Difference between revisions of "Arch boot process"

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In order to boot Arch Linux, a Linux-capable [[Boot Loaders|boot loader]] such as [[GRUB]] or [[Syslinux]] must be installed to the [[Master Boot Record]] or the [[GUID Partition Table]]. The boot loader is responsible for loading the kernel and [[mkinitcpio|initial ramdisk]] before initiating the boot process. The procedure is quite different for [[Wikipedia:BIOS|BIOS]] and [[UEFI]] systems, the detailed description is given on this or linked pages.
 
In order to boot Arch Linux, a Linux-capable [[Boot Loaders|boot loader]] such as [[GRUB]] or [[Syslinux]] must be installed to the [[Master Boot Record]] or the [[GUID Partition Table]]. The boot loader is responsible for loading the kernel and [[mkinitcpio|initial ramdisk]] before initiating the boot process. The procedure is quite different for [[Wikipedia:BIOS|BIOS]] and [[UEFI]] systems, the detailed description is given on this or linked pages.
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== Firmware types ==
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=== BIOS ===
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A BIOS or Basic Input-Output System is the very first program (firmware) that is executed once the system is switched on. In most cases it is stored in a flash memory in the motherboard itself and independent of the system storage. BIOS launches the first 440 bytes ([[Master Boot Record]]) of the first disk in the BIOS disk order. Since very little can be achieved by a program that fits into the 440-byte boot code area, usually a common boot loader like [[GRUB]] or [[Syslinux]] or [[LILO]] would be loaded by the BIOS, and it would load an operating system by either chain-loading or directly loading the kernel.
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=== UEFI ===
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UEFI has support for reading both the partition table as well as understanding filesystems. Hence it is not limited by 440 byte code limitation (MBR boot code) as in BIOS systems. It does not use the MBR boot code at all.
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The commonly used UEFI firmwares support both MBR and GPT partition table. EFI in Apple-Intel Macs are known to also support Apple Partition Map besides MBR and GPT. Most UEFI firmwares have support for accessing FAT12 (floppy disks), FAT16 and FAT32 filesystems in HDDs and ISO9660 (and UDF) in CD/DVDs. EFI in Intel Macs can also access HFS/HFS+ filesystems, in addition to the mentioned ones.
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UEFI does not launch any boot code in the MBR whether it exists or not. Instead it uses a special partition in the partition table called '''EFI System Partition''' in which files required to be launched by the firmware are stored. Each vendor can store its files under {{ic|<EFI SYSTEM PARTITION>/EFI/<VENDOR NAME>/}} folder and can use the firmware or its shell (UEFI shell) to launch the boot program. An EFI System Partition is usually formatted as FAT32 or (less commonly) FAT16.
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Under UEFI, every program whether it is an OS loader or a utility (e.g. a memory testing app or recovery tool), should be a UEFI Application corresponding to the EFI firmware bitness/architecture. The vast majority of UEFI firmwares, including recent Apple Macs, use x86_64 EFI firmware. The only known devices that use IA32 (32-bit) EFI are older (pre 2008) Apple Macs, some Intel Cloverfield ultrabooks and some older Intel Server boards are known to operate on Intel EFI 1.10 firmware.
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An x86_64 EFI firmware does not include support for launching 32-bit EFI apps (unlike x86_64 Linux and Windows versions which include such support). Therefore the UEFI application must be compiled for that specific firmware processor bitness/architecture.
  
 
== Boot process ==
 
== Boot process ==

Revision as of 20:27, 10 January 2014

In order to boot Arch Linux, a Linux-capable boot loader such as GRUB or Syslinux must be installed to the Master Boot Record or the GUID Partition Table. The boot loader is responsible for loading the kernel and initial ramdisk before initiating the boot process. The procedure is quite different for BIOS and UEFI systems, the detailed description is given on this or linked pages.

