NEC Express5800 User Manual page 173

10.
Put the CPU on the socket carefully and slowly.
NOTE:
Note the direction of the CPU. To prevent the CPU from being
inserted into the mating socket incorrectly, a pin mark is put on each of
the CPU and the socket. Check the pin marks to insert the CPU into the
socket correctly.
11.
Push the CPU lightly to the socket, and push down the CPU socket holder.
12.
Push down the lever to secure the CPU.
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