Taking a stroll around the socket
Eight DDR4 DIMM slots are one of the more impressive features of high-end boards built on Intel's X99 platform. That's even more true when we consider that boards like the X99-A II can support up to 128GB of RAM when fully populated. These slots take up a great deal of real estate on the board, which can push slots and sockets closer together than on mainstream platforms. Take the top-most PCIe slot on the X99-A II, for instance. It's close enough to the CPU socket that there's potential for clearance issues with large aftermarket CPU coolers. To help you figure out which components can safely fit together on the board, we've provided the measurements below:
Five fan headers are within easy reach of the CPU socket: two for CPU fans, one for use with a CPU liquid-cooler pump, one for a system fan, and one high-amperage header that can supply 3A of current.
Unlike many X99 boards, the X99-A II doesn't have the DIMM slots butting right up against the top edge of the motherboard. This can make installing and removing DIMMs easier since it grants better access to the locking mechanisms on each slot. This breathing room comes at the cost of the top-most expansion slot, however, leaving the board with six PCI Express expansion slots.
The X99-A II gives us four PCIe x16 slots. Three of these are fed with Gen3 lanes from the CPU: the left-most and two right-most x16 slots. SLI and CrossFire configurations with up to three video cards are supported with both 40-lane and 28-lane CPUs from the Haswell-E and Broadwell-E families alike.
For dual-card setups, the first and third x16 slots from the left should be used, and each of those will get the full 16 lanes with 40-lane CPUs. This arrangement gives breathing room not only for a pair only double-wide cards, but also for two triple-slot behemoths. In the unlikely event that you can find a reason to use two triple-slot coolers, you won't be left with any remaining PCIe slots, but you'd be the talk of the town. The right-most x16 slot comes in to play for three-card configs.
The second PCIe x16 slot from the left is fed with Gen2 lanes from the chipset. Normally, this slot gets by with a single lane of bandwidth, but if you're willing to disable the board's right-most PCIe x1 slot and the USB 3.1 controller, it can be fed with four lanes of Gen2 PCIe goodness. My take is that it would have been preferable to have this lane-sharing happen with the left-most PCIe x1 instead, because it's far more likely to be covered up by a graphics card.
The silver cladding on the left-most PCI Express x16 slots is more than just some bling. Similar to other boards that we've looked at lately, Asus has reinforced this primary PCIe slot with a metal shroud that is soldered to the board at multiple points. This setup, which Asus calls SafeSlot, should reduce the chance of damage to the PCIe slot if you're transporting a system that has a massive video card, but we'd still recommend removing any expansion cards rather than chancing damage to the board with a bump or jostle. It's somewhat surprising to see this reinforcement on the first PCIe slot only. It would have been nice to have all three of the PCIe slots that are connected to the CPU protected in this manner.
On the subject of bling, the PCIe slots don't leave us wanting. The translucent locking mechanisms on each of the x16 PCIe slots actually have embedded RGB LEDs in them. This lighting works in conjunction with the audio trace path lighting as well as any LED strips connected to the onboard 4-pin RGB headers. More on this later.
The number of lanes going to each slot for the different possible multi-GPU configurations will depend on whether your little slice of Broadwell-E silicon has 40 or 28 PCIe lanes enabled. Rather than attempting to paint you a picture with prose, we've instead mapped out how PCIe lanes are assigned to slots in diagrams. Click the buttons to toggle between the two possible processor options:
All four x16 slots are usable, no matter how many PCIe lanes your CPU provides. The processor choice just governs how many lanes each x16 slot gets: for models with 40 lanes, the three CPU-fed slots will have lane arrangements of x8/16/x8, while those running Core i7-6800Ks are looking at x8/x8/x8.
Between the third and fourth x16 slots we find the tail end of the board's M.2 socket. We'll look at it in more detail on the next page when we examine the X99-A II's storage features.