MSI MS-6135 User Manual page 12

2.1-4 CPU Terminator
The CPU terminator is used, when only one CPU is installed. You must
always install the CPU terminator on the empty CPU 2 slot.
Important:
To use dual CPU, it is strongly adviced that same CPU speed and
CPU stepping is used: which means that if you install 400MHz in the
1st CPU slot, then you must also install 400MHz on the 2nd slot.
CPU Terminator
2-7