i'm wondering what an overclock on a cpu (say fsb from 133 to 166, fixed multi) has to do with the ram speed (e.g. that was set to 166 before overclocking). because i'm reading everywhere that ram can be a weak spot when overclocking your cpu / fsb. so does an increase on fsb actually have any influence on RAM speed ???
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relation between overclocking the CPU and RAM.
- Thread starter omega3112
- Start date
For XP system, FSB is a parameter that directly determines how fast the memory modules can commuicate with the CPU, or the operating frequency of the memory modules.
The following describes a more detailed picture. Under the SYNC mode of memory operation (the preferred mode for XP systems),
CPU_frequency = FSB x CPU_multiplier
memory_frequency = FSB
In the formula, FSB stands for the FSB clock frequency.
The specification and quality of memory module and motherboard chipset determine how high in frequency the FSB can go.
Once the max FSB is determined, for multiplier unlocked CPU, the CPU frequency can be set according to the above formula, also as high as possible.
For system with PCI/AGP lock, FSB can be adjusted as high as possible, so is the memory frequency, independent of the PCI/AGP frequency.
FSB connects the CPU and the north bridge, in which the memory controller is located. Memory modules are connected to the memory controller where I/O and control for the memory are carried out. Data is transferred between the CPU and the memory via the FSB and the memory controller.
As FSB comunicates between CPU and the northbridge chipset, it also carries video data between the CPU and the northbridge which dispatches them to the AGP bus then the video subsystem (video card), as well as all other data between the CPU and the chipset northbridge, so FSB frequency is crucial not just to memory performance, but the overall performance of the entire system.
For XP system, the memory bus is made up of 64-bit data path, i.e. 64-bit of data is moved each time or in each transfer of data.
For one FSB clock, there are two transfers of 64-bit memory data using both the rising edge and the falling edge of the clock pulse (called DDR or double data rate transfer), and so 2 x 64 = 128-bit of data is transferred. 128-bit is the same as 16 byte of data, as 1 byte = 8 bit by definition.
Typically, for a Nforce2 system rev 2 (aka Ultra 400), FSB is rated to run at 200 MHz, hence the max data transfer rate (or called bandwidth) is 16 x 200 MB/s = 3200 MB/s. This is the max amount of data that can be passed between the CPU and the memory per second.
Typically, for XP Nforce2 system, the CPU can absorb about 90-95% of that memory bandwidth, or called effective bandwidth. That % number is usually called memory bandwidth efficiency and it measures how effective a system setup is on memory performance.
Related link:
Single Channel vs Dual Channel DDR Memory
Dual Channel, Nforce2, P4 and AMD FSB
What is cycle time and frequency
Frequency, clock, period of synchronous operations, latency
Latency
Analogy on Bus Speed, Bandwidth and Latency
Analogy for FSB, CAS2, CAS3 latency and bandwidth for DRAM memory
The following describes a more detailed picture. Under the SYNC mode of memory operation (the preferred mode for XP systems),
CPU_frequency = FSB x CPU_multiplier
memory_frequency = FSB
In the formula, FSB stands for the FSB clock frequency.
The specification and quality of memory module and motherboard chipset determine how high in frequency the FSB can go.
Once the max FSB is determined, for multiplier unlocked CPU, the CPU frequency can be set according to the above formula, also as high as possible.
For system with PCI/AGP lock, FSB can be adjusted as high as possible, so is the memory frequency, independent of the PCI/AGP frequency.
FSB connects the CPU and the north bridge, in which the memory controller is located. Memory modules are connected to the memory controller where I/O and control for the memory are carried out. Data is transferred between the CPU and the memory via the FSB and the memory controller.
As FSB comunicates between CPU and the northbridge chipset, it also carries video data between the CPU and the northbridge which dispatches them to the AGP bus then the video subsystem (video card), as well as all other data between the CPU and the chipset northbridge, so FSB frequency is crucial not just to memory performance, but the overall performance of the entire system.
For XP system, the memory bus is made up of 64-bit data path, i.e. 64-bit of data is moved each time or in each transfer of data.
For one FSB clock, there are two transfers of 64-bit memory data using both the rising edge and the falling edge of the clock pulse (called DDR or double data rate transfer), and so 2 x 64 = 128-bit of data is transferred. 128-bit is the same as 16 byte of data, as 1 byte = 8 bit by definition.
Typically, for a Nforce2 system rev 2 (aka Ultra 400), FSB is rated to run at 200 MHz, hence the max data transfer rate (or called bandwidth) is 16 x 200 MB/s = 3200 MB/s. This is the max amount of data that can be passed between the CPU and the memory per second.
Typically, for XP Nforce2 system, the CPU can absorb about 90-95% of that memory bandwidth, or called effective bandwidth. That % number is usually called memory bandwidth efficiency and it measures how effective a system setup is on memory performance.
Related link:
Single Channel vs Dual Channel DDR Memory
Dual Channel, Nforce2, P4 and AMD FSB
What is cycle time and frequency
Frequency, clock, period of synchronous operations, latency
Latency
Analogy on Bus Speed, Bandwidth and Latency
Analogy for FSB, CAS2, CAS3 latency and bandwidth for DRAM memory
Last edited:
- Joined
- May 8, 2004
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You ROCK! This forum needs more people like you describing fuctions in-depth and providing links to support your explanation.
Keep up the good work, your helping the noobz like myself!
Keep up the good work, your helping the noobz like myself!
- Thread Starter
- #4
wow, that IS a detailed explaination ! very good indeed
but i have one question:
i have the abit at7 kt333 mobo which is specified to run at 133 fsb. but it is compatible with ddr333 tho.
so, when i set the dram speed of my ddr333 certified ram to self-timing (166) and my fsb to 166 with 5:2:1 divider, do i have anything overclocked besides the fsb (166) ???
sry if you already answered the question and i'm not aware of it
but i have one question:
i have the abit at7 kt333 mobo which is specified to run at 133 fsb. but it is compatible with ddr333 tho.
so, when i set the dram speed of my ddr333 certified ram to self-timing (166) and my fsb to 166 with 5:2:1 divider, do i have anything overclocked besides the fsb (166) ???
sry if you already answered the question and i'm not aware of it
omega3112 said:wow, that IS a detailed explaination ! very good indeed
but i have one question:
i have the abit at7 kt333 mobo which is specified to run at 133 fsb. but it is compatible with ddr333 tho.
so, when i set the dram speed of my ddr333 certified ram to self-timing (166) and my fsb to 166 with 5:2:1 divider, do i have anything overclocked besides the fsb (166) ???
sry if you already answered the question and i'm not aware of it
This is not a Nforce2 board, so there is no PCI/AGP lock. The PCI frequency will vary with FSB at a fixed ratio.
The FSB, memory, PCI ratio is given by the 5:2:1.
In other words, if FSB = 166 MHz
the system is running with
FSB at 166 MHz
Memory at 166 MHz, 333 DDR
PCI at 166 / 5 = 33.3 MHz
When FSB at 133 with 4:1 ratio, or when FSB at 166 with 5:1 ratio,
the PCI is still running in specification at 33.3 MHz.
If memory is a PC2700 module, it is still running within spec, as PC2700 is specified to run at 166 MHz. Higher than FSB 166 MHz, overclock of memory and PCI begins.
You may overclock the FSB a little bit to around 180 MHz and assuming the memory used can support that speed, at such the PCI would become 36 MHz which is at the safe limit that most people would not cross.
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