Content deleted Content added
Fix reference access date (was in hurry) |
→Bootloader stage: The MBR boot sector (MBR boot code) is only used on x86 BIOS systems. However, the MBR partition table can be used on modern UEFI PCs, as well as non-x86 embedded devices (while the boot loader is not MBR boot sector). Tags: Mobile edit Mobile web edit |
||
| (26 intermediate revisions by 18 users not shown) | |||
Line 1:
{{Short description|Multi-stage initialisation process of operating system}}
{{Technical|date=August 2025}}
The Linux [[booting]] process involves multiple stages and is in many ways similar to the [[BSD]] and other [[Unix]]-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components,{{Sfn|M. Tim Jones|2006|ps=, "The process of booting a Linux® system consists of a number of stages. But whether you're booting a standard x86 desktop or a deeply embedded PowerPC® target, much of the flow is surprisingly similar."|loc="Introduction"}} including system startup, [[bootloader]] execution, loading and startup of a [[Linux kernel]] image, and execution of various [[startup scripts]] and [[Daemon (computing)|daemons]].{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process.{{Sfn|M. Tim Jones|2006|ps=|loc="Linux booting process are grouped into 4 stages, based on IBM source"}} When a Linux system is powered up or reset, its processor will execute a specific firmware/program for system initialization, such as the [[power-on self-test]], invoking the reset vector to start a program at a known address in flash/ROM (in embedded Linux devices), then load the bootloader into RAM for later execution.{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} In [[IBM PC–compatible]] [[personal computers]] (PCs), this firmware/program is either a [[BIOS]] or a [[UEFI]] monitor, and is stored in the mainboard.{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} In embedded Linux systems, this firmware/program is called [[boot ROM]].<ref>{{Cite book |last1=Bin |first1=Niu |title=2020 International Symposium on Computer Engineering and Intelligent Communications (ISCEIC) |last2=Dejian |first2=Li |last3=Zhangjian |first3=LU |last4=Lixin |first4=Yang |last5=Zhihua |first5=Bai |last6=Longlong |first6=He |last7=Sheng |first7=Liu |date=August 2020 |isbn=978-1-7281-8171-4 |pages=5–8 |chapter=Research and design of Bootrom supporting secure boot mode |doi=10.1109/ISCEIC51027.2020.00009 |chapter-url=https://ieeexplore.ieee.org/document/9325327 |s2cid=231714880}}</ref>{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} After being loaded into RAM, the bootloader (also called first-stage bootloader or primary bootloader) will execute to load the second-stage bootloader{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} (also called secondary bootloader).{{Sfn|M. Tim Jones|2006|loc=, "Stage 1 boot loader"}} The second-stage bootloader will load the kernel image into memory, decompress and initialize it, and then pass control to this kernel image.{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} The second-stage bootloader also performs several operation on the system such as system hardware check, mounting the root device, loading the necessary kernel modules, etc.{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} Finally, the first user-space process (<code>init</code> process) starts, and other high-level system initializations are performed (which involve with startup scripts).{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}}▼
The Linux [[booting]] process involves multiple stages and is in many ways similar to the [[BSD]] and other [[Unix]]-style boot processes, from which it is derived. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components,{{Sfn|M. Tim Jones|2006|ps=, "The process of booting a Linux® system consists of a number of stages. But whether you're booting a standard x86 desktop or a deeply embedded PowerPC® target, much of the flow is surprisingly similar."|loc="Introduction"}} including system startup, [[bootloader]] execution, loading and startup of a [[Linux kernel]] image, and execution of various [[startup scripts]] and [[Daemon (computing)|daemons]].{{Sfn|M. Tim Jones|2006|ps=, "Figure 1. The 20,000-foot view of the Linux boot process"|loc="Overview"}} Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process.{{Sfn|M. Tim Jones|2006|ps=|loc="Linux booting process are grouped into 4 stages, based on IBM source"}}
▲
For each of these stages and components, there are different variations and approaches; for example, [[GNU GRUB|GRUB]], [[coreboot]] or [[Das U-Boot]] can be used as bootloaders (historical examples are [[LILO (boot loader)|LILO]], [[SYSLINUX]] or [[Loadlin]]), while the startup scripts can be either traditional [[init]]-style, or the system configuration can be performed through modern alternatives such as [[systemd]] or [[Upstart (software)|Upstart]].▼
▲For each of these stages and components, there are different variations and approaches; for example, [[GNU GRUB|GRUB]], [[systemd-boot]], [[coreboot]] or [[Das U-Boot]] can be used as bootloaders (historical examples are [[LILO (boot loader)|LILO]], [[SYSLINUX]] or [[Loadlin]]), while the startup scripts can be either traditional [[init]]-style, or the system configuration can be performed through modern alternatives such as [[systemd]] or [[Upstart (software)|Upstart]].
