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Boot process: BIOS vs UEFI

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Boot process: BIOS vs UEFI

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Boot process: BIOS vs UEFI

  1. 1. Boot process BIOS vs UEFI
  2. 2. Concepts about booting a computer: the test BIOS EFI,UEFI MBR GPT SECURE BOOT ESP FAT32
  3. 3. BIOS 1. POST(Power-On Self Test): gets a list of the available boot devices. 2. Add-On ROMs: RAID, Video Card, etc.. 3. Allow to modify settings. 4. Load into memory the MBR(512 bytes) form the first hard disk, start the bootloading process
  4. 4. BIOS/MBR Boot process First stage: loads bootstrap code in the MBR. It looks up the active partition from the partition table and load the bootsector (512 bytes) into memory. Second stage: loads the bootstrap section in the partition bootsector. It looks up a file stored on the partition itself. Last stage: loads this file who loads the SO
  5. 5. MBR partition table limitations ● Can manage up to 2TB disk space. ● Up to 4 primary partitions. Hack: use 1 extended partition with 4 logical partitions inside. ● The OS communication with BIOS is thought interruptions. GPT partition table improvements ● Can manage up to more the 9ZB: (TB->PB->EB->ZB) ● 128 primary partitions
  6. 6. UEFI SPECIFICATION ● Based in EFI which was developed by Intel. ● Released in 2005. Specification manage by UEFI Forum, http://uefi.org/members ● Requires the firmware to interpret GPT partition table but also MBR for BIOS compatibility mode. ● Defines a new GPT partition type: ESP (EFI System Partition), accessible by both the firmware and the OS ● Defines the UEFI boot manager who load UEFI drivers and UEFI bootloader at boot time. ● Provides efibootmgr, a tool to configure the system boot behaviour from a booted OS.
  7. 7. UEFI BOOT MANAGER [root@system directory]# efibootmgr -v BootCurrent: 0002 Timeout: 3 seconds BootOrder: 0003,0002,0000,0004 Boot0000* CD/DVD Drive BIOS(3,0,00) Boot0001* Hard Drive HD(2,0,00) Boot0002* Fedora HD(1,800,61800,6d98f360-cb3e-4727-8fed-5ce0c040365d)File(EFIfedoragrubx64.efi) Boot0003* opensuse HD(1,800,61800,6d98f360-cb3e-4727-8fed-5ce0c040365d)File(EFIopensusegrubx64.efi) Boot0004* Hard Drive BIOS(2,0,00)P0: ST1500DM003-9YN16G Disk boot targets can be defined but also non-disk targets as PXE server. Disk boot targets: ● BIOS compatibility boot entries: 0000,0004. Autogenerated by the firmware. ● “Fall backpath” native UEFI entries: 0001. The firmware will look through each ESP partition in the disk, in the order they exist, and look for EFIBOOTBOOT{machine type short-name}.EFI: BOOT64.EFI (x86-64)BOOTIA32.EFI (x86-32)
  8. 8. ● Full native UEFI entries: 0002,0003. When you do a native UEFI OS install, the operating system install a bootloader which loads the OS kernel and so on to an EFI system partition, and add an entry to the UEFI boot manager configuration with a name and the location of the bootloader (in EFI executable format) that is intended for loading that operating system. UEFI/GPT BOOT PROCESS
  9. 9. SECURE BOOT The firmware can contain a set of signatures, and refuse to run any EFI executable which is not signed with one of those signatures. Computers complying with the requirements must: ● Ship with Secure Boot turned on (except for servers) ● Have Microsoft’s key in the list of keys they trust ● Disable BIOS compatibility mode when Secure Boot is enabled (actually the UEFI spec requires this too, if I read it correctly) ● Support signature blacklisting ● X86 computers must allow a physically present person who can disable Secure Boot and also can enable Custom Mode, and modify the list of keys the firmware trusts ● ARM computers must NOT allow a physically present person who can disable Secure Boot and also can NOT enable Custom Mode, and modify the list of keys the firmware trusts
  10. 10. Concepts about booting a computer: the solution BIOS : type of firmware EFI,UEFI: type of firmware MBR: Type of partition table GPT: type of partition table SECURE BOOT: An extension of the UEFI specification ESP: type of partition FAT32: type of partition

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