pick up where the maximum 1:1 overclock leaves off, and still scales as it did while running 1:1... if that makes any sense

Still don't quite get it, but I completely understand everything else. Thanks.

 

[Jeff7181]

If you can score 4500 at 3.0 Ghz at 1:1... and 5000 at 3.2 Ghz at 1:1... but can't get any higher and still be stable at 1:1... then you can use a RAM divider and score 5500 at 3.4 Ghz at 5:4... and 6000 at 3.6 Ghz at 5:4

BTW... I pulled these scores out of my arse, they don't mean anything, just used them to illustrate my point.

 

[Chu]

Originally posted by: VIAN
I have a lot of trouble understanding what you just said. Can you separate them into sentences.

An example would be helpful. Let's say you have some memory that you KNOW that your memory will run stable up to 170 mhz. Your processor has a multipler of 14x, and usually runs at a fsb of 166MHz. 14*166 = 2324MHz. You want to overclock it, and it's running stable at 170MHz, i.e. 14*170 = 2380MHz. Your processor can probably go higher, but your memory has topped out. By setting your ratio to 4:5, your memory is now running at 170*(4/5)MHz, or 136MHz. You can now push your processor FSB even higher since your memory now has headroom again.

-Chu

 

[VIAN]

Everything is clear, thanks.

 

[oldfart]

A little late, but....
CPU MHz means A LOT more to total system performance than meme speed does. P4 is multiplier locked. You overclock by raising the FSB speed. P4C starts @ 200 MHz FSB (DDR400). If you overclock to say...250 MHz FSB, you are looking at running DDR500 (PC4000) with a 1:1 ratio. Many types of ram just wont do that. If they do, they use pathetic slow timings to get there and offer no extra performance. You dont want to limit CPU speed for memory speed. In the 250 MHz FSB example, running a 5:4 ratio brings the mem speed down to a manageable DDR400 (PC3200). At this speed, you can run decent timings and have good performance as well.

 

[Fricardo]

I understand the need for different RAM/CPU timings on the P4, but why does it have to be in a ratio like that; 1:1 or 5:4 or 4:3, etc. Why can't it just be whatever is the highest clock achievable for both, maybe 193 RAM and 221 CPU or something random like that?

 

[vshah]

Originally posted by: Fricardo
I understand the need for different RAM/CPU timings on the P4, but why does it have to be in a ratio like that; 1:1 or 5:4 or 4:3, etc. Why can't it just be whatever is the highest clock achievable for both, maybe 193 RAM and 221 CPU or something random like that?

probably to maintain something as close as possible to synchronous operation?
but if running 1:1, they'll always be in sync.
If running 5:4, every 20 cycles they'll be in sync
etc.
just a guess. i know nothing.

-Vivan