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BIOS & UEFI confusion!!

  1. May 28, 2015 #1
    For me BIOS and UEFI is all now confusing :-)

    Is there any element similar to BIOS in UEFI? When a UEFI based PC starts, is there any BIOS component, 16 Bit real mode starting from Reset Vector that still runs and then it actually loads UEFI or is it the UEFI firmware right from the very first instruction?

    Also whenever a legacy BIOS PC starts, it does POST and Memory detection etc and displays the total RAM which typically is 2 or 4 or even more GBs now. If the CPU starts in real mode and that limits 1 MB addressable memory, how BIOS is able to access and report these high figures of total RAM that is way way more than 1 MB. Thank you.
  2. jcsd
  3. May 28, 2015 #2


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  4. May 29, 2015 #3


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    UEFI is just a standardized way to present the basic IO system (which is what BIOS means) plus additional capabilities. You boot in the UEFI. There is no BIOS in a machine with UEFI unless you want to say "the bios is UEFI".

    All machines need to present basic IO operations to start, so you could sort of say it is running a bios, but again, the whole package is called UEFI.

    Whether UEFI starts in 16 bit real mode, or if so how long it stays there is beyond what I know. It's not like there is a separate BIOS that boots UEFI. It is all UEFI.
  5. Aug 26, 2015 #4


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    UEFI uses a dedicated disk partition to hold boot-strap code, and unfortunately, it is completely incompatible with the older BIOS boot methods, where all boot-strap code was on a chip on the device itself. UEFI was devised because the BIOS boot procedure needed more memory. I was against UEFI from the get-go because there was no way to do it to preserve what we had always had, which was device compatibility. With UEFI, it is not as easy (or as likely) for devices to be universally compatible across hardware brands, because that dedicated disk partition has to be present. But, nobody listened to me. Now we have crap compatibility and confusion that is widespread and not easy to break through. Humph. Grump. And yes, I am a curmudgeon.
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