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Thread: Memory / CPU FSB speeds...

  1. #1
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    Memory / CPU FSB speeds...

    Hi Everyone,

    I figured I'd post this in the general forum, as I think it spans a number of areas, so hopefully you'll be able to help set my mind at rest...

    I'm suffering a little confusion to say the least with the the relationship between FSB, CPU bus speed and memory speed.

    My set up is as follows,

    AMD Athlon 2100XP
    Giga-byte GA7 VAX Mobo
    256Mb PC3200 DDR Ram.
    Abit GeForce4 Ti200

    My confusion stems from the fact that I cant seem to get my PC3200 memory running at 400Mhz.

    I've got the FSB set to to 133Mhz on the mobo straps. The FSB for the 2100XP is supposed to be 266Mhz, but my understanding is that this in INTERNAL to the CPU via doubling the FSB, hence the 133Mhz FSB.

    At this speed what should I be seeing as the speed of the memory ? I've benchmarked it using SiSoft Sandra, and i'm getting around 2100Gb/sec which equates to around PC2100 I believe. Easy Tune ( Gigabyte o/clocking / monitoring util ) reckons the memory is running around 333Mhz ( PC2700 ? ). I can force the memory to run at 400Mhz through the bios, but have to up the voltage to 2.8V, and even then its totally unstable.

    How is the DDR memory clocked to these higher speeds ? If it only doubles the FSB ( makes sense to me ) then surely I cant use the memory at 400Mhz unless I have a 200Mhz FSB CPU. TBH, I think from reading other posts, that this might be the way of things... <curses self for falling for the marketing hype ! >

    I've done a search through the forum but cant find the answer that really stops me pondering the questions...

    Thanks in advance....

    Mark.

  2. #2
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    hey there.

    ok

    the fsb is what ur mobo runs. which in ur case is 133Mhz (i will just call it M from now on)

    ok 133M

    the cpu runs at 133Mhz plus the cpu multiplier (e.g. 13x133).

    DDR runs at the FSB speed times 2. so 133x2 is 266M. which is 2100GB memory. but the memory controller isnt that effective and u will never get the 2100Gb.

    but u can run ur memory asynchronious (dam spelling) which means it isnt the same as the mobo FSB but higher.
    With ur mobo u can run the memory up to 166M which in ddr terms is 333Mhz which in turn is a maximum capability of 2700GB. which again you will never archive stupid isnt it.

    if you overclock your processor via the FSB (133M) you will also increase the Memory Bus (166 at the moment) by each 1Mhz you increase the FSB for. if your mobo can overclock the FSB.

    the pci slots and hard drivers run at 33Mhz (133 diveded by 4). Thats why we have dividers. If you overclock the FSB you also increase the PCI bus and most hardware doesnt like this if you go over 40Mhz. If you run a 166FSB processor you will need a 5 divider.

    the agp slot runs at 66Mhz times the AGP speed which is for you 4! so 4 times 44 is 264Mhz. this will aslo increase if you overclock.

  3. #3
    EX Moderator-May He Rest in Peace rangeral's Avatar
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    Also upping your vcore to 2.8 is not good if this is what you mean and can do damage, usually 1.85 is the most you want to go maybe 2.0 but not wise, your system is probably around 1.70 thereabouts so 1.85 is safe to use.

    My shuttle ak35gtr can go higher than 2.0 which is why I selected it over an epox at the time but wouldn't go that high myself, besides the extra heat your building up as well.

  4. #4
    Ultimate Member stix_kua's Avatar
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    Originally posted by germanjulian
    hey there.

    ok

    the fsb is what ur mobo runs. which in ur case is 133Mhz (i will just call it M from now on)

    ok 133M

    the cpu runs at 133Mhz plus the cpu multiplier (e.g. 13x133).

    DDR runs at the FSB speed times 2. so 133x2 is 266M. which is 2100GB memory. but the memory controller isnt that effective and u will never get the 2100Gb.

    but u can run ur memory asynchronious (dam spelling) which means it isnt the same as the mobo FSB but higher.
    With ur mobo u can run the memory up to 166M which in ddr terms is 333Mhz which in turn is a maximum capability of 2700GB. which again you will never archive stupid isnt it.

    if you overclock your processor via the FSB (133M) you will also increase the Memory Bus (166 at the moment) by each 1Mhz you increase the FSB for. if your mobo can overclock the FSB.

    the pci slots and hard drivers run at 33Mhz (133 diveded by 4). Thats why we have dividers. If you overclock the FSB you also increase the PCI bus and most hardware doesnt like this if you go over 40Mhz. If you run a 166FSB processor you will need a 5 divider.

    the agp slot runs at 66Mhz times the AGP speed which is for you 4! so 4 times 44 is 264Mhz. this will aslo increase if you overclock.
    ahhhhhh! brain fry......but i get it....


    this means that if you over o/c, then BAM!!! you crash and burn!
    "I'm no technical supervisor, I'm a supervising technician."
    --Homer Simpson

  5. #5
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    no it doesnt mean that. thinks become unstable!!!! and u get errors, and the hardware damage ismore over time then sudden, but it can happen. Most systems work fine with a 44PCI BUS, and nowadays most mobos have a 5 divider, so if u got good cooling for ur cpu u can run the FSB at 166Mhz instead of 133Mhz, with the 5 divider everything will run in spec except ur cpu, which will need some seriously cooling and higer voltage

    wouldnt do it. my friend send more v-core though his memory and now he cant set it back to its default voltage cause it just crashes

  6. #6
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    Thanks for the Info...

    You've confirmed most of what I figured, and now I guess I'd bett go think about way of maximising the bandwidth...

    I've managed to get it running at 333Mz async, but it kind of defeats the object as you say... Hmmm wonder if that Athlon can take 166Mhz FSB.....

    Thanks again,

    Mark.

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