grub_and_linux_boot

# firmware history from MBR to UEFI



The firmware is the very first program that is executed once the system is switched on.

Tip: The words BIOS and (U)EFI are often used instead of firmware




UEFI

The Unified Extensible Firmware Interface has support for 
reading both the partition table as well as file systems. 
UEFI does not launch any boot code from the Master Boot Record (MBR) whether it exists or not, 
instead booting relies on boot entries in the NVRAM.

The UEFI specification mandates support for the FAT12, FAT16, and FAT32 file systems 
(see UEFI specification version 2.9, section 13.3.1.1), 
but any conformant vendor can optionally add support for additional file systems; 
for example, HFS+ or APFS in some Apple's firmwares. 
UEFI implementations also support ISO-9660 for optical discs.

UEFI launches EFI applications, e.g. boot loaders, boot managers, UEFI shell, etc. 
These applications are usually stored as files in the EFI system partition. 





# understanding [U]EFI GRUB boot loader boot process

## this one is a good explanation of 2 stages loading
https://en.wikipedia.org/wiki/GNU_GRUB

## other worth reading material
https://wiki.archlinux.org/title/GRUB
https://wiki.archlinux.org/title/Arch_boot_process#Boot_loader
https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface


# something to know about efivars


UEFI defines variables through which an operating system can interact with the firmware. 
UEFI boot variables are used by the boot loader and used by the OS only 
for early system start-up. UEFI runtime variables allow an OS to manage certain settings 
of the firmware like the UEFI boot manager or managing the keys for UEFI Secure Boot 
protocol etc. You can get the list using:

$ efivar --list

Linux kernel exposes UEFI variables data to userspace via efivarfs





# initramfs

The root file system at / starts out as an empty rootfs, which is a special instance of 
ramfs or tmpfs. This is the temporary root file system where the initramfs (initial RAM 
file system) images will be unpacked to.

The purpose of the initramfs is to bootstrap the system to the point where it can access 
the root file system (see FHS for details). It does not need to contain every module 
one would ever want to use; it should only have modules required for the root device 
like IDE, SCSI, SATA or USB/FW (if booting from an external drive) and encryption. 
The majority of modules will be loaded later on by udev, during the init process.

First, the kernel unpacks its builtin initramfs into the temporary root. Arch Linux's 
official kernels use an empty archive for the builtin initramfs, which is the default 
when building Linux. Then, the kernel unpacks external initramfs files specified by 
the command line passed by the boot loader, overwriting any files from the embedded 
initramfs. These external initramfs images can be generated with mkinitcpio, dracut 
or booster, and are Arch's preferred method for setting up for early userspace. 







# delete uefi boot entry
efibootmgr -b 0 -B

# add uefi boot entry
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=ARCH




# removable disk may cause internal hard disk to get a new order number

GRUB2: error: disk `(hd0,)' not found



# read this -> https://wiki.archlinux.org/title/GRUB #4.3 section  -> Using the rescue console



# creating Gparted bootable usb failure, maybe it's because Gparted iso's problem

bootable usb created by using Unetbootin seems can't be recognized by UEFI only machine

https://wiki.archlinux.org/title/USB_flash_installation_medium#Using_the_ISO_as_is_(BIOS_and_UEFI)

# using ventoy to boot iso





// not important

https://serverfault.com/questions/857255/what-is-grub-install-actually-doing