LEGACY Booting is no longer supported past SKYLAKE. [URL]  UEFI mode requires a 64-bit operating system be installed (ex, Windows 10 Pro 64-bit, Windows 10 Home 64-bit, Ubuntu 20.04 LTS 64-bit).  32 BIT OS and Drivers are no longer supported by anyone. 64 BIT windows 10 or Linux in UEFI mode is the only thing supported going forward. This means ancient Linux that uses GRUB/MSDOS MBR booting is also not supported. CSM with MBR legacy booting is gone as is support for windows 7. Optiplex GX390 GX620, 745, 755, 760, 780, 790, 960, 980,990 all work fine from MSDOS all the way thru Windows 10. 3010 3020 7010 7020 9010 9020 also work fine. These are the last units that will boot and RUN XP.  
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How to change UEFI to lagecy in Bios 1.0.7 Dell optiplex 3080?