A little confused on the whole memory timing with FSB

[tw0k1ngs]

Ok, so if you have DDR400 ram running on a mobo with a cpu that has 800mhz FSB, when you increase your FSB, how does the 400mhz increase at a 1:1 ratio with the 800mhz? Shouldn't default CPU FSB to RAM Freq be 2:1? I'm a little confused on how all this works, how people can say that the current today standards for most mobo's are DDR400, but you see cpu's having FSB's of about 800. Is there any guide or something somewhere where I can understand where the extra 400mhz for the ram comes from? Or if it is running at the same as the FSB, why do they even label it DDR400 and not DDR800?

[timpanogos]

Let’s take my 55 for an example – fsb default is 200. The CPU has a “multiplier”, which on the 55 is “unlocked” both up and down (meaning I can change it in the bios). The default multiplier for the 55 is 13. Thus
200 * 13 = 2600mhz cpu stock rating.

Now if I oc my fsb to say 250, and drop my default multiplier of 13 down to 11, I get 250*11 = 2750mhz cpu oc. This all along with the 1:1 “divider”.

I can also hit a nice 2880 with a fsb of 240 and multiplier of 12.

Or
2800 with 200 fsb * 14.

Now if/when your fsb * multiplier become too much for your cpu to handle on mhz, then you can start throwing in dividers, to bring your cpu mhz down to runnable levels with high fsb clocks.

This is all in a nut shell, there are very good guides around that spell this out much better .. look for amd oc guides

[tw0k1ngs]

After reading a little, I realized that the FSB runs at ONLY the freq of the Memory. So if it only runs at say 400mhz, then why do companies go around putting 2000mhz FSB on the new x2's? Is there any memory that can run 2000mhz fsb? So your FSB would be capped at the memorys speed. Why do they give the CPU 2000mhz fsb if it will only run at 1/5 that?

[(sin)morpheus]

Ok here's my little ASCII pic on it:

[cpu-fsbx4]====[northbridge-fsb200MHz]====[memory-fsbx2]

So you see, the ram speed is still 200MHz(DDR400) it doesn't get faster, the cpu end of the bus is just faster so that it can improve bandwidth.

[alexio]

A cpu with an FSB of 800 is quad-pumped. So 200mhz *4 = 800mhz. On AMD cpu this pumping doesn't really boost performance. We don't know about Intel because you can't change that setting on Intel cpu's.

Memory that runs at 200mhz is called DDR400 because it is effectively running at 400mhz (DDR = dual data rate).

[(sin)morpheus]

After reading a little, I realized that the FSB runs at ONLY the freq of the Memory. So if it only runs at say 400mhz, then why do companies go around putting 2000mhz FSB on the new x2's? Is there any memory that can run 2000mhz fsb? So your FSB would be capped at the memorys speed. Why do they give the CPU 2000mhz fsb if it will only run at 1/5 that?

The reason they say that is because the athlon64 has an on die memory controller which effectively eliminates the need for an fsb. They say 2000MHz because that is the maximum speed that the HTT bus can go. The HTT bus is what they use to carry data between the cpu and other components such as the memory since the fsb no longer exists to do this job.

[tw0k1ngs]

So default on all CPU's is an fsb of 200?

[tw0k1ngs]

And the HTT bus on the AMD and just the "bus" on intel processers are the same? and the FSB and the "memory controller" for intel and AMD respectively are at 200mhz which is what the memroy runs at? And does the motherboard have any part in this FSB/HTT/BUS transfer besides using its BIOS to increase/monitor them?

[tw0k1ngs]

Also, what is the point of Quad Pumping? (or in the x2 case deca-pumping) If the transfer speeds will only be as fast as the DDR's freq, what is the point of having such a high theorectical value?

[[XC] leviathan18]

because you can have greater bandwith more fsb more that you can stream is like a freeway you have all the cars runing @ 200km/h but in just 800lines (intel) and 2000lines (amd) so will have more cars sent in the amd system @ 200km/h

[tw0k1ngs]

So if my ddr400 is running at 400mhz, why does CPU-Z say my FSB is at 200mhz, and my "Bus" is at 800?

[tw0k1ngs]

Ok after reading some OC guides I think I have a better understanding. But one question, if the AMD 64's and up are not double pumped, and use HTT to achieve effective FSB's of 2ghz, then what is the "true" fsb that I will want to sync my RAM with? And what "true" fsb will I overclock when I want to OC my CPU?

[(sin)morpheus]

The 200HTT is what you want to sync you ram with, though syncing the ram isnt that important with an athlon64. The on die memory controller allows you to use memory dividers without a penalty. The athlon64 doesn't have an fsb as stated before, the 2000MHz is just the speed of the HTT link between the memory controller and the RAM. When you overclock you want to overclock the HTT. By the way, DDR 400 doesn't run at 400MHz it runs at 200MHz. It's double data rate, which means that it works on the raise and fall of the sin wave. This effectively doubles the speed of the memory.

[tw0k1ngs]

So how do you get clock speeds if there is no FSB? If I bought a 4800 @ 2.4ghz, wouldnt that have to be a 12x multiplier on a 200mhz FSB? If there is no FSB with the HTT technology, how do they arrive at the 2.4ghz?

[tw0k1ngs]

Also, since the HTT is 200mhz, the LDT multiplier must be 10 right now adays right? Or is the HTT still double pumped like FSB used to be on AMD XP's? So it would be LDT 5 (2000mhz effective).

[coZmic]

hi :)

i hope u all can read (and understand :P) my english, i should work at it, hehe:

- a stick of 400 MHz DDR-SDRAM (2 signals per clock ) is physically clocked at 200 MHz. 200 x 2 = 400 effective

- an 800 MHz FSB of a P4 (4 signals per clock) is physically clocked at 200MHz. 200 x 4 = 800 effective

in a p4 system the FSB (800MHz) is also used by the memory controller in the p4 chipset, the mem runs at equal higher or lower speedor higher or lower with dividers. any change in fsb frequency results in a change in mem frequency. the same happens in a socket a system.

an a64 sys works a little bit different, there is no FSB anymore.
the cpu has an integrated memory controller. the mem frequency is generated by dividers from the core (cpu) frequency.

the core frequency is generated by the cpu's default multi x HTT frequency (which is 200MHz by default).
so an a64 with a core frequency of 2000MHz and a cpu/10 divider runs the mem at 200MHz (400 MHz effective because its DDR-SDRAM).

the LDT Multiplier is a different story. it controls another bus, the HyperTransport Bus. the frequency of the HT-Bus is generated as following:
HTT frequency x LDT multi. so usually: 200 x 5 = 1000 (for cpu's and boards that support that)
there also exists 800MHz HT Bus (x4) and 600MHz HT Bus (x3) chipset/cpu combinations.

sorry again for my english ;) .

[tytlyf]

i think you guys are making it sound more confusing than it really is. Just read alot of the stickies in the forum. You are making it more difficult to understand. I basically ignore the HTT speed when oc'n, it's not important.

[EMC2]

Maybe this will help you "see" it better ;)

http://img8.echo.cx/img8/382/a64system4tg.png (http://www.imageshack.us)

Peace :toast:

[[XC] leviathan18]

is not 2000mhz is 2000mts or 1000mhz