Front side bus speed vs Ram speed?

Sorry for the stupid question, but I am trying to fix a friends computer. I need to replace the motherboard and cpu. My
friend currently has 512mb of PC100 SDRAM. I found a motherboard/CPU combo on ebay thats inexpensive, BUT.. I'm not sure it
will work with her ram. The cpu in the combo I found runs with a FSB of 133 (its a p3 933mhz). Will my friends current 100mhz
RAM work with the new 133mhz FSB cpu?

 

As far as I know, memory and FSB are operated at 1:1 only. That
is, if the board you are buying uses the 440BX Intel chipset.

http://homepage.hispeed.ch/rscheidegger/p2b_procupgrade_faq.html
http://homepage.hispeed.ch/rscheidegger/ram_bx_faq.html

One option would be to slow the processor down, from 933 to 700,
by using any DIP switches placed on the board for such mods.
You can then play with the clock and test whether the RAM is
still stable or not.

With the 440BX, there is an AGP divider that is set to either
1/1 or 2/3. At 133MHz, AGP runs out of spec at 89MHz. At
100Mhz, AGP runs in spec at 66MHz. At FSB 66MHz, the 1/1 setting
should be selected by the user, to get AGP running in spec at
66MHz. The 89MHz seems to be accomodated by most old video cards,
so this should not be a limitation.

As for PCI bus speed, there are a couple of possibilities.
Some clock generators have a 1/4 divider that operates at
133MHz, and that gives standard 33MHz PCI bus operation.
There are also some that only have 1/3 divider, leaving the
PCI bus at a non-stanaard and non-working 44MHz.

I'm guessing this motherboard was designed to run 133Mhz
processors, in which case if the 440BX chipset is used,
the AGP runs at 89MHz, and that should be OK. Dialing down
the clock that feeds the processor, should allow you to
run with the PC100 RAM. Make sure the motherboard has
adjustments, such as the DIP switches mentioned above.

Take a look at page 18 here, to see what an adjustable
motherboard looks like:
http://www.asus.com/pub/ASUS/mb/slot1/440bx/p3b-f/p3bf-104.pdf

For more precise info, posting the motherboard brand and model
number might be a good idea. It is possible, if some other
chipset is on your new motherboard, that other things are
possible. The 440BX just happens to be the "king of stability".
I have a board with 440BX and it still works.

HTH,
Paul

 

Hi there, actually the motherboard is using the Intel 815 chipset. I was under the assumption that the RAM speed and
motherboard FSB had to be the same. But, I see motherboard/cpu combos all the time on ebay where the cpu is 133 FSB and the
ram that comes with it is PC100. Normally I would just make sure the speed is the same, but as I said Im trying to fix my
friends computer at minimal costs, and if her current PC100 SDRAM will work with a cpu that has 133mhz FSB, (intel 815
chipset) then that is fine by me. So is it possible on the i815?

 

You are in luck. From the 815 datasheet (and presumably applies
to 815E as well):

"System Clocking

The 82815 GMCH has a new type of clocking architecture. It has
integrated SDRAM buffers that run at either 100 or 133 MHz,
independent of the system bus frequency. See table below for
supported system bus and system memory bus frequencies. The
system bus frequency is selectable between 66 MHz, 100 MHz,
or 133 MHz. The GMCH uses a copy of the USB clock as the DOT
Clock input for the graphics pixel clock PLL.

Table 1. Supported System Bus and System Memory Bus Frequencies

FSB System Memory Bus Display Cache Interface
66 MHz 100 MHz 133 MHz or DVMT
100 MHz 100 MHz 133 MHz or DVMT
133 MHz 100 MHz 133 MHz or DVMT
133 MHz 133 MHz 133 MHz or DVMT"

So, go ahead and buy it. Your 512MB will use up all of the
capability of the 815. You won't be able to stuff more
memory into it than what you have got.

For more info on chipsets, click the MCH part number links on here:
http://developer.intel.com/design/chipsets/mature/index.htm

Paul

 

Depends. Some motherboards will allow you to slow down the memory in
the BIOS setup, and some won't. It's hard to say unless you can
actually get your hands on the board, or if someone else has used it
and can tell you.

 

Thanks paul! That's exactly what I needed to know