Matching CPU and Memory Speed

[PCMan]

Hello all,
I have a question regarding the mixing of Front Side Bus speeds of the CPU and memory modules.

My previous motherboard burnt out last week so I am in the market for a new motherboard. I have decided on an Asus P4P800-VM; however, my old CPU is a Pentium 4 clocked @ 1.5 GHz and a FSB of 400. Now, the memory that I am looking at is a kit of 2 DDR 400 (PC3200) 512mb Corsair value sticks.

The manual for the Asus P4P800-VM states to use PC2100 for CPU's with a FSB of 400 (my case)...and higher memory speeds for CPUs with higher FSBs.

My question is, "Can I use the faster PC3200 memory with my P4 1.5GHz CPU, even though the manual says to use PC2100?"

Here is a link to the manual (look on p.24 for memory listings):
Link to Manual (http://dlsvr03.asus.com/pub/ASUS/mb/sock478/p4p800-vm/e1338_p4p800-vm.pdf)

I have heard that if you mix the wrong speeds between the CPU and Memory the two will be out of sync. But this is usual for memory slower than the CPU which causes the whole computer to slow down...right?

[saphalline]

Technically speaking, you can mix RAM speed and FSB speed however you like, as long as the mobo & CPU support it. You'll get a working system either way. The only time you have to worry about matching it all up is for performance optimization. And really, there's no way to tell if you're using a single-channel or dual-channel capable system if you're just surfing the web. Gaming and other CPU-intensive tasks require more performance optimization.

To answer your question in terms of actual numbers :p your new mobo is capable of dual-channel RAM. So the RAM bandwidth will equal the FSB bandwidth in most cases. For a 400 FSB, you want dual-channel PC1600 because 2 x DDR200 = 400 = FSB. However, that mobo probably doesn't like to go all the way back to DDR200 speeds, so the manufacturer (like many 478 mobo manufacturers) only refers to PC2100 = DDR266. Likewise, for the 533 FSB, you want dual-channel PC2100 also because 2 x DDR266 = 533 = FSB. For the 800 FSB, you want dual-channel PC3200 because 2 x DDR400 = 800 = FSB.

Lower numbers hurt performance, but only if the RAM is the lower number. If you use dual-channel PC2700 for the 800 FSB, you only get 2 x DDR333 = 667 < 800. That hurts. However, if the FSB is the lower number, you don't lose any performance. So if you use dual-channel PC2700 for the 533 FSB, you've got 2 x DDR333 = 667 > 533. That gives you more RAM speed in relation to your FSB which is good.

This is just a simple explanation, but it gives you an idea of how to match up the numbers.

[PCMan]

Thank you, saphalline!

So basically what you are saying is that if you get faster RAM than what your CPU sets the FSB as (in my instance 400MHz), the northbridge controller will slow down the memory to the speed of the lowest item on the FSB (so will reduce the PC3200 memory down to the speed of PC2100 [or PC1600] to match my CPU FSB).

In other words, the memory will slow down to the weakest link in the chain, right?

[saphalline]

No no, the RAM will be at whatever speed you set it to. If you have DDR333, that's what speed it will run at. It will just be faster than the FSB, that's all. Same with slower RAM. If you had an 800 FSB P4 in there and DDR333 RAM, then the RAM would be slower than the FSB. The RAM doesn't actually slow down, it's just slower (or faster) than the FSB.

It's all relative.

[PCMan]

All right, Thank you, saphalline!

That cleared things up. I just wonder why the motherboard manual said to match the CPU FSB with the certain type of memory, and not say that you can use the faster memory...Oh, well as long as it works together and it gets me back a computer I will be happy!

Thanks again for all your help!

[PCMan]

Saphalline,

I just wanted to check with you (or any other person that may have an answer). I currently am still without a comptuer (new motherboard failed to post as screen...see my other post "Motherboard - Lights, fans...no action "), and I am trying to isolate the problem.

I just want to rule out the possible problem of different speeds of CPU and RAM...I just found a link to a website Link - Second FAQ from Bottom (http://www.upgradememory.com/Performance_Q_A_W11.cfm?NBP=1) that states that an Intel CPU can not be ran with faster memory.

Does anybody have any evidence that you can use faster memory with a slower Intel P4 CPU?

My Pentium 4 has a 400MHz bus speed, and I am using DDR 400 (800MHz) on a 800MHz rated Motherboard.

Any input is greatly appriciated!

[rond36]

The Crucial Memory Advisor™ tool lists only the upgrades that are guaranteed compatible with your ASUS P4P800-VM.

These 184-pin DIMM modules are compatible with your system. (http://www.crucial.com/store/listparts.asp?model=P4P800%2DVM&tabid=AM)

These Ballistix 184-pin DIMM modules are compatible with your system. (http://www.crucial.com/store/listparts.asp?model=P4P800%2DVM&tabid=PM)

Crucial lists modules from DDR266 PC 2100 all of the way up to DDR500 PC4000

Crucial recomends this 184-pin DIMM DDR PC3200 module (http://www.crucial.com/store/listparts.asp?model=P4P800%2DVM&tabid=CR) guaranteed to be compatible with your system and deliver the performance you need.

