Vista with 3 GB RAM vs. 2 GB Ram on a Core 2 Duo?

[RayLopez99]

I hear that Vista can run on an odd multiple of RAM, in other words 3
GB versus 2 GB or 4 GB. How does this work? I thought RAM comes in
pairs--so how would you get two pairs giving you 3 GB?

And does anybody think the extra 1 GB RAM will make a noticeable
difference on a Core 2 Duo?

RL

 

[Dave]

If you are using the 32bit version of Vista,
though you can use 4 gigs of RAM...

the operating system (this applies to all 32 bit operating systems...not
just Vista)
will only be able to use about 3.25 gigs of it.

If your motherboard can only support RAM when used in pairs...

you can use two, one gig sticks
and two 512 meg sticks to give you a total of 3 gigs.

Depending on what type of RAM your motherboard takes, that might not be
possible. I was surprised to discover recently that several major brands of
RAM do not have 512MB sticks in the types I was looking at (DDR2 800, DDR2
1066)

I would advise the OP to run exactly two sticks of RAM to total either 2GB
or 4GB. 2GB should be sufficient. But RAM is so cheap now, 4GB is not a
terrible idea. -Dave

 

[John Weiss]

I hear that Vista can run on an odd multiple of RAM, in other words 3
GB versus 2 GB or 4 GB. How does this work? I thought RAM comes in
pairs--so how would you get two pairs giving you 3 GB?

With DDR2 RAM, use RAM pairs of different sizes if you have 4 slots (2 x 1 GB +
2 x 512 MB). However, in many rigs this will actually slow down memory
performance.

With DDR3 RAM, use 3 slots for best performance (3 x 1 GB).

And does anybody think the extra 1 GB RAM will make a noticeable
difference on a Core 2 Duo?

Up to 4 GB, more is better for 32-bit Vista. The system will recognize
something around 3.3 - 3.5 GB, depending on gfx RAM and a couple other things.

 

[Paul]

I hear that Vista can run on an odd multiple of RAM, in other words 3
GB versus 2 GB or 4 GB. How does this work? I thought RAM comes in
pairs--so how would you get two pairs giving you 3 GB?

And does anybody think the extra 1 GB RAM will make a noticeable
difference on a Core 2 Duo?

RL

Just so this discussion doesn't get derailed...

The operating system is not aware of the intricate details
of the hardware. All it knows about is things like
TOM (Top Of Memory) register. That tells it how much memory
the BIOS has validated as existing and ready to use. The
BIOS has the option to disable one or more sticks, if they
don't meet the requirements. (Some dual channel motherboards,
sometimes disabled a pair of slots, just for fun.)

The OS has some amount of address space to work with.
It also has a division of spaces for kernel and user space.
All of those things, affect how much of the physically
installed memory, actually gets used.

(Memory Limits)
http://msdn.microsoft.com/en-us/library/aa366778.aspx

If you have a four slot motherboard, with two slots per
each of two channels, it is still possible to do things
like install 3 x 1GB DDR2 or DDR3. Intel chipsets
have Flex memory mode, and some other chipsets support
virtual single channel mode. There could be some
reduction in memory bandwidth, but otherwise, the
OS is not aware of how the memory is implemented.
The 3GB total is still seen, but memory operations
average a lower speed.

The X58/Nehalem is the first processor I'm aware of,
to have the memory interface on the processor and offer
three channels. It means the magic number for that new
hardware, is three or six sticks of memory. (Each set
of three should be matched.) The need for three
matched sticks, means some of the memory manufacturers
have introduced new "kits" just for Nehalem users.

Previous motherboards with four slots, worked best with
two or four sticks, where pairs of sticks were matched.
You could use 2x1GB + 2x512MB, for example, if you wanted
3GB total. If you installed 3x1GB, memory bandwidth would
be reduced.

HTH,
Paul

 

[Conor]

With DDR3 RAM, use 3 slots for best performance (3 x 1 GB).

Only valid advice if your motherboard is a P58 chipset.

 

[Paul]

Another thing manufacturers have changed for X58 triple
channel RAM is the voltage is now 1.5 or 1.65V. According
to the Asus P6T Dlx manual, the DDR3 configurations can
be 2,3,4, or 6 DIMMs.

According to my Asus P5B Dlx MB manual, if you install,
say, 3 1GB DIMMs, one in each of channel A1, B1, and A2,
the A1/B1 pair will work in dual channel mode and A2 will
operate in single channel mode. This will also work for
different sizes in the dual channels.

Yes, and that feature of Flex memory, is similar to the
way my Nforce2 motherboard used to do it. The bottom part
of memory, ended up faster than the top part, if
the RAM in the channels wasn't balanced.

For simplicity, I've opted to always run uniform
memory speeds, by matching sticks were required. I
did try the mixed mode for some testing, but didn't
leave it that way.

I downloaded the source for memtest86+ and made
a three line mod to the program, so I could measure
memory bandwidth at different points in the memory
space. That is how I was able to verify that the
Nforce2 works that way. (I.e. Install 3x512MB, and
the bottom 1GB is faster than the top 512MB.)

A virtual single channel mode, such as you'd get on
a pre revision E Athlon64, is uniformly bad from
top to bottom. It would be the same as installing
RAM in only one physical channel.

Paul