You can run a free OS pretty effortless, but when wanting 100% free software, you have to dig deeper and replace the proprietary BIOS firmware.
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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There can be. There are certainly Bios' that don't give options that motherboards are perfectly capable of changing. I had an old Phenom II that I managed to patch NVME support into the bios so I could boot off of a PCIe Riser.
Granted, I was patching UEFI stuff and none of it was open source -- but the idea is the same. Open source bios in theory, could unlock features.