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Front side bus speed

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December 2, 2009 9:49:15 AM

i was wondering how a cpu can have a set fsb speed does it not depend which speed and size ram you use?

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a b à CPUs
December 2, 2009 10:22:45 AM

The speed of the FSB, along with the CPU Multiplier and Memory Divider, determines the speed of your RAM and CPU.
Think of it more as a master clock used by the RAM and CPU to determine how fast they run.
December 2, 2009 10:26:28 AM

It's more like the speed of ram depends on the fsb of a cpu.

so you set 400 mhz FSH for CPU then if your ratio is 1:1 your mem speed will be 400mhz (800mhz effective)
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a c 108 à CPUs
December 2, 2009 11:38:25 AM

A 'bus' is simply electrical pathways that by way of 'hubs' (the north bridge and south bridge) connect the various components of the motherboard.

Most importantly: Hyper Transport (HT) or Quick Path (QP) 'links' (or 'interconnects') replace the front- (and back-side) 'bus'.

The 'speed' (actually, 'transfer rate') of the 'bus' depends upon the width of the path (8-bit, 16-bit, 32-bit, etc) times the speed (frequency) of the bus (33Mhz, 66MHz, 100MHz, 133MHz, etc) times the transfers per clock cycle (... double- or quad-pump).

With HT and QP (I assume - Intel is still a little pricey for me) the speed of your RAMs is dependent upon the integrated memory controller on the CPU.



December 2, 2009 12:09:29 PM

so what is the point of faster ram
a b à CPUs
December 2, 2009 12:16:13 PM

Faster RAM allows you to overclock further as it is stable at higher clock speeds.

As most CPUs require you to raise the FSB in order to overclock, the RAM speed will raise along with it.
If you are running slower RAM, you may not be able to raise the FSB high enough to get your desired CPU overclock without loosing RAM stability.
The actual performance difference between 'slow' and 'fast' RAM is quite low, usually a 0-2% difference.
December 2, 2009 12:52:40 PM

i thought external speed was how fast it was able pull data off the ram and because ram was a bottleneck and there for faster ram = faster external speed but if im wrong what are the determining factors of external speed? thanks
a c 108 à CPUs
December 2, 2009 2:42:02 PM

shlib92 said:
i thought external speed was how fast it was able pull data off the ram and because ram was a bottleneck and there for faster ram = faster external speed but if im wrong what are the determining factors of external speed? thanks


Wellllll ..... :na: 

Ultimately, it's the time it takes RAM to recharge (or 'refresh') an active memory cell.

This is generally defined by access time and cycle time. When the data stored in memory is not ready to be accessed, then the processor has to wait a few cycles until the memory is ready. This is called a 'wait-state'.

There is a rather simple equation to calculate this but I'm too brain-dead at the moment to recall --- lol (i.e.: lazy). The point I guess I should make is that the faster the memory when compared to the speed of the CPU, the fewer the overall wait-states.
a b à CPUs
December 2, 2009 3:04:27 PM

Yup, and in situations where you are waiting for data the Latency is more important than pure Bandwidth.
The Latency reflects how many cycles from requesting data you will receive the data.
High Bandwidth is more useful in a server or workstation environment when you need to quickly load large amounts of data into the RAM.

Here are a few things you should read through as you are interested in the subject:
Toms Memory FAQ
Understanding RAM Timings
How memory frequency affects latency - Tighter timings vs. Higher speeds?
!