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WHAT do I need to use dual double data rate (DDR) random access memory
(RAM) on my motherboard? A: What most people mean when they say “dual DDR RAM” is actually what is meant as “dual-channel” DDR RAM. “Dual channel” is basically what its name implies – instead of one “channel” to the memory controller, it has two. Let’s look at an example, Intel’s 865PE chipset. If one looks at the diagram at www.intel.com/design/chipsets/865PE/pix/865PE_schematic.gif, one will notice that there are two DDR “groups” (the jade-green “bubbles” marked “DDR”). One will also notice that these groups are connected to the memory controller hub (the one labelled “86865PE MCH”) by yellow lines. These yellow lines represent data channels – the roads that connect the DDR RAM to the memory controller. Look closely and one will notice there are two yellow lines that connect
the DDR groups to the memory controller hub. This implies (but does
not necessarily guarantee) that this chipset is capable of operating
in a dual-channel mode. To confirm, one will have to look for supporting
The description near the controller confirms that this chipset is capable of dual-channel memory access. In this mode, the two banks are accessed separately and independently. This effectively doubles the bandwidth of the memory subsystem. To see how this actually works, consider a housing area. Most housing areas have “access roads”. These roads connect the housing area to the major thoroughfares and are often the only ways to enter and exit the area. Now let’s assume that a housing area has only one access road. As the area becomes popular, the traffic on that particular road increases. At some point, the road will be congested and traffic jams may become
a Dual-channel RAM works very much the same way – there are two “roads” in and out of the memory banks, so the memory controller(s) is no longer forced to wait if that one “road” is congested or not immediately available. The actual implementation of this dual-channel technology differs from
manufacturer to manufacturer; some manufacturers use a single controller
with additional pins while others use two separate controllers. Whichever
the implementation, the end result is the same – memory bandwidth
is The fact that the two RAM banks can be accessed simultaneously, however, imposes some constraints on the type and specification of the RAM that can be used together on the same board. These constraints are usually documented on the manufacturer’s Web site. A summary of these constraints may also be included in the user manuals for some mainboards. Using the earlier example chipset (Intel’s 865PE), one discovers that the correct way to ensure that the mainboard utilises the RAM in a dual-channel configuration is documented in a Portable Document Format (PDF) document. A page on Intel’s site (www.intel.com/design/chipsets/applnots/253036.htm) contains a link to the correct document. According to this document, to enable dual-channel mode, one must have matched pairs. This means that one must either install two or four dual in-line memory modules (DIMMs), not one or three. A DIMM is what is colloquially known as a “stick” of RAM. If one is installing two DIMMs, both of them must be identical. If one is installing four DIMMs, one must install them in matched pairs. This means that two of them must be identical, and the third identical to the fourth. If one looks at a board using this chipset, one will find that, in most cases, the DIMM slots are grouped in twos. Let’s look at an example, Intel’s D865PERL board. A picture of this board can be found at www.intel.com, selecting Motherboards from the first drop-down combo box, and selecting Intel Desktop Board D865PERL from the list of links on the right of the page. This will load a page about the mainboard. The picture of the mainboard is actually a Java applet. This picture can be manipulated – the slider on the lower left of the picture can be used to zoom (magnify) and shrink the picture. Clicking on the button next to it, and then clicking and dragging on the picture will move the board left and right, and up and down. By using these controls, it’s possible to obtain a close-up view of the board without the board being physically present. By manipulating the controls, one will be able to obtain a detailed view of the RAM slots. Looking closely, one will see two DIMM slots, and then a space, and then two more DIMM slots. The first two DIMM slots are connected to the first channel, and the other two to the second channel. To enable dual-channel mode with two DIMMs, both DIMMs must be identical and installed in either the first and third slots, or the second and fourth slots. To enable dual-channel mode with four DIMMs, two of them must be identical, and the third identical to the fourth. The first set of two identical DIMMs should go into the first and third slots, and the second set into the second and fourth slots. For the best results, all four DIMMs should be identical. If all four DIMMs are identical, it doesn’t matter where the DIMMs go, any configuration will work since they are all the same. Some manufacturers have even coloured the DIMMs differently so that one can see where the first set of two DIMMs should be installed. One example is Asus’ P4P800 Deluxe (www.asus. |
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