BIOS and UEFI are types of firmware that control the boot process. BIOS uses the MBR partition table and boots by loading the MBR, then the partition bootsector. UEFI uses the GPT partition table and ESP partition, and its boot manager loads UEFI drivers and bootloaders. Secure Boot is an UEFI extension that verifies signatures of boot components for security.

1. Boot process
BIOS vs UEFI

2. Concepts about booting a computer: the test
BIOS
EFI,UEFI
MBR
GPT
SECURE BOOT
ESP
FAT32

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. 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. 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. 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. 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. ● 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. 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. 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