What is the difference between FSB and system bus?

You need to look at bandwidth numbers, not MHz. The FSB is the limit on the speed that the CPU can communicate with the system (I/O & RAM), e.g. a 800MHz FSB have a maximum system bandwidth of 6.4GB/s to communicate with the memory. PC3200 (DDR400) has a maximum memory bandwidth of 3.2GB/s in single channel mode, and 6.4GB/s in dual channel mode. For instance the Pentium M/Centrino supports a 400MHz FSB with a maximum system bandwidth of 3.2GB/s, but only supports single channel PC2700 (DDR333) with a maximum memory bandwidth of 2.7GB/s.

All systems have a system bus, but not all use a FSB. Systems with external memory controller (e.g. Athlon XP, P4 & Pentium M) uses a FSB to communicate with it, while systems with integrated memory controller (e.g. Athlon 64) communicate with the memory directly without using the system bus. Systems with a FSB must share all the system bandwith between RAM & I/O (GPU, hard disk, optical drive, etc), while systems without a FSB the system bus is dedicated for I/O only. For instance the Mobile Athlon 64 supports a 800MHz HyperTransport system bus with a maximum system bandwidth of 6.4GB/s dedicated for I/O and also supports single channel PC3200 (DDR400) with a maximum memory bandwitdth of 3.2GB/s that is also dedicated. This translates to a maximum processor-to-system bandwidth (total available I/O & RAM bandwidth) of 9.6GB/s for the Athlon 64 and only 3.2GB/s for the Pentium M.

See also this comparison sheet for more information:
http://www.amd.com/us-en/Processors/...E11029,00.html

With your example the system bus is actually 200mhz (P-4C and above). The FSB is "Quad Pumped" so 4x200=800 and the RAM is DDR so 200mhz x 2 = 400. If you look into Overclocking this starts to make more sense. As a 50 system bus overclock (at 1:1) will push you RAM to 250 so the DDR will be at 500 (DDR4000) and the FSB will be a 1000mhz. Also the processor speed is a multiple of the system bus so a P4 2.4C is using a 12x multiplier for the processor, a 2 multiplier for the RAM and a 4x for the FSB. Now older P4's used a 133mhz system bus also quad pumped but now only to 533, so the RAM runs at 266mhz and so forth. So when looking at P-4's you can find several diferent 3.2Ghz chips some with 533mhz FSB's and some with 800mhz FSB's so the 533. is running at a 24x multiplier where the 800 is running at a 16x multiplier (which puts less stress on the chip).

The P-M is a quad pumped 100mhz FSB (based on the P3) so the RAM is not running 1:1 since it is running at 333mhz DDR normally (so 166mhz).

A-64 get rid of this concept and effectively have 800 or 1600mhz FSB (they do not actually have a FSB I think they call it a Hypersomethingortheother) which is more efficient since it doesn't have to share.

But think of everything (CPU speed, DDR speed, AGP speed, PCI speed) as being a multiple or fraction of the system bus. Hope this helps.

Still confused after reading this thread...but a few questions that maybe you could clear up..


The maximum the CL56 (or any Pentium M Chipset for that matter) can take is a PC2700? Correct?

Well it benefit from 2x256 (or 2x512) more than 1x512 (or 1X1024) since it can then run using that dual channel thing or can it not do that? 1 or 2 sticks better? (I read two is better in some systems, i think they were desktops, cuase it can run dual something)...

Thanks.

The CL56 can't run in dual channel mode. The chipset (the 855) don't support it, only single channel.

PC2700 is the max in this laptop.

I'd go with 1 stick of RAM. It'll give you room to add more later if you want.

Pentium M will get dual channel memory, but not until 2005.

How could it "get" dual channel memory in 2005? If I buy memory now, is there some type I can purchase to prepare for the 2005 upgrade so I dont have to repurchase RAM all over again? What is the best manufactur of RAM in yoru opinion?

When the P-Ms "get" dual channel, what is meant is the new P-M chips will get dual channel. Older P-Ms will not upgrade. To upgrade, you would have to replace the chip.
You could buy dual channel memory now and it would then be useful when you replaced your current single channel memory chip with the newer dual channel chip, but the added cost now probably wouldn't save you much by then. Its probable the newer chips will also be going to DDR2 by late 2005 so your RAM wouldn't work with them anyway. Or the price on RAM will have dropped so much it wouldn't be much of a savings anyway if at all.

Its a good idea to think about the future, but unfortunately, the future in computers is changing so fast and so radically all the time, its almost impossible to prepare for it. Its possible you might score, but just as possible you will miss because something nobody has heard of comes out.

At best, you can think about the future 12 months from now. 6 months is more reliable.
Its better by far to think about the here and now with maybe timing your purchases in a 3 to 6 month slot when you know for sure something is about to happen. You can plan purchases of peripherals pretty good because the basic I/O isn't going to change radically (there's too big of a market out there for a manufacturer to ignore, why do you think nearly everything has a serial port on it?). But the basic components: CPU, RAM and motherboard, are all changing fast. Even GPUs are difficult to think about for more than a year in advance.

Basically, computers are similar to skiing or sailing, expensive and you are continually dumping money into them. At least on the enthusiast or professional level. Any other level, it doesn't matter, you can get by with stuff as much as 2 to 4 years old, even more than that.

In short, they don't call it the bleeding edge for nothing. Just the blood is green.

It will have a new motherboard chipset. It's code named Alviso and will either be designated at a 915 or a 925 chipset. Probably something like a 915M. In order to upgrade, you'd need a new motherboard, which most likely means a new laptop.

The type of RAM you'd neeed will depend on what Intel decides to do with Alviso. Currently it's slated to use DDR2, which means that any DDR memory you have right now will be useless for it. If Intel makes a plain DDR version, which is possible as the desktop 915 can use DDR and DDR2 (but not both at the same time), you may be ok.

Memory I'm less picky about in a laptop as you can't overclock with it. Something good quality like Corsair, Crucial, Kingston, Kingmax, etc would be good.

i dunno why? name me something that uses a serial port..(my TI-89 connect cord did, but now they have a USB version)


anyways, i am kinda pissed cause i got a great deal on the campus edition cl 56 at velocity micro, but it comes with 512x1 pc2100...is it worth upgrading? it will probably cost me $100 to get a stick of 512x1 pc2700...will i notice the upgrade? i doubt i could even sell the old ram anyways...ugh. hmm.