[PCMan]

I understand that they guarantee it will work with the motherboard...and I have full faith that the memory will work with the motherboard; however, will the memory work the the CPU is another matter.

Has anyone specificly mated a Intel Pentium 4 with a lower bus speed to memory with a higher bus speed?

According to the link I posted previously, AMD processors do fine with faster memory, but it says the Intel processors do not work with memory at a higher bus speed.

[jlreich]

that states that an Intel CPU can not be ran with faster memory.

The link is a little misleading. What it's trying to say has to do with performance. Basically that it's best to have the ram synced with an Intel CPU's FSB for best performance. It's not really important performance wise for an AMD CPU.

Yes you can run faster ram with either Intel or AMD. If for instance your FSB is 533MHz and the ram is pc3200 DDR400MHz it will slow down by default to to 333MHz. But many motherboards will allow you to set the ram speed manually to it's full speed if you like. And as rond36 said you can run even faster ram if you wanted to.

Just so you know, my kids machine has a Celeron D 533MHz with 1GB DDR400. It runs just fine whether the ram is running at DDR333 or DDR400 speeds.

BTW, DDR is double pumped, meaning DDR400 is actually 200MHz x 2 = 400MHz. And your CPU is quad pumped running at 200MHz x 4 = 800MHz. So they are perfectly matched.

Sorry I just looked at your first post again and see that your CPU is 400MHz. It's still matched though just in a different way. 100MHz x 4 = 400MHz. 100MHz and 200MHz are matched.

[PCMan]

BTW, DDR is double pumped, meaning DDR400 is actually 200MHz x 2 = 400MHz. And your CPU is quad pumped running at 200MHz x 4 = 800MHz. So they are perfectly matched.

Sorry I just looked at your first post again and see that your CPU is 400MHz. It's still matched though just in a different way. 100MHz x 4 = 400MHz. 100MHz and 200MHz are matched.

Thank you jlreich. One correction that I would like to make is that the RAM is DDR 400 (PC3200) so effectively the memory runs at 800MHz (400MHz x 2 = 800MHz)...so really the memory is 400MHz faster than my Pentium 4 (100MHz base frequency x 4 frequency multiplier = 400MHz). Right?? This is how I understand it.

[jlreich]

No, the ram is 200MHz double pumped to run at an effective 400MHz. The effective MHz is what gives the numbers for DDR.

[PCMan]

I see...thank you for clearing that up (I am kind of new to this DDR and "pumping" thought pattern ;) ...what ever happend to "you get what you get"? LOL)!

[saphalline]

Ooh! Lots of activity here today!

The link you posted is very outdated and doesn't tell the whole story. Back when that FAQ was writtten, most mobo chipsets for Intel CPU's couldn't run the RAM faster than the CPU's FSB. This has nothing to do with any incompatibility with the CPU itself, merely the chipset. The memory controller in the chipset is responsible for communication between the CPU and RAM, and at the time, no memory controller for Intel CPU's could successfully run the RAM faster than the CPU's FSB. This has obviously been fixed since then. ;) I hate old data on the web, though - it can be very misleading and can actually result in big mistakes. You have to realize that computer technology changes daily so it was a good idea for you to ask that question here.

In terms of that mobo, if PCMan is using 2 or 4 sticks of RAM, that probably means dual-channel. In which case dual-channel DDR400 gives an effective RAM speed of 2 x 400 MHz = 800 MHz. I'm sure you all just got lost in the numbers. Be careful. The rest of the bit about DDR being double data rate was good, though.

For the ultimate in performance, the Pentium 4 actually receives a healthy boost with a slightly excessive amount of RAM bandwidth. The P4's microarchitecture in general is very sensitive to RAM bandwidth. Making sure the numbers match is good, but you can actually get a boost from increasing the RAM bandwidth in relation to the FSB. So if you have a 533 FSB P4 (or Celeron D) then dual-channel DDR333 is better than dual-channel DDR266, which would normally be in-sync. Of course you'd only notice this boost in gaming or video editing, but it's there. This is also why it's so convenient to recommend dual-channel DDR2-533 for the new LGA 775 Intel systems, because it provides more bandwidth than the P4/PD needs. In the case of DDR2, it also helps to have more bandwidth to offset the higher latencies, but that's... getting too deep...

[PCMan]

Thank you SO much, Saphalline! Your input is always welcome and great to read!

When I read and posted that link, I wondered if it were outdated; I just couldn't imagine it could be true without having the forums flooded with posts about incompatible memory.

I thought that the link was fairly recent, for I looked for the date of writing when I read it...then I saw the 2006 copyright at the bottom and assumed it was very recent (most likely it was the website copyright date).

Thank you for clearing everything up, Saphalline. I will be receiving my new motherboard today...fingers are crossed! :cool:

[jlreich]

Good point Saph. I wasn't even thinking about dual channel doubling the effective MHz. Don't you just love dual channel ram. :D