What's the deal with RAM running dual channel? 6

[blackranger]

I recently talked to a sales guy at ibuypower.com.(internet computer building company) I wanted to know why they don't offer any options in their custom built "gaming" laptops for more RAM than 2GB. There are a ton of other laptops out there that offer 3GB or even 4GB of RAM. He said that Vista 64 bit doesn't have any drivers out yet and that's why they don't use 4GB, and he said that when you run 3GB in dual channel you will be unstable and your pc will run slower than if you were just running 2GB RAM dual channel.
When I asked him why other people are doing it, he said that it is just advertising to draw people in and that they probably disable dual channel before they sell the computer to people.????

What am I missing about how dual channel works and if he has a point, why are big name manufacturers selling their high power laptops with 3GB or 3.5GB (that's 4GB in 32bit Vista minus the 512mb video card) like there's no problem? Alienware and Dell XPS have laptops with 3 and 3.5 RAM.

Any insight would be appreciated...

 

[cdogg]

I'm a little late getting back to this, but wanted to address some of the comments. Sorry if this gets kind of long...

1) Dual-channel
If you have 2 x 2GB modules, dual-channel will work fine. What Windows can or can't see of that 4GB does not matter. Dual-channel is a method of data transfer. Windows can't mess with it!
This is a good read about the way dual-channel works:

http://www.kingston.com/newtech/MKF_520DDRwhitepaper.pdf

Dual-channel will beat single-channel.

In most cases, yes. In every case, no. You always have to compare the speed of the memory to the speed of the CPU's FSB. When they match at 667MHz for example, then dual-channel is not needed and will not make any difference.

2) Address Space
Brian Madden's link is a popular one, but it doesn't really go into much detail when you think about it. I don't know why a lot of people reference it to be honest. It talks about how 32-bit is limited to addressing 4GB (something you already knew), mentions the 2GB app & kernel limits (without explaining what the difference is), and brings up "Terminal Server" environments. Hmmm...OK! If you were even concerned about a Terminal Server environment in the first place, I doubt this would be anything new to you or at least I'd hope not!

The term "address space" really needs to be understood first before you can make any good sense of what's going on here. Think of it as being a very detailed table of contents (TOC) that is written before the rest of the book. The TOC is a list of unassigned pointers. The book on the other hand represents what is being pointed to, assigned and in use. Both the TOC and the book are authored (managed) by the BIOS.

As soon as you flip the computer switch to "on", the BIOS allocates address space for the hardware. More specifically this is the I/O memory manager called MMIO, whose goal is to set aside enough space for communication. So within an instant, the TOC is written!

Because everything is fighting over the same 4GB of space in the TOC, the maximum amount of addresses reserved for RAM is under 4 gigs (4GB minus the amount used by MMIO). On the "book" side of things, RAM will also be held to that same number. So even before Windows rears its ugly head, the size of available RAM has already been crippled to something like 3.25GB or 3.5GB.

...because the OS will only recognize 3GB to 3.5GB RAM due to the kernel requiring address space?

It's not the kernel that takes up space. You'll find out more about the kernel below.
The correct answer: MMIO

3) Now you're ready for kernel vs. applications
Of the remaining address space that is available to Windows at bootup, half goes to the kernel (kernel space) and the other half goes to apps (user space). Each can have a maximum of 2GB. The kernel represents things like device drivers, thread scheduling, the Windows GUI (Graphical User Interface), and the HAL (Hardware Abstraction Layer). It is NOT just hardware-related! This article explains the difference between kernel and user mode:

http://www.freewebs.com/four-f/KmdTut/kmd01.html#k1d1d2

The main point to realize after all this is that the answer is not as simple as it may seem!
I disagree with comments that say the video card steals a lot of address space. Remember, it's more the interface (AGP or PCI-E) causing space to be reserved for "communication". You could have a 128MB card or a 512MB card in that slot, it doesn't matter. You would probably still see the same amount available to Windows.

As a final word, read multiple articles. Don't let just one or two make you question the reliability of another. Always get a good sample.


~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Einstein
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[blackranger]

cdogg,

That was by far the best explanation I've got so far. I find myself asking why some of you take the time to give such informative responses, I haven't really figured it out but I sure am grateful there are still some considerate people left in the world.

On a side note, I got sick and tired of researching every laptop in the known universe and comparing every detail of each one. I just bought the Gateway P6831FX the other day. It's the cheapest laptop ($1399.00) with the best video card out. (8800GTS) I got the 2 year service agreement with Best Buy for another $200. I probably won't need it but some people warned me that Gateway has had problems with quality in the past. What finally sold the computer for me besides my dwindling budget and need of a laptop now was reading how other people have upgraded this laptop already. The 1.66ghz CPU is upgradeable so in the future I hope to move up after some current prices have dropped.

