IBM 6643 - NetVista X40 - 128 MB RAM User Manual page 62

6. Position the new DIMM over the socket, making sure that the notches in the
DIMM are aligned with the tabs on the socket. Push the DIMM straight down
into the connector until the retaining clips close.
7. Replace the system board cover.
8. Replace the hard disk drive.
9. Replace the rear cover.
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User Guide
Notches