Socket LGA1156 CPUs, like socket LGA1366 and AMD processors, have an embedded memory controller. All other Intel CPUs use an external memory controller, which is located on the north bridge chip (a.k.a. MCH or Memory Controller Hub) from the chipset. This means that with other Intel CPUs the chipset (and thus the motherboard) is the component that says what memory technologies and the maximum amount of memory you can have on your PC.
Since now the memory controller is inside the CPU, it is the processor, and not the chipset, that defines the memory technologies and the maximum amount of memory you can have. The motherboard, however, may have a limitation as to how much memory can be installed.
At the moment, the integrated memory controller of socket LGA1156 processors supports only DDR3 memories up to 1,333 MHz under dual-channel architecture, however ASUS says P7P55D PRO supports DDR3 memories up to 2033 MHz through overclocking. P7P55D PRO has four DDR3 sockets and since each DDR3 memory module can have up to 4 GB, you can have up to 16 GB with this motherboard.
The first and the third sockets are dark gray, while the second and the fourth are light blue. In order to achieve the maximum performance, you should install two or four memory modules to enable the dual-channel architecture. When only two modules are used make sure to install them on the blue sockets. If you install them on the dark gray ones the computer won’t turn on and the memory diagnostics LED will be permanently turned on.
If you pay close attention in Figure 5 you will see that ASUS is using a different kind of socket on this motherboard. Instead of each socket having one latch at each side of the socket, the sockets have only one latch each. On the other side of the socket there is only a slot for inserting the memory module.
ASUS P7P55D PRO comes with a built-in memory compatibility tester called MemOK!. To activate this feature all you need to do is to push a button located near the main motherboard connector (see Figure 6) for a few seconds after you turn on the computer until you see the red LED next to it start blinking. Then the motherboard will check if your memory modules are compatible with your CPU and will display a message on the screen after some seconds (see Figure 7). If this LED is permanently turned on and the system does not show a message even after you wait one minute, then your installed the modules on the wrong sockets (see text above), your modules are not installed correctly/have bad contact with the sockets, they are incompatible or they are damaged.