Question on FSB and RAM bandwidths

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pm

78 posts

Master Geek

# 21613 2-May-2008 10:16

If a motherboard quotes an FSB (front side bus) of say 1066MHz, does that mean it can support 1 ram stick @ 1066MHz and/or 2 ram sticks at 533MHz (1066/2=533)?

I'm trying to decide on the different DDR2 ram's to go for - there is no point going for 4 sticks at DDR2-800MHz since that equals 800*4 = 3200MHz thus, there ends up being 2134MHz (3200-1066) of unused bandwidth - A waste of money.
I figure with 4 sticks the max speed I need is 267MHz for a 1066FSB.

Please correct me.

Mark

1487 posts

Uber Geek

# 127871 2-May-2008 12:11

No it doesn't :-)

The FSB is basically the speed at which the CPU can talk to the chipset on the motherboard. The chipset then is the part that communicates to attached peripherals (USB, IDE etc) and memory.

A good article is in wikipedia:

http://en.wikipedia.org/wiki/Front_side_bus

So basically if you motherboard supports it, go for the fastest RAM you can .. also if you mother board supports Dual channel (or quad channel) then put in two or 4 sticks respectivly.

Regards

Mark

pm

78 posts

Master Geek

# 127891 2-May-2008 12:58

So is the FSB speed divided amongst ALL the attached componenets on the motherboard eg. ide/memory/usb etc...?
If so, then there must be a defined speed/bandwidth allocated to the ram slots right?
If not, that implies that the FSB speed quoted is for EACH ram slot in which case the RAM closest or higher than the FSB is good?

pm

78 posts

Master Geek

# 127903 2-May-2008 13:39

After reading the wiki article I more questions:

The northbridge/southbridge layout figure shows ONE bus line connecting all four RAM slots. Too say each RAM slot has a bandwidth equal to the FSB would mean the northbridge can handle 4*FSB. Going back the 1066MHz FSB example:
4*1066 = 4264MHz! Faster than any CPU speed...

If the CPU is connected via FSB to the northbridge, whats the point in making/buying fast CPU's if the FSB can only transfer data at a rate LESS THAN the CPU speed??

Mark

1487 posts

Uber Geek

# 127915 2-May-2008 14:04

Yes it is shared :-)

Each chipset has 1 or more channels to the memory on the motherboard. then say you had a stick of memory, and that stick had a 64bit address/data bus on it, that means you send and recieve data to that bit of memory in 64bit chunks for every clock tick (not going to get into DDR here hurts my head) .. that would be an example of a single channel.
Dual channel is where you'd have a 2nd 64bit bus to the RAM from the chipset.
quad would be 4 channels.

For a single channel, 64bit bus that does 64bits for every clock tick on a 500Mhz bus you could send or recieve (no concern to overhead or parity done) :

64 * 500,000,000 = 32,000,000,000 bits per second
in gigabytes per second
((32,000,000,000 / 8) /1024)/1024)/1024 = 3.7 Gigabytes per second

Dual Channel double it to 7.4 Gigabytes per second.

You motherboard manual would tell you if it is Dual Channel capable or not.

But then not all devices will be reading/writing to RAM at the same time, some are idle 99.99999999% of the time and others are just talking to other devices on the bus without going to RAM.

Just remember RAM is nowhere near fast enough to feed your CPU anyway. In the CPU there is 2nd level cache (about 2Mb ish on most computers these days) this RAM is MUCH faster than the DIMMs you put in .. but even it is still not quick enough, right in the heart of the CPU are data and instruction caches .. this is actually where all the work is done by the CPU, all the other cahces and main memory are just staging areas before getting to here. (roughly ) :-)

Have a good look over Wikipedia, some really good articles in there that are far more detailed (and accurate) than me :-)

Mark

pm

78 posts

Master Geek

# 127942 2-May-2008 15:39

Mark: .. that would be an example of a single channel.
Dual channel is where you'd have a 2nd 64bit bus to the RAM from the chipset.
quad would be 4 channels.

I understand dual channel, and my motherboard will support it. I'm not too concerned about that anyway.

Mark:
Just remember RAM is nowhere near fast enough to feed your CPU anyway.

Thats my point..essentially everything in your pc is limited by the FSB (assuming the wiki m/b architecture is up to date).
So if I buy a 1066MHz FSB motherboard and I want all four channels filled - I only need each RAM stick too be max 267MHz for single channeling or each stick too be max 533MHz for dual channeling right?

I guess I'm looking for a solid reason as too why faster RAM = better, whereas I don't see how it is due to the FSB rate.