ASROCK H81M-VG4 R4.0 Manual page 16

2.3 Installing Memory Modules (DIMM)
This motherboard provides two 240-pin DDR3/DDR3L (Double Data Rate 3)
DIMM slots, and supports Dual Channel Memory Technology.
1. For dual channel configuration, you always need to install identical (the same
brand, speed, size and chip-type) DDR3/DDR3L DIMM pairs.
2. It is unable to activate Dual Channel Memory Technology with only one memory
module installed.
3. It is not allowed to install a DDR or DDR2 memory module into a DDR3/DDR3L
slot; otherwise, this motherboard and DIMM may be damaged.
The DIMM only fits in one correct orientation. It will cause permanent damage to
the motherboard and the DIMM if you force the DIMM into the slot at incorrect
orientation.
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