System Bottleneck/Bus Speed Confusion

[Raptor9]

I've been considering buying a new computer, and have accordingly been doing some research into what various specifications and components mean within the context of a desktop PC. While I have found some good information, I'm still confused about how all of these components work together by means of various buses, and about the ?bottlenecks? I?ve read can occur. I don't understand how system buses and RAM speed effect overall performance--it seems like having a processor any faster than the RAM or FSB would be pointless, because it can only process information as it receives it; yet, that can't be true because the DDR RAM I've seen seems to run at 400Mhz maximum (which if I understand correctly, means it effectively runs at 800Mhz) yet processors run much, much faster. Does the speed rating of the processor correspond solely to its clock cycle, while the measure of RAM relates more to actual data output capacity?

I also have some other misc. questions:

How many important busses are there to consider when building a PC? (I understand that PCI and AGP busses are all essentially generic, and not really a factor, but what about the memory or FSB? How do they work together, and are they the only ones to look out for?)

What is the significance of a motherboard?s chipset with regards to performance?

How does dual channel RAM work and why is it faster?

Does the CPU solely determine FSB speed, or does the motherboard and/or RAM have to support it as well? (I'm thinking specifically of the AMD 64 Hypertransport technology?does it only work with certain motherboards?)

Thanks in advance for any clarification.

 

[stevty2889]

The CPU and the memory run on the same speed front side bus, they just run at multipliers of that bus. 400mhz DDR, is really running at 200mhz, but uses both the high and low single for carrying data, making it 400mhz effectivly. Thats exactly what DDR stands for, double data rate. Your CPU also runs at multiplier of the FSB. Say you have an 800mhz FSB 2.8ghz P4. The 800Mhz FSB is actualy a quad pumped 200mhz FSB. The CPU has a 14x multiplier. 14x200=2.8ghz. The memory bus, is basicly just transfering data to and from the CPU, while the CPU is actualy performing instructions and calculations. Having fast memory, helps to feed the proccessor information faster, but if it were to run at the same speed as the CPU, it wouldn't be all that effective, because then the memory data would be waiting for the CPU to finish doing the work on the data it already has..a bus is just a connection between components that transfers data to where it needs to go. You'll also notice that 400mhz DDR is called PC3200..the 3200=the bandwidth of the ram, 3.2gb/s tranfer rate. Dual channel doubles it, so if you have PC3200 in dual channel mode, it has a 6.4gb/s transfer rate. Pentium 4's are a lot more bandwidth dependant than A64's, due to A64's having an onboard memory controler that reduces latencies, and also because it has a shorter pipeline than a P4. Motherboards and CPU's both have to have a compatible FSB in order to work together. If you had a P4 with an 800mhz FSB, but your motherboard only has a 533mhz FSB, then they won't work together, or if they do, the CPU won't run at rated speed.

 

[ribbon13]

Here... look at this to see a good example of how memory effects performance
http://forums.anandtech.com/messageview...atid=28&threadid=1475190&enterthread=y

The important factors in buying a PC are what your going to be doing with it. Rendering machines sshould really be a dually. Gaming machines can get away with a slower cpu and need a good video card. AGP is depricated by the way. PCIe (PCI Express) is replacing it. The chipset effects performance greatly... think of it as the spinal column, and the CPU is the brain. Dual channel works in 128-bit interleaved mode. Dual channel gives a real world ~4% performance increase. Not that much.

HyperTransport is how the AMD64 cpus communicate with everything besides the ram. So an AMD64 motherboard by definition would have support.

 

[imported_Kensai]

As stevty2889 pointed out.

 

[ribbon13]

Originally posted by: Kensai
As stevty2889 pointed out.

Post count +1? This isn't OT.

 

[Raptor9]

So, if I'm understanding this correctly, RAM sends data slower, but it can send more at once? If this is correct, then is there a certain number of gb/s any given CPU can process? Also, I'm not sure I understand what multipliers are--is there any place I can read more about all of this?

 

[ribbon13]

Are you wanting to just learn about PCs or are you building a computer?

Technically, an AMD64 would be limited to 6400MB/s by the memory bus, but how fast it processes data, really depends on what you are processing.

 

[akugami]

In layman's terms a bus on a computer is the same as a real life bus with wheels. It's there to transfer something from one place to another. In the case of a real bus it's to transfer people from one location to another. In the case of a computer it transfers data from one place to another (eg: hard drive to the cpu).

Now, some of those buses on a computer don't like going faster than a certain speed so overclocking friendly motherboards have bus locks that lock the bus to a certain speed and it can't go past that. Think of it as having a bus that will have it's tires blow out if you go past the 65mph but will be fine if you're driving it slower than that. In the case of the computer bus, data can be corrupted if you go beyond a certain speed.

There are various buses that are used but you really don't need to know what they are in detail. A general knowledge is good enough if you just plan on building, using and maintaining your computer.