Hi, I am trying to get my head around overclocking. I have done a fair bit of research and I understand how to do it in practice and I almost understand the theory but I am getting confused. I understand that the CPU Multiplier x Mobo FSB = True CPU Speed. So, how can a CPU running at say 1500Mhz chuck out data at the motherboard when the bus the data is travelling on might only run at a 133MHz?? I imagine the simplest way to overclock is to get an unlocked CPU (or unlock one) and up the muliplier.... so that others components are not in the equation (like PCI / AGP / RAM). Is it right that the FSB of the mobo is the speed at which the motherboard communicates to the CPU? If so, how can the CPU communicate at anything but the top speed of the mobo (say 133MHz) I would really appreciate some expert help here because it's driving me nuts!! Thanks Shaun

Come on jam, help this guy out !!

LOL! Hey, why lay this one one me??

Then where is johnoh?

Good question...johnoh hasn't been heard from in a while... I'll do my best in his absence...hopefully some of the other regulars will jump aboard & help out. The Front Side Bus would be better described as the communication path between the CPU, main memory, & chipset. All data processed within the CPU runs at full processor speed, but external communications run on the FSB. That alone should tell you that the FSB is the bottleneck in most systems. The PCI bus is the path that the hard drive & PCI cards use to communicate with the motherboard chipset. It's default speed is 33mhz. The AGP bus is a dedicated path that video cards use to communicate with the main memory. It's default speed is 66mhz, twice the speed of the PCI. Generally, the PCI & AGP bus speeds are a ratio of the FSB. For instance, on a 133mhz FSB system, the PCI is 1/4 FSB & AGP is 1/2 FSB. You can see that by raising the FSB, you'd also be raising the PCI/AGP speeds. Although many of the newer boards now allow you to lock the PCI/AGP at their default speeds, most previous boards do not. All components, whether it's the CPU, memory, PCI, AGP, etc have a bit of a "buffer" or speed tolerance designed into them. If that wasn't the case, we wouldn't be able to overclock at all...PCI cards would HAVE to run at 33mhz & no higher...PC133 memory would HAVE to run at 133mhz...a 2.0ghz CPU would HAVE to run at 2.0ghz...& so on. It's been determined that the safe max PCI speed is about 37.5mhz, so that number should be kept in mind when overclocking via the FSB. Overclocking via the multiplier increases the CPU speed only, but does little for overall system performance. Raising the FSB however, increases the speed of entire system, so ideally, this is what you wanna work with. Let's take an example using a 2.0ghz CPU on a 133 FSB system. The CPU would have a multiplier of 15 (15 x 133 = 2000). Using the info that I gave above, the theoretical max FSB should be about 150mhz. Why? Because 150/4 = 37.5mhz, & that's the predetermined safe limit for the PCI bus. But will the CPU run at this bus speed? Maybe, maybe not...15 x 150 = 2250mhz...that's a pretty fair overclock...it may actually be necessary to LOWER the multiplier, provided you have a CPU that's unlocked (or unlockable). Will PC133 memory handle this speed? Once again...maybe, maybe not...chances are, you'd either have to upgrade to PC150, or lower the FSB to a speed that PC133 is more likely to tolerate...usually about 142mhz. Basically, once you know the theory & the limiting factors, it's all trial & error from there. One thing I neglected to mention was CPU voltage. Different CPUs run at different voltages...it's best to keep it at the default voltage in the beginning & see what speeds you can hit by manipulating the FSB & multipliers. Once you hit a point of instability, increasing the voltage a little may help stabilize the system again. Increasing the voltage will also increase the CPU temp, so the temp should be monitored constantly. I didn't wanna get into the whole DDR (double data rate) issue because that can really get confusing...lol!

jam, Yes, it does get confusing real fast. Here's my shot at this...hope I didn't try to over simplify this. Shaun, Your comments pretty well describe the reasons overclockers cry for help all the time. Using your example of a 133Mhz front side bus 64bits wide would give us a bandwidth (or data transfer rate) of 1.06 Gigabytes/second. So, wtf is up when the processor is working at 1.3Ghz and the front side bus is working at 133Mhz? Now, I'll take a stab at this starting with some definitions: Hz=cycles per second. US house current runs at 60hz Byte=8bits. Modern front side bus is 64bit, processor is 32bit, AGP is 32bit Bytes/sec=the rate data is transferred Front Side Bus (FSB) - This is how fast your motherboard chipset communicates with your CPU, and is also one of the two factors that determine overall CPU speed in MHz. The FSB also controls every other clock in the computer in older motherboards, though in newer ones, some of the clocks are independent. CPU Clock Multiplier - This is a unitless value, which's value increases or decreases in set increments, depending on the type of cpu (such as 10.0, 10.5, 11.0, etc). This value only affects the cpu, and is the other determining factor in overall cpu speed in Mhz, when multiplied together with the fsb. We are measuring the speed things happen in Mhz or Ghz but we are at the same time measuring data transfer in Mb/sec or Gb/sec. Increasing the cpu multiplier makes the processor faster i.e. 133Mhzx10multi is faster than 133Mhzx9.5 multi but, data transfer speed does not change. If we leave the multiplier at 9.5 and up the fsb to 140 we get the same speed as 133x10 but we also get faster data transfer all over the motherboard circuits. This is possible in varying amounts depending on however your machine is configured and is different from any one pc to any other. In the three above examples we have: 1. Speed at 1330Mhz, fsb capable of transferring at 1064Mb/sec 2. Speed at 1263.5Mhz, fsb capable of transferring at 1064Mb/sec 3. Speed at 1330Mhz, fsb capable of transferring at 1120Mb/sec So, overclock choice #3 is preferable to either 1 or 2. Upping the fsb gives you the most bang for the buck until other data paths (memory, AGP, PCI) begin to fail. Multiplier increases also aid number crunching, video rendering and so forth. I'm sure it's possible to completely clog the fsb bandwidth (the 1064Mb/sec thing above) but this is seldom the case as the AGP bus is set to run at 66Mhz and PCI at 33Mhz. You'd have to be pumping a hell of a lot of info through the fsb at the same time. So to recap, cpu speed reflects how fast information can be processed and the fsb determines how much data can actually be moved from processor to memory to agp card during each clock cycle. All these theoretical data transfer rates don't mean all that much either. Example of a modern IDE harddrive running at ATA133 would lead you to believe it's gonna transfer data at 133Mb/sec when around 45-50Mb/sec is the norm. Well, this should get something started. Jump on in folks. Skip

Gee,jam and skip, my eyes hurt. :-) Are your fingers sore? Shaun, you are lucky we have these guys around. Yes, where is johnoh?