Dell OptiPlex 990 (Early 2011) - Manual (Page 28)

Installing The Memory
1. Insert the memory modules into the connectors on the system board. Install the
memory module in the order of A1 > B1 > A2 > B2.
2. Press down on the memory modules until the retention clips spring back to secure
them in place.
3. Install the cover .
4. Follow the procedures in After Working Inside Your Computer .
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