Boot Loaders

  • BOOT LOADERS
    • The computer’s firmware will read the boot loader and put it into memory from the hard disk, where it is executed.
    • The boot loader will load the Linux kernel (or Windows equivalent) into memory and execute it.
    • From the BIOS options, your first disk that houses the MBR will give the first bit of code to be loaded
      • The code in the MBR can be literally anything to include an OS, or instructions for an OS (a kernel)
      • The code run from the MBR is the Boot Loader code
    • Windows boot loaders are very simple and basically point to the Windows OS
      • Windows will write its standard MBR boot loader when installed
      • If a new OS is installed with a boot loader, it will cause problems for that original installation
      • If Windows is installed after Linux has been installed, it will most likely rewrite the MBR taking away the Linux option for booting.
    • Linux boot loaders are a bit more complicated and provide several options
      • Linux uses a boot loader called LILO or GRUB
      • A linux boot loader can be configured to boot to multiple OSs, or multiple Linux kernels
      • You can modify the existing boot loader configuration instead of performing a new install
    • EFI Boot Loaders
      • EFI, as compared to a traditional BIOS, is much larger in size and allows for multiple boot loaders if desired.
      • EFI System Partition (ESP) is a FAT filesystem that houses files that act as the boot loaders.
      • The .efi files on the ESP are named for each boot loader option allowing a separate boot loader  for each OS that is installed on the computer.
      • A built in EFI utility allows you to register each boot loader with the firmware so that it knows it exists
  • Other Boot Loaders
    • [ Syslinux ] — smaller and more specialized than GRUB.  In this family is ISOLINUX which is used for booting CDs.  Also in this family is EXTLINUX used for booting ext2/3/4 filesystems.
    • [ LILO ] — Linux Loader (LILO) is primitive and works only on BIOS-based systems.
    • [ ELILO ] — EFI Linux Loader (ELILO) is used by most systems as the default boot loader for EFI systems.
    • [ Linux Kernel ] — On an EFI system, you can boot directly to the kernel through a .esp file in the ESP
    • [ rEFIt ] — Popular on Intel Macs and presents icons instead of texts for the OS options.  This is technically a boot manager and is no longer being used.
    • [ rEFInd ] — Is currently replacing rEFIt and is also a icon based boot manager (not boot loader)
    • [ gummibot ] — EFI boot manager similar to rEFInd, but uses text instead of icons
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