== System startup ==
Line 11 ⟶ 14:
In BIOS systems, the BIOS will respectively perform power-on self test (POST), which is to check the system hardware, then enumerate local device and finally initialize the system.{{Sfn|M. Tim Jones|2006|loc=, "System startup"}} For system initialization, BIOS will start by searching for the bootable device on the system which stores the OS. A bootable device can be storage devices like floppy disk, CD-ROM, USB flash drive, a partition on a hard disk (where a hard disk stores multiple OS, e.g Windows and Fedora), a storage device on local network, etc.{{Sfn|M. Tim Jones|2006|loc=, "System startup"}} A hard disk to boot Linux stores the [[Master boot record|Master Boot Record]] (MBR), which contains the first-stage/primary bootloader in order to be loaded into RAM.{{Sfn|M. Tim Jones|2006|loc=, "System startup"}}
In [[UEFI]] systems, the Linux kernel can be executed directly by UEFI firmware via
If [[UEFI#Secure_Boot|UEFI Secure Boot]] is supported, a "shim" or "Preloader" is often booted by the UEFI before the bootloader or EFI-stub-bearing kernel.<ref>{{Cite web |title=Using a Signed Bootloader - Arch Wiki |url=https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot#Using_a_signed_boot_loader |access-date=2024-12-05 |website=wiki.archlinux.org}}</ref> Even if [[UEFI#Secure_Boot|UEFI Secure Boot]] is disabled this may be present and booted in case it is later enabled. It merely acts to add an extra signing key database providing keys for signature verification of subsequent boot stages without modifying the UEFI key database, and chains to the subsequent boot step the same as the UEFI would have.
The system startup stage on embedded Linux system starts by executing the firmware/program on the on-chip boot ROM, which is stored on the storage device of the system like USB flash drive, SD card, eMMC, NAND flash, NOR flash, etc.{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} The sequences of system startup in on-chip boot ROM varies by processors{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} but all include hardware initialization and system hardware testing steps.{{Sfn|M. Tim Jones|2006|loc=, "System startup"}} For example in a system with an i.MX7D processor and a bootable device which stores the OS (including U-Boot, an external bootloader), the on-chip boot ROM sets up the [[DDR SDRAM|DDR memory]] controller at first which allows the boot ROM's program to obtain the SoC configuration data from the external bootloader on the bootable device.{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} The on-chip boot ROM then loads the U-Boot into RAM for the bootloader stage.{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=29}}▼
▲The system startup stage on embedded Linux system starts by executing the firmware / program on the on-chip [[boot ROM]], which
== Bootloader stage ==
In x86 PC, first- and second-stage bootloaders are combined into the [[GNU GRUB|GRand Unified Bootloader]] (GRUB), and formerly Linux Loader ([[LILO (bootloader)|LILO]]).{{Sfn|M. Tim Jones|2006|loc=, "Stage 2 boot loader"}} [[GRUB 2]], which is now used, differs from GRUB 1 by being capable of automatic detection of various operating systems and automatic configuration. The stage1 is loaded and executed either by the [[BIOS]] from the [[Master boot record]] (MBR). The intermediate stage loader (stage1.5, usually core.img) is loaded and executed by the stage1 loader. The second-stage loader (stage2, the /boot/grub/ files) is loaded by the stage1.5 and displays the GRUB startup menu that allows the user to choose an operating system or examine and edit startup parameters. After a menu entry is chosen and optional parameters are given, GRUB loads the linux kernel into memory and passes control to it. GRUB 2 is also capable of chain-loading of another bootloader. In [[UEFI]] systems, the stage1 and stage1.5 usually are the same UEFI application file (such as grubx64.efi for [[x64]] UEFI systems).
Line 30 ⟶ 35:
* [[LILO (boot loader)|LILO]] does not understand or parse filesystem layout. Instead, a configuration file (<code>/etc/lilo.conf</code>) is created in a live system which maps raw offset information (mapper tool) about ___location of kernel and ram disks (initrd or initramfs). The configuration file, which includes data such as boot [[Disk partitioning|partition]] and [[Kernel (computer science)|kernel]] pathname for each, as well as customized options if needed, is then written together with bootloader code into MBR bootsector. When this bootsector is read and given control by BIOS, LILO loads the menu code and draws it then uses stored values together with user input to calculate and load the Linux kernel or [[Chain loading|chain-load]] any other [[Booting#Boot-loader|boot-loader]].
