Extra ram in dual channel mode


G

Gabriel Knight

Hi all, I have two sticks that are both 2gig sticks in dual channel mode
my question is If I put another two sticks in do they have to be the
same as the sticks I started with as I don't have quad channel support
on my motherboard, If I buy the same speed and voltage but different
brand will it effect the dual mode support or not?

im using:
G.Skill DDR2-1066 PC2-8500 CL5-5-5-15 2.0~2.1v

Thanks GK.
 
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Paul

Gabriel said:
Hi all, I have two sticks that are both 2gig sticks in dual channel mode
my question is If I put another two sticks in do they have to be the
same as the sticks I started with as I don't have quad channel support
on my motherboard, If I buy the same speed and voltage but different
brand will it effect the dual mode support or not?

im using:
G.Skill DDR2-1066 PC2-8500 CL5-5-5-15 2.0~2.1v

Thanks GK.
Well, some variations are allowed, and some not.

For example, say I had a "low voltage RAM" kit, mixed with a
"high voltage RAM" kit. I might not be able to find a VDimm setting,
that makes both of them work (meet their overclocked timing).
If I ran only 1.9V, the 2.1V kit might not meet timing
(might have to run CAS6 perhaps).

If they were rated for the same upper voltage range, then you
might have a better immediate handle on what speed and timing they
can be run at. If you run mixed voltages, the speed and timing
are established by testing them.

If we went back to an era where voltage was not such an issue,
what else would we notice ? Say I had a PC2-6400 CAS6 kit mixed
with a PC2-6400 CAS5 kit. All four sticks end up running at CAS6.

If I had a PC2-6400 kit, mixed with a PC2-8500 kit, then the
computer runs at PC2-6400 rate. The "lowest common denominator",
for speed and timing is selected.

Now, in the latter example, it's possible that when the PC2-8500
is run at PC2-6400, it doesn't need quite as much voltage. So you
may be able to run a mixture of low and high voltage DIMMs, at a
lower voltage (and a lower speed). As long as you don't expect
them to be meeting their peak speed/timing at the time. So if
you're mixing "extremely different" kits, you're best off doing
extensive testing, before booting into Windows. That includes
memtest86+ for a first attempt at stability, as well as running
Prime95 from a Linux LiveCD, to prove it's really stable.

There is one other parameter, called Command Rate, but it was
probably already set to 2 (slow), due to the clock rate of the RAM.
There is a small penalty associated with running four sticks
instead of two sticks (bus loading), and command rate can be
set to 1 or 2 to compensate. When set to 2, the command sits on
the bus for two cycles, but is strobed into the memory on the second
cycle, giving an entire cycle for Tsetup.

On older RAM technologies (PC3200), there is a more significant
penalty with added bus loading. And typically, a user might go
from DDR400 to DDR333. DDR2 and later technologies, aren't affected
to the same degree. If there's any problem at all with running
the four sticks, it might only require a minor adjustment (say, bump
up Trcd by 1), to fix it. Or even, run the Northbridge memory
controller at a slightly higher voltage (to improve the timing).

*******

Considering the speed (1066), CAS (CAS5), and 4GB total of your current
kit, I might be tempted to leave well enough alone. You're doing
pretty good for DDR2 there, as it is. Adding more RAM, you might
have to adjust for a lower performance setting. So there had better
be a good reason for wanting something like an 8GB configuration.

If you had a slow setup like I've got (CAS6 PC2-6400), then there'd
be no penalty with going to 8GB. But if you have high performance
RAM already in place, there's a small risk you're going to need
to detune it a bit, for stability. And you can't really buy good RAM
these days for DDR2 anyway - the enthusiast companies have moved on,
leaving "slow RAM" for sale.

Paul
 
G

Gabriel Knight

Thanks for the detail and information Paul, I need the extra 4 gig to
see if rendering in Blender will be faster I have to test it on two
operating systems one win7 and BlenderBuntu 2.5 both 64 bit but why do
you say that if I add more ram I might have to adjust for a lower
performance setting? Isn't more ram going to make things faster?

GK.
 
P

Paul

Gabriel said:
Thanks for the detail and information Paul, I need the extra 4 gig to
see if rendering in Blender will be faster I have to test it on two
operating systems one win7 and BlenderBuntu 2.5 both 64 bit but why do
you say that if I add more ram I might have to adjust for a lower
performance setting? Isn't more ram going to make things faster?

GK.
Let's say you buy 4GB DDR2-1066 PC2-8500 CL5-5-5-15 2.0~2.1v to
go with the RAM you have currently. Will it run CAS5 ? Will
you need to back off the timings slightly ? I don't know the
answer to that. A slightly timing change, doesn't affect performance
that much. If you had to drop the clock rate, that still isn't
the end of the world.

I have one computer, where there was no difference between running
with two DIMMs and four DIMMs. But I suspect that is the exception
rather than the rule.

Can you still find DDR2-1066 PC2-8500 CL5-5-5-15 2.0~2.1v products ?

Usually what happens, is the manufacturers of enthusiast RAM, leave
the market, and the RAM that remains for sale is stuff like PC2-6400 CL6.

Paul
 
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B

BW

cycle, giving an entire cycle for Tsetup.

On older RAM technologies (PC3200), there is a more significant
penalty with added bus loading. And typically, a user might go
from DDR400 to DDR333. DDR2 and later technologies, aren't affected
to the same degree. If there's any problem at all with running
the four sticks, it might only require a minor adjustment (say, bump
up Trcd by 1), to fix it. Or even, run the Northbridge memory
controller at a slightly higher voltage (to improve the timing).
I still have some DDR2 PCs. There is one with Intel G33 chipset that
falls back to 667 speed
if you put 4 faster DIMMs in it. Although the manual gives no mention
of this crippling.
 

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