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Thread: Please help clarify boot priority selection.




  1. #1
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    Default Please help clarify boot priority selection.

    As an educational project I have just built a computer using a Z77 Pro4 motherboard which will be a linux only computer dedicated to learning as much about the linux OS and computers in general as a senior citizen (me) can absorb.


    When I start my computer I get a large colorful screen with several options. Of these, F2 and F11 are of interest to me. When I press F2, the UEFI Setup utility described on page 45 of the on line manual comes up. At the top of this screen two options are offered. One for the hard drive (called AHCI P1: WDC WDI0EALX-009BA0)nd one for the optical drive (called AHCI P0: ATAPI 1HBS112 2). If I have put a live Ubuntu disk in the optical drive before I start the computer, a third option is offered (called UEFI P0: ATAPI 1HBS112 2). I see that when I highlight one of the options, a dialog opens in which I can select one of the 2 or 3 available devices as described above or “DISABLED”. In this way, I can match options with the various drives. I assume that in this way I have set the priority for where to look for an operating system during bootup with the first place to look being option one. Below the above mentioned options I see two Icons (semi open books) that suggest file locations. One opens up to a description of the optical drive and the other for the hard drive. My questions so far are: Is the screen that I mentioned in the first sentence above what I have seen referred to as the “Splash” screen? Is my assumption regarding priority by option correct? If this is the case, what is the purpose of the open book icons and how are they used and what does BBS stand for?


    When I press F11 (Boot menu) I get a dialog box that allows you to pick one of the 2 or 3 devices described above. Below that are buttons “configure” and “Boot default”. If the boot order is setup as described above is this dialog necessary? If so, how is it to be used?


    What might follow in the future will be questions regarding my ubuntu installation which is giving me some trouble. For now, I think I know that UEFI is the new way (longer addresses??) and AHCI is the old way and that they are different types of bios. Is this right? Must you choose one or the other? If so which is recommended?



    Whew! Thanks,

  2. #2
    parsec's Avatar
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    Default Re: Please help clarify boot priority selection.

    Yes, the first screen you see with the function keys is the Splash Screen

    The BBS options, or BIOS Boot Specification (your semi-opened books), has existed for a while. They are what might be called boot profiles, vaguely similar to a BIOS/UEFI profile. The BBS settings allow you to create several different boot orders, and boot orders for different boot devices. One sets the order for hard disk drives, another for optical/CD/DVD drives. You also have network boot devices, Ethernet cards or onboard chips, and detachable devices like USB flash drives. It's a bit much for most users, but can come in handy for advanced users, really a convenience, rather than setting boot orders specifically every time you need to do something different.

    UEFI and AHCI are not related in the respect you implied. While a UEFI boot device must be formatted in the newer GPT format, AHCI is a SATA I/O protocol, that would still be used with GPT formatted disks. It becomes complicated at this point with UEFI, and has more to it than I am willing to type in, or completely understand. Most people do not use the full capabilities of a "UEFI boot", beyond the UEFI type of BIOS. Many people consider the GPT disk format type to be a failure, or overly complex for no return. It can be necessary for very large HDDS, over three TeraBytes.

    I cannot recommend something that I have never used, or completely understand, which is what true UEFI booting and disk formatting is to me.

  3. #3
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    Default Re: Please help clarify boot priority selection.

    Minor correction:
    With a non-GPT setup, the maximum accessible disk size is almost always ~ 2.2 TB.
    A web search using mbr maximum disk size will provide more details.
    You could also search using mbr vs gpt

    A quick and easy, non technical read is http://www.uefi.org/learning_center/..._Limits_v2.pdf

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  4. #4
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    Default Re: Please help clarify boot priority selection.

    I beg your pardon sir, the Cap of Dunce you say?

    When approximate is not good enough, as in ~2.2TB, the actual value is 2,199,023,255,040 bytes (yes, I counted them... )

    'Tis not only myself being sloppy, the utility used to allow very large HDDs to be used on non-GPT formatted systems, as provided by ASRock, is called "ASRock 3TB+ Unlocker Utility ver:1.1".

    But I do appreciate your proof reading of my ramblings, it is important to keep misinformation out of forum posts, which as we know can be littered with that.

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    Default Re: Please help clarify boot priority selection.

    My $.02 just to clear up some terminology and actually more likely to muddy the waters in general, which I seem to have a habit of. hehe

    MBR and GPT are partition tables. MBR uses old 32bit numbers to define a "partition" (2^32 - 1 x 512 byte sectors.) GPT uses 64bits. that's like 4billion times larger, so it's fairly significant. GPT is also not limited to 4 primary partitions as is MBR. Although I would be surprised to see anyone who finds this to be a problem (since primary partitions can be sliced into extended partitions)

    UEFI can and does support BOTH GPT and MBR partition tables. The limit being that the OS boot loader needs to be within the first 8GB (a BIOS limitation due to legacy CHS limits) when using MBR. Not so with GPT.

    UEFI also has a mode where it can detect a UEFI "application", boot or whatever. This would be "full" UEFI.

    In Linux, stick with UEFI's emulated BIOS for now. lots of system libraries don't support full UEFI yet, including GRUB and LILO. (Linux boot loaders). GRUB 2 does support full UEFI now... but I have no experience with that yet.

    caution: Linux "fdisk" does not support GPT at all. (well it does "detect" it if it's there and tells you to use parted instead)
    Use "parted" to partition your disk up.

    Syn

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    Default Re: Please help clarify boot priority selection.

    Some more info on GPT disk formatting, this time for Windows. BTW, Intel came up with this GPT format, and in a similar way with the Intel developed AHCI (not related to GPT) it will take years for it to become more commonly used.

    When Windows 7 formats a disk as GPT, it still puts the standard MBR at the beginning of the disk. This is known as "Hybrid GPT", and Windows loaded on a Hybrid GPT disk will still boot with the MBR unless the PC is explicitly booted in EFI mode, which does not mean simply using a UEFI type BIOS.

    There are alignment issues with Hybrid GPT formatting, really bad for SSDs, although why anyone would use GPT on a SSD, when the main motivation for GPT is for HDDs greater than 2.2TB in size, makes no sense.

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