Odd Divisor Correct – The Tech ARP BIOS Guide

Odd Divisor Correct - The Tech ARP BIOS Guide

Odd Divisor Correct

Common Options : Enabled, Disabled

 

Quick Review of Odd Divisor Correct

The memory clock speed of AMD Athlon 64 / Opteron processors is determined using this formula :

Memory Clock Speed = Processor Clock Speed / Memory Divisor

But instead of giving you the option of selecting any memory divisor you want, each Athlon 64 / Opteron processor has a fixed number of divisors designed to meet compatible memory standards. You can, of course, select any memory divisor you like even if you are using slower memory modules. However, it is still quite frustrating as each processor only comes with three memory divisors.

Perhaps with that in mind, AMD implemented a new feature called Odd Divisor Correct in all Athlon 64 / Opteron processors from Revision E onwards. It automatically rounds up any odd memory divisor to the next number, which is always an even number. It has no effect if the memory divisor is already an even number.

The odd memory divisor is always rounded up, thereby resulting in a lower memory clock speed. This rounding-up is done quietly in the background. So, you will not realize that your processor is using a higher memory divisor, unless you actually benchmark and compare your memory system’s performance.

While it may sound like a useless feature, it can be used to gives you additional memory divisors that would not have existed otherwise. In the example of the Athlon 64 X2 6000+, enabling this feature would give you an extra memory divisor of 10 (when you select a divisor of 9).

As such, Odd Divisor Correct is mainly a feature for overclockers. Judiciously enabling and disabling this feature will give you one or more additional memory divisors to play with. The number of extra memory divisors you can obtain using this feature depends entirely on the number of odd memory divisors in your processor.

However, if you do not overclock, you should disable this BIOS feature. Enabling it certainly will not improve performance. In fact, it will do quite the opposite.

 

Details of Odd Divisor Correct

The memory clock speed of AMD Athlon 64 / Opteron processors is determined using this formula :

Memory Clock Speed = Processor Clock Speed / Memory Divisor

But instead of giving you the option of selecting any memory divisor you want, each Athlon 64 / Opteron processor has a fixed number of divisors designed to meet compatible memory standards. Socket 939/940 processors come with divisors that allow them to support PC2100, PC2700 and PC3200 DDR memory, while Socket AM2 processors have divisors that support PC2-4200, PC2-5300 and PC2-6400 DDR2 memory.

If we take the AMD Athlon 64 X2 6000+ processor with a clock speed of 3000 MHz as an example, it has memory divisors of 12, 9 and 8. These allow it to support the following memory speeds :

Memory Clock Speed = 3000 MHz / 12 = 250 MHz (500 MHz DDR)
Memory Clock Speed = 3000 MHz / 9 = 333 MHz (667 MHz DDR)
Memory Clock Speed = 3000 MHz / 8 = 375 MHz (750 MHz DDR)

As you can see, the Athlon 64 X2 6000+ processor’s memory divisors won’t allow you to achieve the full potential of PC2-4200 (533MHz DDR) or PC2-6400 (800MHz DDR) memory. This is because all Athlon 64 / Opteron memory divisors must be whole numbers.

The only way to reach the proper memory clock speed would be to overclock the processor. For the Athlon 64 X2 6000+ processor to run with a 400MHz memory clock, you will need to overclock the processor to 3200MHz.

You can, of course, select any memory divisor you like even if you are using slower memory modules. For example, you can use the memory divisor of 8 with PC2-5300 memory, essentially running them at 375MHz instead of 333MHz. However, it is still quite frustrating as each processor only comes with three memory divisors.

Perhaps with that in mind, AMD implemented a new feature called Odd Divisor Correct in all Athlon 64 / Opteron processors from Revision E onwards. It automatically rounds up any odd memory divisor to the next number, which is always an even number. It has no effect if the memory divisor is already an even number.

The odd memory divisor is always rounded up, thereby resulting in a lower memory clock speed. For example, if you select a memory divisor of 9 in the AMD Athlon 64 X2 6000+ processor, this feature will automatically modify it to a memory divisor of 10.

Memory Clock Speed (Odd Divisor Correct disabled) = 3000 MHz / 9 = 333 MHz
Memory Clock Speed (Odd Divisor Correct enabled) = 3000 MHz / 10 = 300 MHz

This rounding-up is done quietly in the background. So, you will not realize that your processor is using a higher memory divisor, unless you actually benchmark and compare your memory system’s performance. Software utilities like CPU-Z will continue to report the odd memory divisor even when it has been “corrected” by this feature to an even number.

While it may sound like a useless feature, it can be used to gives you additional memory divisors that would not have existed otherwise. In the example of the Athlon 64 X2 6000+, enabling this feature would give you an extra memory divisor of 10 (when you select a divisor of 9).

As such, Odd Divisor Correct is mainly a feature for overclockers. Judiciously enabling and disabling this feature will give you one or more additional memory divisors to play with. The number of extra memory divisors you can obtain using this feature depends entirely on the number of odd memory divisors in your processor.

However, if you do not overclock, you should disable this BIOS feature. Enabling it certainly will not improve performance. In fact, it will do quite the opposite.

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