Comment on Of BIOS and UEFI: A shim??

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apparentlymart@beehaw.org ⁨5⁩ ⁨days⁩ ago

Indeed, I was thinking about OSes like DOS that use the BIOS API even at runtime, for tasks like accessing disks.

As you say, Linux is built for the same world that UEFI was built for, where the firmware is mostly used only to boot the system and for low-level stuff like power management. In that case, the “boot services” in UEFI help to get the kernel loaded and then that takes over most of the hardware interactions. Linux uses BIOS in the same limited way it uses UEFI.

But the IO.SYS in DOS (on IBM PC-compatible platforms, at least) is effectively a wrapper around the BIOS interrupts, and applications running under DOS also expect to be able to interact with BIOS directly sometimes, so I think to do what was asked would mean the OS effectively running inside the UEFI “boot services” environment, rather than the usual approach of the UEFI application only dealing with early boot and then transferring control fully to the OS.

(UEFI does have a legacy compatibility layer that I’ve been ignoring for the sake of this discussion because it’s something normally built in to your firmware rather than something you can add yourself. But it is technically possible for a BIOS implementation to run in that environment. I don’t think it’s possible for a normal UEFI application to use that facility, but I might be wrong about that.)

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