AMD Chipsets and PCI Express

When AMD introduced the FX series of processors in 2011, it also introduced three supporting desktop chipsets: the 990FX, the 990X, and the 970. These chipsets differ in a number of aspects, but they way they allocate PCI-E lanes is probably of the most interest to an enthusiast.

In the Intel world, CPUs provide a number of PCI-E Gen 3 lanes from the processor– 16 in the case of LGA1150 CPUs such as the Core i7-4790, and 28 or 40 lanes for LGA2011 and LGA2011v3 CPUs. The supporting Z97 or X99 chipset provides 8 additional PCI-E Gen 2 lanes. With LGA1150 systems limited to 24 PCI-E lanes, it’s possible to run out of lanes depending on how your system is set up. In fact some motherboards provide software or hardware switches to control the allocation of PCI-E lanes.

In the AMD world, the CPU provides no PCI-E lanes: it’s all on the chipset. The top-end 990FX chipset has a total of 42 lanes, with 32 lanes dedicated for graphics, blocked into a PCIE-GFX and PCIE-GFX2 block, each with 16 lanes.

The 990 and 970 chipsets don’t have the PCIE-GFX2 block and thus can provide only 16 PCI-E lanes for graphics cards out of 26 lanes total. The 990 can split these up as 2×8, but the 970 cannot, and is thus cannot run an SLU or CrossFire setup…or so AMD says. MSI begs to disagree:

Although MSI does not say how they accomplished this, I’d guess they found some way to split the PCIE-GFX block into a 2×8 configuration. AMD seems completely OK with this since this is the motherboard AMD themselves used to introduce their new FX CPUs like the FX-8320E. So, let’s take a look at it starting in the next section.