Nabin Chettri :
Hello, jewel and thanks for the assistance, I am pursuing 3d Animation so i need to use multiple softwares at a time. As you said i had installed the 8GB modules in the blue DIMM slots (DIMM_A1/DIMM_B1) and the 4GB module in one of the black slots (DIMM_A2/DIMM_B2) but the problem remains same , i also wanna know if there is any tweaking to enable it. thanks
First thing to check is to make sure that the 8GB modules are working properly. Remove the 4GB module and see if Windows recognizes roughly 16GB as available. If not, there could be a problem with the memory. Run Windows Memory Test or Memtest86 to see if there are any errors. If there are then it would be best to do a warranty replacement.
Second, make sure your BIOS is up to date, the latest version available is 1801, the previous released version 1503 states it improved memory compatibility which could factor into this issue (http://www.asus.com/Motherboards/M5A78LMUSB3/HelpDesk_Download/).
Third, try the memory in alternate slots; the 4GB in each black slot, then swap the 8GB into the black slots and try the 4GB in each blue slot.
Fourth, check out the maximum memory settings in Windows and if memory remapping is enabled in BIOS. More info on these steps under "Things to Try" in this KB article:
http://support.microsoft.com/kb/978610
The memory remapping setting according to your motherboard manual should be under "DRAM Controller Configuration" and is called "Memory Hole Remapping" on pg 2-14 (http://dlcdnet.asus.com/pub/ASUS/mb/SocketAM3+/M5A78L-M_USB3/E8017_M5A78L-M-USB3.pdf)
Hope this helps