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== System startup ==
'''System startup''' has different steps based on the hardware that Linux is being booted on, especially between embedded Linux and Linux PC.{{Sfn|M. Tim Jones|2006|loc=, "System startup"}} As mentionmentioned earlier in the introduction part, during system startup stage, the [[BIOS|'''BIOS firmware''']] is called. [[IBM PC compatible]] hardware is one architecture Linux is commonly used on; on these systems, the BIOS plays an important role. 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"}} IBM PC compatible replaces BIOS by UEFI. In [[UEFI]] systems, the Linux kernel can be executed directly by UEFI firmware via EFISTUB,<ref>{{Cite web |title=EFI stub kernel - Gentoo Wiki |url=https://wiki.gentoo.org/wiki/EFI_stub_kernel |access-date=2020-11-02 |website=wiki.gentoo.org}}</ref> but usually uses [[GRUB 2]] or [[Gummiboot (software)|systemd-boot]] as a bootloader.<ref name="Intel2000">{{cite web |last=Kinney |first=Michael |date=1 September 2000 |title=Solving BIOS Boot Issues with EFI |url=http://systems.cs.colorado.edu/Documentation/IntelDataSheets/xscalemagazine.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070123141151/http://systems.cs.colorado.edu/Documentation/IntelDataSheets/xscalemagazine.pdf |archive-date=23 January 2007 |access-date=14 September 2010 |pages=47–50}}</ref><ref name="ElReg1">{{cite news |date=23 September 2011 |title=MS denies secure boot will exclude Linux |url=https://www.theregister.co.uk/2011/09/23/ms_denies_uefi_lock_in/ |access-date=24 September 2011 |publisher=The Register}}</ref>
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}}
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