* GRUB 1 includes logic to read common file systems at run-time in order to access its configuration file.<ref name="redhat_startup">{{cite web|url=http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/ref-guide/s1-boot-init-shutdown-process.html |archive-url=https://web.archive.org/web/20080830065326/http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/ref-guide/s1-boot-init-shutdown-process.html |url-status=dead |archive-date=2008-08-30 |title=Product Documentation |publisher=Redhat.com |date=
* [[Loadlin]] is a bootloader that can replace a running [[DOS]] or [[Windows 9x]] kernel with the Linux kernel at run time. This can be useful in the case of hardware that needs to be switched on via software and for which such configuration programs are proprietary and only available for DOS. This booting method is less necessary nowadays, as Linux has drivers for a multitude of hardware devices, but it has seen some use in [[mobile device]]s. Another use case is when the Linux is located on a storage device which is not available to the BIOS for booting: DOS or Windows can load the appropriate drivers to make up for the BIOS limitation and boot Linux from there.
== Kernel ==
The kernel stage occurs after the bootloader stage. The [[Linux kernel]] handles all operating system processes, such as [[memory management]], task [[scheduling (computing)|scheduling]], [[I/O]], [[interprocess communication]], and overall system control. This is loaded in two stages – in the first stage, the kernel (as a compressed image file) is loaded into memory and decompressed, and a few fundamental functions are set up such as basic memory management, minimal amount of hardware setup.{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}}
For details of those steps, take an example with [[i386]] microprocessor. When its bzImage is invoked, function <code>start()</code> (of <code>./arch/i386/boot/head.S</code>) is called to do some basic hardware setup then calls <code>startup_32()</code> (located in <code>./arch/i386/boot/compressed/head.S</code>).{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} <code>startup_32()</code>will do basic setup to environment (stack, etc.), clears the [[.bss|Block Started by Symbol]] (BSS) then invokes <code>decompress_kernel()</code> (located in <code>./arch/i386/boot/compressed/misc.c</code>) to
{{Anchor|Early user space}}<code>start_kernel()</code>executes a wide range of initialization functions. It sets up [[interrupt handling]] ([[Interrupt request|IRQ]]s), further configures memory, mounts the [[initrd|initial RAM disk]] ("initrd") that was loaded previously as the temporary root file system during the bootloader stage.{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} The initrd, which acts as a temporary root filesystem in RAM, allows the kernel to be fully booted and driver modules to be loaded directly from memory, without reliance upon other devices (e.g. a hard disk).{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} initrd contains the necessary modules needed to interface with peripherals,{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} e.g SATA driver, and support a large number of possible hardware configurations.{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} This split of some drivers statically compiled into the kernel and other drivers loaded from initrd allows for a smaller kernel.{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} [[initramfs]], also known as early user space, has been available since version 2.5.46 of the Linux kernel,<ref>{{cite web |last1=Corbet |first1=Jonathan |title=Initramfs arrives |date=6 November 2002 |url=https://lwn.net/Articles/14776/ |access-date=14 November 2011}}</ref> with the intent to replace as many functions as possible that previously the kernel would have performed during the startup process. Typical uses of early user space are to detect what [[device driver]]s are needed to load the main user space file system and load them from a [[temporary filesystem]]. Many distributions use [[dracut (software)|dracut]] to generate and maintain the initramfs image.
The root file system is later switched via a call to <code>pivot_root()</code> which unmounts the temporary root file system and replaces it with the use of the real one, once the latter is accessible.{{Sfn|M. Tim Jones|2006|loc=, "Kernel"}} The memory used by the temporary root file system is then reclaimed.{{Clarify|date=March 2010}}
Line 59 ⟶ 64:
* [[systemd]] is a modern alternative to SysV init. Like init, systemd is a daemon that manages other daemons. All daemons, including systemd, are [[background process]]es. [[Lennart Poettering]] and [[Kay Sievers]], software engineers that initially developed systemd,<ref>{{cite web |title=systemd README |url=http://cgit.freedesktop.org/systemd/systemd/tree/README |accessdate=2012-09-09 |website=freedesktop.org}}</ref> sought to surpass the efficiency of the init daemon in several ways. They wanted to improve the software framework for expressing dependencies, to allow more processing to be done in [[Parallel computing|parallel]] during system booting, and to reduce the [[computational overhead]] of the [[Shell (computing)|shell]]. Systemd's initialization instructions for each daemon are recorded in a declarative configuration file rather than a shell script. For [[inter-process communication]], systemd makes [[Unix ___domain socket]]s and [[D-Bus]] available to the running daemons. Systemd is also capable of aggressive parallelization.
== See also ==
{{Portal|Linux}}
* [[SYSLINUX]]
* [[Booting process of Android devices]]
* [[Booting process of macOS]]
* [[Booting process of Windows]]
| |||