Legacy Intel LGA 775 / AMD AMx CPU Overclocking Guide

Before I start the core just keep in mind – what the acronyms below mean(Just in case you are a complete noob like me ):

Legacy Intel LGA 775 Overclocking

Basically Front Side Bus (FSB) is set of wires i.e. a Bus that connects CPU to the “Northbridge(Chipset)”.By the way – Intel also calls the Northbridge as “Memory Controller Hub” or MCH for short.

Wait … what is the Northbridge in the first place?

Now back to where I was …

This FSB is the address and data bus which the CPU uses. The CPU interacts with all the other components of the computer through the FSB via the Northbridge. Since the Northbridge is so central to all the connections in the computer – increasing the FSB will also increase the speed at which other components interact with the Northbridge which in turn will increase the system performance. Overclocking the FSB may be a little tricky for someone new because the base clock rate differs from the effective(actual) clock rate which is termed as ‘Quad-Pumping’ by Intel.

On systems that support Intel processors the FSB is Quad Pumped which can be stated as :

Hence for eg. a system running at 1333 MHz FSB has a base FSB frequency of 333 MHz.

One of the key factors that makes FSB the first choice to be overclocked is that the CPU’s speed is determined by the Base FSB which is then multiplied by the CPU Multiplier which sets the operating frequency of the CPU.This relation of proportionality between the CPU and FSB frequency gives an instant CPU speed boost when the FSB is increased given that the multiplier remains the same.

The formula is given as :

The CPU multiplier is also referred to as CPU to FSB ratio in some BIOSes – its logical because if you divide CPU frequency with FSB freq what we get is the CPU Multiplier.

Lets take an example of Q9300 which has native FSB of 1333 MHz (Quad Pumped) and Multiplier ‘Top-Locked’ to 7.5

Native Processor FSB is the default FSB which is used to generate the CPU frequency. Each processor has its own predefined default FSB. When every thing is set to default – i.e. no Overclocking – the CPU dictates the Northbrigde that it wants to work at the default CPU FSB.

When is comes to FSB we are mainly concerned with max FSB supported by the motherboard and the native FSB of the processor. The Q9xx0 series Quads from Intel may have the edge of being 45nm based and slightly faster than the 65nm based Q6×00 series Quads. But the 45nm Quads have higher native FSB of 1333 MHz which makes it difficult to overclock beyond a limit and it even gets worse for quads like Q9300 which has the multiplier at 7.5 max. Consider today’s p35 , x38 , p45 , x48 chipset based motherboards which can be OCed to 1600MHz easily but not quite beyond 2000 MHz i.e. a Base FSB of 500 MHz is limit for the current generation chipsets. Now with Q9300 the ‘max possible (can be pushed more)’ overclock will be = 500 x 7.5 = 3.75 GHz which does not make it suitable for Overclocking because such a high FSB will tax both the chipset and also the RAM to maintain good FSB : DRAM ratios [ill explain RAM OCing in next Article].

On the other hand lets consider Q6600 which has 1066MHz with multiplier of 9 max. With Q6600 the ‘max possible’ OC will be = 500 x 9 = 4.5 GHz which some have reached. I remember that the Tom’s Hardware team from France managed to push Q6600 up to 5 GHz on liquid nitrogen. Due to this relation between the Base FSB and the CPU Multiplier the CPU having native FSB less then the max FSB supported by the motherboard has a more Overclocking head room than a CPU have native FSB same as the max FSB of the motherboard.

In our consideration of Q9300 and Q6600 I would go with Q6600 because its more Overclockable and will not tax the chipset with extreme high FSB as with Q9300. But on the other side the main advantage of Q9300 is the higher FSB itself. Since the native FSB of the processor dictates the default FSB at which the system will run having higher default FSB will make system run fast due to higher FSB. But the down fall is that further OC is possible from the default FSB , hence more the head room more is the Overclockability.

AMD With HTT

On AMD systems the bus used to interface the CPU with Northbridge uses HyperTransport Technology (HTT) which was earlier called Lighting Data Transport (LDT). This is similar to FSB on Intel systems which uses Assisted/Advanced Gunning Transreceiver Logic (AGTL+). On AMD systems the bus speed is the base HT speed from which we get the HT Link speed.

Here’s the formula :

Usually the HT multiplier swing is limited between 1 and 5(max).
So if we have bus speed of 200 MHz and HT multiplier set to 5x then our HT link speed will be : 200 x 5 = 1000 MHz HT Link

The HT link speed is used to get actual data rate of the system bus. Since HTT can transfer data twice per clock pulse (Double Data Rate) the effective can be calculated as :

In our case we have: 1000 MHz x 2 = 2000 MHz Effective Bus Data Rate

As on Intel systems the CPU speed here is also obtained by multiplying the CPU Multiplier with the bus speed (Base HT) which is as follows :

Lets say we have an AMD X2 5000+ which has its multiplier set to 13 . Hence in our case the CPU would be : 200 x 13 = 2600 MHz = 2.6 GHz