IBM ThinkPad A20m Hardware Maintenance Manual page 96

When installing the CPU, do as shown in the figure.
v Rotate the screw head (A) 180° to the direction of the
arrow b to lock the CPU.
v When attach the CPU to the socket, do as shown in the
step c .
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ThinkPad A20m/p, A21m/p, A22m/p
Removing and replacing a FRU