Location Matters: Fill Your Slots Wisely

It’s a typical rookie PC move: In the excitement of building your machine, you jam the RAM into any slot available. Unfortunately, that’s a practice that can lead to instability or even lower performance.

Here are a few basic tips to help you avoid problems when building any new PC:

• Make sure you’re running on all channels. Even seasoned system builders can get confused by the RAM coloring schemes on motherboards. Asus, for example, used to require that you put the RAM into slots that were the same color to get dual-channel support on its motherboards. MSI, however, used to do the opposite, so you had to have the RAM in slots that weren’t colored the same. This would cause even old salts to pull a brain fart and set a new motherboard in single-channel mode by mistake. The solution is to read the manual before you build the board to find out what the recommended slot config is.

• Put the RAM in the slots closest to the CPU—except when you need to put them in the slots furthest from the CPU. Yup, you got that right. With Athlon 64, Phenom, and Phenom II, it’s generally recommended that you use the slots for each channel that’s closest to the CPU. For Core i7, it’s the opposite. In fact, many motherboards won’t even boot if you put the RAM in slots closest to the CPU first. Again, RTFM.

• void the fourth slot. Intel’s Core i7-based DX58SO motherboard features four slots for RAM. You should fill only three of these for tri-channel mode and avoid filling the fourth, since the company says it will impact overall bandwidth. Intel won’t be the last to design a Core i7 board with just four slots, so consider yourself warned.

 So You Want to Run the Most Memory Possible?

One of the most common questions we get is, why the hell doesn’t Windows XP show 4GB of RAM? The short answer is, 32-bit Windows XP can address only 4GB of RAM. Since address space is also set aside for the graphics card memory and other devices in the system, the majority of 4GB systems end up with 3GB to 3.5GB of RAM available to the OS.

Although you might think that 32-bit Windows Vista overcomes this, it doesn’t. Microsoft just changed the OS to report the total amount of RAM installed—part of that RAM is still being set aside for addressing memory used by videocards and other devices. The only way to fully utilize your 4GB, 6GB, or 12GB of RAM is to run with a 64-bit OS such as Windows Vista 64-bit, which inherently offers additional space for addressing memory other than RAM.

But that’s not all. We’ve found that the still-immature X58 boards and BIOSes aren’t the best at booting 12GB of RAM and seem to get confused on how to configure the memory. Before you fill all DIMM slots, install just three, do your build, and update the system to the latest BIOS. For example, we had to swap 6GB of Patriot RAM for 6GB of Kingston to go with our 6GB of Corsair RAM before we could POST our MSI board. Another trick you might try: Add a tiny bit of voltage. How do you do this if the system won’t POST? Fire it up with just three sticks of RAM and change your BIOS settings. Then power down, unplug, discharge, and install your three other DIMMs and reboot.

Finally, we recommend that if you truly intend to fill every dang DIMM slot on that new Core i7 board, that you go specifically with RAM from the QVL, or Qualified Vendor List, that each motherboard maker publishes on its website. This will increase your chances of having a flawless build. 

Core i7 Memory: Locked or Not?

If you read our Core i7 overclocking guide in the April issue, you know we reported that Intel locked the memory multipliers on its non-Extreme CPUs. We said Core i7-920 and Core i7-940 were simply incapable of running DDR3/1600 at its rated speed without overclocking the bclock. Well, we were wrong.

Most of our Core i7 testing was based on engineering samples of the CPUs. These chips are supplied to hardware vendors and the media to perform tests before the chips go into full production. Normally, the difference is nil between ES parts and production parts, but Intel made a change that unlocked the memory multiplier (and QPI speeds too). The company said it made the change at the last minute in response to “customer requests.” The change was so last minute that PC OEMs and even some Intel field engineers were unaware of it.

This should make cheapskates everywhere smile: Retail Core i7-920s feature unlocked memory multipliers.

This is, of course, good news for folks interested in running higher-speed memory without having to resort to an overclock. Production Core i7-920 and Core i7-940 chips are fully capable of running DDR3/1333, DDR3/1600, or higher-rated RAM, and not locked down at DDR3/1066. This also makes the budget chips just a heartbeat away from the top-end Core i7-965 in capabilities. With the ability to increase the QPI from 4.8GT/s to 6.4GT/s and run the RAM at any speed you want, the only real difference between Extreme and non-Extreme today is the overall multiplier lock of the CPU, fine-grain Turbo Mode settings, and some thermal override settings. This makes the budget chips far more attractive than they seemed just three months ago.