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Melab
Asked: 2014-12-14 19:24:52 +0800 CST

Why are the MBR and GRUB present as files in the Linux filesystem?

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A computer's master boot record consists of raw binary and does not reside on any sort of partition. Legacy GRUB and GRUB 2 are installed to the sectors inbetween the end of the MBR and the start of the first partition. The boot code in the MBR, which is blind to filesystem formatting, loads GRUB from the no man's land residing after it. Why are the MBR (boot.img) and GRUB (core.img) accessible as files through the Linux filesystem?

filesystem

1 Answers

  1. First, in linux, everything is a file. Binaries and libs are all files.

    Second, boot.img and core.img are not in your MBR. The MBR is the first part of your hard drive and contains the initial boot and partition table. The MBR then passes off the boot process to grub, which is located on your boot partition.

    See

    https://wiki.archlinux.org/index.php/Master_Boot_Record

    http://www.thegeekstuff.com/2011/02/linux-boot-process/

    https://www.gnu.org/software/grub/manual/html_node/Images.html

    From the last link

    boot.img

    On PC BIOS systems, this image is the first part of GRUB to start. It is written to a master boot record (MBR) or to the boot sector of a partition. Because a PC boot sector is 512 bytes, the size of this image is exactly 512 bytes.

    The sole function of boot.img is to read the first sector of the core image from a local disk and jump to it. Because of the size restriction, boot.img cannot understand any file system structure, so grub-setup hardcodes the location of the first sector of the core image into boot.img when installing GRUB.

    and

    core.img

    This is the core image of GRUB. It is built dynamically from the kernel image and an arbitrary list of modules by the grub-mkimage program. Usually, it contains enough modules to access /boot/grub, and loads everything else (including menu handling, the ability to load target operating systems, and so on) from the file system at run-time. The modular design allows the core image to be kept small, since the areas of disk where it must be installed are often as small as 32KB.

    See BIOS installation, for details on where the core image can be installed on PC systems.

    both boot.img and core.img are a part of the grub package and are stored in /boot, regardless as to if /boot is a separate partition from / or not

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