Firmware types

BIOS

A BIOS or Basic Input-Output System is the very first program (firmware) that is executed once the system is switched on. In most cases it is stored in a flash memory in the motherboard itself and independent of the system storage. BIOS launches the first 440 bytes (Master Boot Record) of the first disk in the BIOS disk order. Since very little can be achieved by a program that fits into the 440-byte boot code area, usually a common boot loader like GRUB or Syslinux or LILO would be loaded by the BIOS, and it would load an operating system by either chain-loading or directly loading the kernel.

UEFI

UEFI has support for reading both the partition table as well as understanding filesystems. Hence it is not limited by 440 byte code limitation (MBR boot code) as in BIOS systems. It does not use the MBR boot code at all.

The commonly used UEFI firmwares support both MBR and GPT partition table. EFI in Apple-Intel Macs are known to also support Apple Partition Map besides MBR and GPT. Most UEFI firmwares have support for accessing FAT12 (floppy disks), FAT16 and FAT32 filesystems in HDDs and ISO9660 (and UDF) in CD/DVDs. EFI in Intel Macs can also access HFS/HFS+ filesystems, in addition to the mentioned ones.

UEFI does not launch any boot code in the MBR whether it exists or not. Instead it uses a special partition in the partition table called EFI System Partition in which files required to be launched by the firmware are stored. Each vendor can store its files under <EFI SYSTEM PARTITION>/EFI/<VENDOR NAME>/ folder and can use the firmware or its shell (UEFI shell) to launch the boot program. An EFI System Partition is usually formatted as FAT32 or (less commonly) FAT16.

Under UEFI, every program whether it is an OS loader or a utility (e.g. a memory testing app or recovery tool), should be a UEFI Application corresponding to the EFI firmware bitness/architecture. The vast majority of UEFI firmwares, including recent Apple Macs, use x86_64 EFI firmware. The only known devices that use IA32 (32-bit) EFI are older (pre 2008) Apple Macs, some Intel Cloverfield ultrabooks and some older Intel Server boards are known to operate on Intel EFI 1.10 firmware.

An x86_64 EFI firmware does not include support for launching 32-bit EFI apps (unlike x86_64 Linux and Windows versions which include such support). Therefore the UEFI application must be compiled for that specific firmware processor bitness/architecture.

Boot process

Under BIOS

  1. System switched on - Power-on self-test or POST process
  2. After POST, BIOS initializes the necessary system hardware for booting (disk, keyboard controllers etc.)
  3. BIOS launches the first 440 bytes (Master Boot Record) of the first disk in the BIOS disk order
  4. The MBR boot code then takes control from BIOS and launches its next stage code (if any) (mostly boot loader code)
  5. The launched (2nd stage) code (actual boot loader) then reads its support and config files
  6. Based on the data in its config files, the boot loader loads the kernel and initramfs into system memory (RAM) and launches the kernel

Under UEFI

See the main page: Unified Extensible Firmware Interface#Boot Process under UEFI.

Kernel

The kernel is the core of an operating system. It functions on a low level (kernelspace) interacting between the hardware of the machine and the programs which use the hardware to run. To make efficient use of the CPU, the kernel uses a scheduler to arbitrate which tasks take priority at any given moment, creating the illusion of many tasks being executed simultaneously.

initramfs

After the kernel is loaded, it unpacks the initramfs (initial RAM filesystem), which becomes the initial root filesystem. The kernel then executes /init as the first process. The early userspace starts.

The purpose of the initramfs is to bootstrap the system to the point where it can access the root filesystem (see FHS for details). This means that any modules that are required for devices like IDE, SCSI, SATA, USB/FW (if booting from an external drive) must be loadable from the initramfs if not built into the kernel; once the proper modules are loaded (either explicitly via a program or script, or implicitly via udev), the boot process continues. For this reason, the initramfs only needs to contain the modules necessary to access the root filesystem; it does not need to contain every module one would ever want to use. The majority of modules will be loaded later on by udev, during the init process.

Init process

At the final stage of early userspace, the real root is mounted, and then replaces the initial root filesystem. /sbin/init is executed, replacing the /init process. Arch uses systemd as the init process.

See also