Anyway, thanks to everybody who participated in this question, I'm done with it.

 

[cdogg]

blackranger,

I suppose this site is a hobby to a lot of us as well as being a gratifying way to kill time. People that show an authentic interest like yourself often get the most detailed answers. Glad you found what you were looking for...cheers!

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Einstein
[tab][navy]For posting policies, click [/navy]here.

 

[tjsanda]

Just wanted to let you all know, the Gateway P6831FX does run 3gigs of ram in Dual Channel mode.

 

[tjsanda]

Sorry, link didn't work. This one should

 

[blackranger]

tjsanda,

Interesting comment, however I couldn't get your link to work. I would like to know where you got your info, especially since I endend up buying that laptop. I would like to understand my machine better.

thanks for your interest...

 

[Freestone]

http://img.photobucket.com/albums/v618/mistafew/Gatewayp6831fxmemory.jpg

 

[cdogg]

You can check the SPD tab of CPU-Z and it will show you that you have 1-1024MB module and 1-2048MB module running in both slots. The general rule you have to follow for dual-channel to work is to use memory modules that match in size (capacity). When I searched on P6831FX, a lot of sites reported the specs as 3GB running in dual-channel. So you would think there's something going on that we don't know about.

Well, I think the following clears that up:

Certain Intel® chipsets support different capacity chips in what they call Flex Mode, the capacity that can be matched is run in dual channel the remainder runs single channel.

That quote came from the FAQ section on Gateway's notebook support site. You can read it here:

http://support.gateway.com/s/Mobile/SHARED/FAQs/1014142Rfaq9.shtml

So the first 2GB is running in dual-channel while the rest is single. Since I haven't seen any benchmarks on this type of setup, I couldn't tell you if there's any real benefit to Intel's new Flex Mode as opposed to having 4GB in dual-channel (2 x 2048MB). It sounds more like a marketing ploy at this point...but who knows?

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Einstein
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[blackranger]

Thanks for the links guys.

Take care...

 

[HBonner1]

That last post about Flex Mode is interesting... now I have a lot to ponder. I have a Centrino Duo running at 1.83 GHz, 667MHz FSB. Running 3GB of 667MHz RAM, trying to figure out if I should just drop off the 2GB stick and put back in the 1GB stick to match them.
Intel's site lists particular chip sets for desktops that handle Flex Mode:

http://www.intel.com/support/motherboards/desktop/sb/CS-011965.htm

... as well as laptop support for the PM965 chipset...

http://www.intel.com/design/mobile/datashts/31627303.pdf

... but might not apply to my slightly older one.

 

[HBonner1]

Still trying to digest all this...

Control Panel in Windows recognizes the 3GB, as well as does SiSoft Sandra. My current FSB speed is 664 MHz, max is 800 MHz, so maybe I will just migrate to PC6400. TigerDirect has a crazy deal of 2x2GB of PC6400 for $49.99 after rebate...

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3582354&CatId=3633

Sandra also shows that my memory is divided unevenly over 4 logical banks (512MB, 512MB, 1 GB, 1GB). I would assume therefore that Flex Mode is working and, like another site says, "Note: Flex technolgy runs the 1st GB of 2GB stick in dual-channel mode with 1GB stick. Rest 1GB of 2GB stick is used in single-channel." From

http://forum.notebookreview.com/showthread.php?t=239902.

So apparently I have 2x1GB dual-channel and 1GB single-channel.

A comparison from this web site (

http://forum.notebookreview.com/showthread.php?page=4&t=167472,

the one from powerpack on 9-23-2007) shows benchmarks from dual-, flex-, and single-channel configs. Flex is a "bridge" that is closer to DC than SC speed, but not exactly the same.

Great to know - thanks!

 

[jurgen36]

Sett up your memory
1st channel 1Gb
3rd channel 1Gb
2nd channel 500 Mb
4th channel 500Mb

If you do this you will find that you have dual channel access.
Regards


Jurgen

 

[cdogg]

Jurgen,
HBonner has a laptop with only 2 memory slots. He is referring to one slot having a 1GB stick and the other 2GB. As explained above, Flex Mode enables you to "split" the 2GB stick in such a way so that half of it can run in dual-channel, while the other half runs in single-channel.


HBonner,
I'm not sure there is much to gain by upgrading to PC2-6400. Your FSB runs at 667MHz because of the CPU. So if you have PC2-5300 right now, then your memory bus and FSB match exactly. That's what you want. In fact, dual-channel might only make a 0-10% difference in your case depending on the type of activity. Upgrading to 800MHz memory is going to throw your FSB and memory bus out of synch causing slightly more latency which will offset any real benefit.

~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Einstein
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