In summary: The interplay of meaning and axiomatic machine mathematics, captured by the difference between $\models$ and $\vdash$, is a subtle and interesting thing.
The 390 is a crippled system with ATA only. AHCI is not an option. There is a 2018 issued bios patch for Spectre / Meltdown. The 2011 Optiplex 390 was only validated with the following i3/i5 CPUs = CPU socket type: LGA 1155 MHF7R i5-2400 Sandy Bridge, 3.10GHz, 6MB, 95w, 4C, D2 950R3 i3-2130 Sandy Bridge, 3.40GHz, 3MB, 65w, 2C, Q0 972H5 i3-2120 Sandy Bridge, 3.30GHz, 3MB, 65w, Q0 5MH9T i3-2100 ...
DSA-2025-390: Dell Secure Connect Gateway Security Update for Multiple Third-Party Component Vulnerabilities Summary: Dell Secure Connect Gateway contains remediation for multiple vulnerabilities that could be exploited by malicious users to compromise the affected system. Detailed Article Impact
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For example: C:\ OptiPlex_390 _X.Y.Z.exe Where C is the drive letter of the USB device where the executable file is located and OptiPlex_390 _X.Y.Z.exe is the name of the downloaded file. The system restarts automatically and updates the BIOS at the system startup screen. After the BIOS update is complete, system restarts again.
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Optiplex 390 Tech Guide A list of compatible CPU's are on pg. 10, or search using "processor." The top CPU listed is the I5-2400. If you want to center things around having a Xeon, then you might want to look at refurb Dell Precision's.