GRUB

bootloader

MBR
- the first 512 bytes of a disk
- After BIOS, it jumps to MBR
- also holding the partition table
  - A partition can be marked bootable (active). It is used by boot loaders from old days to determine which partition to boot. Not used by grub.
PBR (partition boot sector)
- the first 512 bytes of a partition
- boot loaders can also be installed to PBR, suppose the one installed to MBR knows PBR
grub
- grub-install is a program to install grub to the disk
- it detects the target automatically
  - if the current system uses UEFI, x86_64-efi or i386-efi
  - otherwise, i386-pc
  - --target to override
- it defaults the boot dir to /boot
  - grub dir to /boot/grub
  - --boot-directory to override
  - it copies some binaries to /boot/grub
- grub_install_make_image_wrap generates and copies the core image
- if UEFI
  - grub_install_register_efi updates nvram to add the boot entry
- if BIOS
  - grub_util_bios_setup installs the boot image to the boot sector
    - whether boot.img is installed in MBR or PBR, it loads another fixed sector which holds the first 512 bytes of core.img. It then jumps to it and loads the rest of core.img
    - core.img has the ability to load modules and use filesystems
if BIOS with GPT disk, grub requires a BIOS boot partition
- grub-install writes core image to the partition
- it is not ESP