understanding ram & fsb freq

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ic3dblue

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Oct 29, 2007
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so i have been reading online searches and the memory faq here, but i still don't quite understand how to calculate 1:1 fsb and ram frequencies.

scenario 1:
so on my current system..P4 Prescott 3.0 800 fsb, i have dual-channel PC3200/DDR400

intel fsb = 200 * 4-pumped = 800

so why is the my ram in 1:1 only running at 400?

is it cuz in dual channel you multiply by 2 to get the ddr to 800 mhz?

scenario 2:
on a new system i plan on overclocking intel quadcore with 422 fsb, i want to calculate the HYPOTHETICAL ddr2 ram that would run at 1:1, although i know it doesn't exist.

intel fsb = 422 * 4-pumped = 1688

so does that mean that the HYPOTHETICAL ram has to run at 1688 mhz?

assuming in scenario 1 where i just multiplied by 2 to get ddr to 800, this would mean i need ddr2-844, no?

as u can tell i am totally lost..and i havnt been able to find anyone or an article to explain it so i can understand.

any help is appreciated! thanks!
 
You are on the right track. Look at it this way:

CPU is FSB * 4 200 * 4 = 800
DDR RAM is FSB * 2 200 * 2 = 400

In this case the CPU and RAM is 1:1 because of the FSB. Dual Channel does not affect this in any way. You are correct in scenario 2, but seem to be looking at scenario 1 in reverse. I hope this clarifies it.
 
I will probally just make it worse but:

a) wolfofsin is correct.

b) what is confusing you is the 1:1 , its not really "1:1" every time you see 1:1 think "memory runs at 2x the FSB per initial design so they call that 1':1 or synchronous mode."


This is how we engineers get back at marketing people who do 1/10 the work and make 4 x the money.;) It is unfortunate the end users suffer too. :eek:

so to make it easy on an Intel system where "actual" is the number you type in the FSB speed box.
cpu runs at the cpu multiplier x the "actual" FSB.
memory (default 1:1) runs at an "actual" speed of 2 x the FSB.

(lets not go into data transfer rates, a transfer in the rising edge of the clock pulse and one on the falling edge, double data rate, but it is still just one clock pulse, so just go with the above as the "actual FSB" is what the cpus and memory advertisments and typcial rating use)
 
I think actual speed and effective speed really confuse people as well. Effective speed of DDR400 at stock is 400 MHz, but since it's DDR, it's *actually* running at 200 MHz ( 200×2 )
 
gwai lo said:
I think actual speed and effective speed really confuse people as well. Effective speed of DDR400 at stock is 400 MHz, but since it's DDR, it's *actually* running at 200 MHz ( 200×2 )
Click to expand...

Yep yep I agree, I hate the bios(es) ? that do not show "actual" but instead give the effective. Confusing as hell and I been doing this stuff since Nixon was elected. Guess its just me, either method is probally OK once you grasp what is really going on and then it is just personal perference. Your post made me realize that a lot of bios(es) dont even let you put in "actual" values but show only menus of the "effective" speeds. doh !!
 
You can get DDR2-800 and manually overclock the 800 to 844 and it should work if I'm not mistake, however, it may not be stable. That's a whole other issue. ;)
 
BillParrish said:
Yep yep I agree, I hate the bios(es) ? that do not show "actual" but instead give the effective. Confusing as hell and I been doing this stuff since Nixon was elected. Guess its just me, either method is probally OK once you grasp what is really going on and then it is just personal perference. Your post made me realize that a lot of bios(es) dont even let you put in "actual" values but show only menus of the "effective" speeds. doh !!
Click to expand...
really? the ones i play with seem to..however i don't get to play with too many

<--poor college student :(

but yeah it's annoying when people who work with today's computers post their effective speeds and i don't realize it. -_-

"WHOA! you got 740 MHz out of that thing?!?!.......oh wait..nevermind.."
 
A "800 Mhz" FSB is in fact 200Mhz QDR. (4 data per clock)

PC3200 Ram = "400 Mhz" Memory is in fact 200 Mhz DDR. (2 data per clock)

200:200 = 1:1

PC3200 in dual channel can fill up a "800Mhz" bandwidth.

FSB = 200Mhz x 4 (QDR) x 64bit /8 (bit to byte) /1024 (Mega to Giga) = 6.25 GB/s
PC3200 memory in Single channel = 200Mhz x 2 (DDR) x 64bit /8 (bit to byte) /1024 (Mega to Giga) = 3.125 GB/s
PC3200 memory in Dual channel channel = 200Mhz x 2 (DDR) x2 Dual Channel x 64bit /8 (bit to byte) /1024 (Mega to Giga) = 6.25 GB/s

ic3dblue said:
so if i overlocked an intel quadcore to 422 * 4 = 1688, i just need ddr2-844?

and thanks for all the help..this confusion is getting better lol.
Click to expand...
You are correct.

The explanation I gave above stand for the Core 2 CPU's and DDR2. PC6400 = 400Mhz (real Mhz) DDR so "800Mhz Effective". You'd need a "1600Mhz effective" FSB (400mhz QDR) to even get in 1:1 withPC6400 ram. Double channel ram at those speed will fill the bandwidth needed by the FSB.
 
Cyant said:
FSB = 200Mhz x 4 (QDR) x 64bit /8 (bit to byte) /1024 (Mega to Giga) = 6.25 GB/s
PC3200 memory in Single channel = 200Mhz x 2 (DDR) x 64bit /8 (bit to byte) /1024 (Mega to Giga) = 3.125 GB/s
PC3200 memory in Dual channel channel = 200Mhz x 2 (DDR) x2 Dual Channel x 64bit /8 (bit to byte) /1024 (Mega to Giga) = 6.25 GB/s
Click to expand...

wow thanks for that. that was written in the sticky too, but i didnt know what each # meant.

haha i know it was easier to follow a guide and just buy what's in the guide but i wasnt mentally satisfied lol.
 
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