FSB and memory bus conundrum.

  dagwoood 04:18 03 Oct 04

I've come across something and hope someone could shed some light on it please.

I've had a cpu upgrade that facilitated the need for a new motherboard and RAM.

I noticed that my bios wasn't auto detecting the RAM's correct speed(showing 166Mhz instead of 200Mhz)and so manually changed it to 200.

After running some benchmark tests, I couldn't understand why the memory scores hadn't improved and so downloaded SiS Sandra to check out if my memory was PC3200, as per the labels on the memory sticks.

SiS Sandra confirmed that the memory is PC3200, but I noticed something that caught my eye. The FSB is showing as 2x200, which is correct(as per bios), but there was another entry called the memory bus. This showed a value of 2x133, and in brackets, 266Mhz data rate.

From what I'd read, I thought the memory bus and FSB were one and the same thing, or aren't they? As the memory bus is showing a value of 266Mhz, am I correct in assuming my RAM may only be running at this speed and not 400Mhz?

Hope you understand all that :0)

I'd REALLY be grateful for some input as I've Googled but couldn't find anything useful.

Cheers, dagwoood.

P.S.Asrock K7VT6 motherboard, Athlon XP 3200+ cpu and 2x512 pc3200 RAM, running on XP.

  temp003 05:44 03 Oct 04

Yes, memory bus should mean FSB (it basically just means the line of communication between the CPU and main memory), but use of computer terminology is never consistent. I guess in SiSoft Sandra FSB means the operating frequency of the CPU, and the memory bus just means the operating frequency of the RAM.

If you have checked the settings in BIOS, under the Advanced tab, and both the CPU frequency and the DRAM frequency are shown as 200MHz, then there's not much you can do.

The only thing to try is to use manual settings. Your motherboard online manual suggests that you can use jumper settings to determine the FSB. On the motherboard, to the right of the PCI slots, just underneath the AGP slot, there are 3 sets of jumpers. Consult the manual and double check the jumper settings for FSB400 (i.e. 2x200MHz). Then go into BIOS and select Manual for CPU, set the frequency to 200MHz, multiplier to 11. Then set DRAM frequency manually to 200MHz. Save settings, and see if it makes any difference.

If you have done all this already, then that's it. Or try another utility to test the memory speed.

  georgemac 09:15 03 Oct 04

On new motherboards the cpu and memory buses are separate and can run at different speeds

my xp3000 is running at 167/333 and my ram at 200/400 on an msi board

click here everest home edition is a useful tool for identifying your system components - have not run sandra for a while now

  georgemac 09:18 03 Oct 04

it may be a sandra bug? in my bios I always leave the memory setting set to auto to set speed etc

everest shows you the manufacturer of each ram module with all the details - cannot remember if sandra does this.

  dagwoood 16:45 03 Oct 04

Thanks for the input guys. Sorry for the delay in posting back.

I found out where the problem was, and it was to do with a bios setting. In my bios there's an option called "Flexibility Option". It describes it in the bios as follows "Allow better tolerance for DDR memory". This should read cripple faster RAM ;0)

This option was enabled, and when I disalbed it and ran SiS Sandra, it showed the memory bus was running at 2x200. Ran the benchmarks in Sandra and now the memory is performing as it should.

Temp003, georgemac was correct in saying the memory bus is diffent to the FSB. I went on the VIA site and looked at a diagram for the KT600 chipset. The FSB connects to the north bridge, and this in turn connects to the memory via the memory bus. You were right when you said the memory bus reading is a reflection of what speed your memory is running at.

Georgemac, I downloaded Everest but found the Sandra software more comprehensive in that there are more benchmarks you can perform and it shows bus settings as they are set, Everest just shows what the hardware can support.

Thanks again guys for the advice/info, it was appreciated, dagwoood.

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