RAM speed on AMD 64

  • Thread starter Thread starter EdG
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EdG

Does any one know the formula for figuring out ram speed MHz on the AMD
64's? At DRR400 setting the math is simple, clock speed / CPU multiplier
, but once you change to a lower setting like DDR33 it's no longer
valid.

I don't want to download some program, I want to know how to do it with
paper and pencil. :)

Thanks,
EdG
 
EdG said:
Does any one know the formula for figuring out ram speed MHz on the AMD
64's? At DRR400 setting the math is simple, clock speed / CPU multiplier
, but once you change to a lower setting like DDR33 it's no longer
valid.

I don't want to download some program, I want to know how to do it with
paper and pencil. :)
Click to expand...


M = Memory Multiplier.
F = DDR actual speed
R = DDR rated speed
C = Processor speed

F x M = C, obviously

M is chosen such that the actual DDR speed determined by F = C/M
is as large as possible without exceeding R.

I cannot recall whether M must be integral or whether half
integrals are also acceptable.

Example:

C = 1800 MHz, R = 166.67 MHz (for DDR333)

Start with an initial value for M determined by M = C/R = 10.79.
That is unacceptable because it is neither integral nor
half-integral. Any value of M smaller than 10.79 would result in
an F that exceeds R, so obviously we have to use next largest
integer (or half integer ?), which is 11.

So with M = 11, we get F = 1800 MHz /11 = 163.64 MHz.
 
M = Memory Multiplier.
F = DDR actual speed
R = DDR rated speed
C = Processor speed

F x M = C, obviously

M is chosen such that the actual DDR speed determined by F = C/M
is as large as possible without exceeding R.

I cannot recall whether M must be integral or whether half
integrals are also acceptable.

Example:

C = 1800 MHz, R = 166.67 MHz (for DDR333)

Start with an initial value for M determined by M = C/R = 10.79.
That is unacceptable because it is neither integral nor
half-integral. Any value of M smaller than 10.79 would result in
an F that exceeds R, so obviously we have to use next largest
integer (or half integer ?), which is 11.

So with M = 11, we get F = 1800 MHz /11 = 163.64 MHz.
Click to expand...

Initial versions of the Opteron and Athlon64 (the "Sledgehammer" and
"Clawhammer" chips) did require integral multipliers for memory. As
such, as you correctly state above, you ended up with slightly less
then ideal speeds for memory other than DDR400. This often lead to a
difference of up to 10MHz on the memory clock speed, depending on your
processor speed.

However more recent versions of AMDs chips have changed the rules
somewhat. Unfortunately they also seem to have removed the public
documentation as to what those rules are (or at least they've hidden
it such that I can't find it!). You can find a bit of info here:

http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2469&p=2
 
Initial versions of the Opteron and Athlon64 (the "Sledgehammer" and
"Clawhammer" chips) did require integral multipliers for memory. As
such, as you correctly state above, you ended up with slightly less
then ideal speeds for memory other than DDR400. This often lead to a
difference of up to 10MHz on the memory clock speed, depending on your
processor speed.

However more recent versions of AMDs chips have changed the rules
somewhat. Unfortunately they also seem to have removed the public
documentation as to what those rules are (or at least they've hidden
it such that I can't find it!). You can find a bit of info here:
Click to expand...

No, the public docs are still there for the s754 (31410.pdf) and s940
(31412.pdf), which are pretty old but the s939 (31411.pdf) has it removed.
I thought it might have moved to the Bios & Kernel Developer's Guide --
which is fairly recent -- like many other things but I can't find anything
there about dividers, though it *is* heavy going.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2469&p=2
Click to expand...

I wish they'd say where they got this from... even if it's a secret line to
internal docs.:-)
 
EdG said:
Does any one know the formula for figuring out ram speed MHz on the AMD
64's? At DRR400 setting the math is simple, clock speed / CPU multiplier
, but once you change to a lower setting like DDR33 it's no longer
valid.

I don't want to download some program, I want to know how to do it with
paper and pencil. :)
Click to expand...

IIRC, Kingston has some info on this. Corsair's 'Ask Ram Guy' and their
forums may be a good source.
 
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