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Project:
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Overclocking 102: Taking Your Overclock to the Next Level |
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Author:
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BigDaddy
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Date:
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November 27th, 2002
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Introduction:
Welcome to the second in MTB’s series of overclocking articles. In our previous article,
Overclocking 101: Overclocking Fundamentals,
we introduced the reader to the most basic concepts of overclocking. Multiplier adjustments and front side bus increments are basic
elements to tweaking a processor beyond its specifications. However, modifying these settings alone will not result in high-level
overclocks. In this instalment in our series, we will cover how clock dividers can help increase the total overclock on a given
system.
NOTE: The information being provided in this article is for information purposes only. The author and the
MODTHEBOX.com website are not liable for any damage to person or property caused by attempting to modify any device
beyond the manufacturer's specified design.
Overclocking Obstacles
There are three brick walls that any overclock will hit which can be compensated for. As was previously discussed, the first obstacle
to overcome is heat. This is the easiest obstacle to overcome, simply by employing some form of cooling solution. That can be as
simple as a CPU cooler upgrade, or as complex as a state-of-the-art phase-change cooling system.
The other scenarios that will hinder your overclocking progress need some understanding of motherboard and CPU design. After
explaining how the system works at stock speeds we will identify why it hinders higher overclocks, and then describe how to
compensate for it. This article will address higher front side bus values and their effect on the entire system, while a future
article will look at processor design and how to compensate for running a CPU beyond it’s designed frequency.
The Front Side Bus
As was previously outlined, the front side interconnects major components on the motherboard. These include, but are not limited
to, the CPU, RAM, the AGP bus and the PCI bus. The front side bus co-ordinates communication among these system components. Some
of these components are fairly recent entities, while others have been around for quite some time. Needless to say, newer system
elements run at significantly faster speeds than their more antiquated comrades do. Also, some of these buses were designed to run
at an absolute speed, while others at a speed relative to another component. For example, the PCI bus was designed to run at one
absolute stock speed: 33 MHz. The memory bus, however, can run either at the same speed as the FSB (synchronously) or slightly faster
than the FSB (asynchronously). Memory ratios and memory timings will be discussed in a future article. For the purpose of this article,
we will assume the memory bus is running synchronously.
Here is some of the absolute stock values commonly found on modern motherboards:
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Front Side Bus Ratings
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| PCI Bus: | 33 MHz |
| AGP Bus: | 66 MHz |
| AMD Duron FSB: | 100 MHz |
| Intel FSB (excluding Northwood B): | 100 MHz |
| Intel Northwood B FSB: | 133 MHz |
| AMD FSB (excluding Durons & T-Breds): | 133 MHz |
| AMD T-Bred FSB: | 166 MHz |
One thing to note about these speeds is that they are the base value FSB settings. Intel and AMD both market their chips as running on
much faster bus speeds (P4 systems are marketed as running at 400 MHz or 533 MHz, T-Breds boast 333 MHz operation). This is the
external frequency, which is derived after things like cache, chipset and processor design are taken into consideration.
You will notice that these values are mathematically similar to each other. The common base value is 33 MHz. The only exception is the
default Intel FSB of 100MHz, which is actually rounded up from 99 MHz. The reason for this is that a single clock using a quartz
crystal ultimately controls all of the timings on the motherboard.
The relationship between the front side bus and the other buses is called the clock divider. The clock divider determines the speeds
of the other buses based on the value of the front side bus. Typical clock dividers are 3:2:1 and 4:2:1.
Here’s how it works. The first number in the sequence is the divider itself. Some people will refer to the entire system having a
1/3 divider, others a 3:2:1 divider. Essentially, they are the same, since most dividers end with a 2:1 option. Applying
the divider to the FSB gives you a base value. Then, that base value is multiplied by a second value to determine the speeds
of the other two buses.
So, here’s an example of how the stock Intel clock divider works
FSB = 100 MHz
Clock Divider = 3:2:1
We start off by taking the FSB and dividing it by 3, the divider. That gives us a value of 33 MHz (rounding it out). This is the base
value. The second and third numbers in the clock divider are multipliers for the AGP and PCI buses. By taking the base value and
multiplying it by the individual bus multipliers, we get the bus speeds.
So in our above example, to determine the AGP bus, we take the base value we got by dividing the FSB by the divider (33 MHz) and
multiply by the AGP multiplier (2).
33 MHz * 2 = 66 MHz
The PCI bus follows the same procedure, but using the PCI multiplier of 1.
33 MHz * 1 = 33 MHz
Adjusting the Clock Divider
Armed with the knowledge of the speeds buses expect to run at and how the motherboard derives bus speeds, the second overclocking
roadblock won’t seem so cryptic.
As you adjust your front side bus to obtain a higher overclock you start moving some of the "absolute value" buses further and further
from their stock values. While the buses may be able to handle some overclocking, they will eventually reach their limit, and keep the
system from booting. The biggest culprit is the PCI bus. The PCI bus cannot handle being pushed too far beyond its stock speed of 33
MHz. As well, some add-in cards may not handle even slight overclocks on the PCI bus (Adaptec SCSI adapters come to mind). Take a look
at the following example of how easily the PCI moves well out of spec.
You decide to increase the front side bus on your Intel P4A motherboard from stock 100 MHz to 150 MHz. Assuming the board is using the
default clock divider of 3:2:1, here’s what the AGP and PCI buses are running at:
FSB=150
Clock Divider=3:2:1
Base Value=50MHz
AGP bus = 50 MHz * 2 = 100 MHz
PCI bus = 50 MHz * 1 = 50 MHz
Now, you might think "Big deal, it’s only overclocked from 33 to 50 MHz". Think again. If you look at it from the percentage of the
overclock, your PCI bus is running at 151%. That may be more than your motherboard (specifically the PCI bus) can handle.
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