When you turn on a computer, it goes through a "boot up" process – a term that comes from the word "bootstrap". This is what happens in the background – whether you're using a Windows computer, Mac or Linux system.
Hardware forces on
When you press the power button, the computer delivers power to its components – the motherboard, processor hard drives, solid state devices, graphics processors, and anything else in the computer.
The hardware that supplies power is called "power supply". Inside a regular desktop computer, it looks like a box in the corner of the case (the yellow case in the picture above), and that is where you connect the power cord.
The CPU loads UEFI or BIOS
Now that it has electricity, the CPU initiates itself and looks for a small program that is usually stored in a chip on the motherboard.
Previously, the computer loaded something called BIOS (Basic Input / Output System.) On modern computers, the CPU loads the UEFI (Unified Extensible Firmware Interface) firmware instead. This is a modern replacement for the old-fashioned BIOS. But to make it extra confusing, some PC manufacturers still call their UEFI software "BIOS".
RELATED: What is UEFI and how does it differ from the BIOS?
UEFI or BIOS test and initiates hardware
Firmware for BIOS or UEFI loads configuration settings from a particular location on the motherboard – traditionally, this is stored in the memory of a CMOS battery. If you change some low-level settings on your BIOS or UEFI setup screen, this will store your custom settings.
The CPU drives UEFI or BIOS, which tests and initiates system hardware, including the CPU itself. For example, if the computer has no RAM, it beeps and shows you an error and stops the startup process. This is called the Power On Self Test (POST) process.
You can see that the PC manufacturer's logo appears on your screen during this process, and you can often press a button to access your BIOS or UEFI setup screen from here. However, many modern computers fly through this process so fast that they do not disturb to display a logo and require access to their UEFI setup screen from the Windows Boot Options menu.
UEFI can do much more than just initiate the hardware; It's really a small operating system. For example, Intel processors have Intel Management Engine. This provides a variety of functions, including running Intel's Active Management Technology, which enables remote control of corporate PCs.
UEFI or BIOS Hands Off to a Boot Device
After testing and initializing your hardware, UEFI or BIOS will refrain from starting your computer to the operating system boot loader.
UEFI or BIOS is looking for a "startup device" "Starting the operating system. This is usually the computer's hard drive or solid state drive, but can also be a CD, DVD, USB drive or network location. The boot device can be configured from the UEFI or BIOS installation screen If you have multiple boot devices, UEFI or BIOS tries to disable the boot process for them in the order they are listed, so if you have a bootable DVD in your optical drive, for example, the system may attempt to boot it before attempting it boot from the hard disk.
Traditionally, a BIOS looked at the master boot record (MBR), a special boot sector at the beginning of a disk, the MBR contains code that uploads the rest of the operating system, known as a "bootloader". that takes it from there and starts the current operating system, such as Windows or Linux.
Computers with UEFI can still use this old-fashioned MBR boot method for to start an operating system, but they usually use something called an EFI executable instead. These do not need to be stored at the beginning of a disc. Instead, they are stored on something called an "EFI system partition".
Anyway, the principle is the same. BIOS or UEFI examines a storage device on your system to look for a small program, either in the MBR or on an EFI system partition, and run it. If there is no bootable boot, the boot process fails and you will see an error message that says on your monitor.
On modern computers, the UEFI firmware is usually configured for "Safe Start". This assures the operating system that it has not been tampered with and does not load malicious software at a low level. If Secure Boot is enabled, UEFI checks if the boot loader is properly signed before it is started.
Bootloader loads the entire operating system
The boot manager is a small program that has the great task of starting the rest of the operating system. Windows uses a bootloader named Windows Boot Manager (Bootmgr.exe), most Linux systems use GRUB, and Mac uses something called boot.efi.
If there is a problem with the bootloader – for example, if its files are damaged on the disk -You see an error message about the bootloader and the boot process ending.
The boot reader is just a small program and does not handle the boot process itself. In Windows, Windows Boot Manager finds Windows OS Loader. The OS reader loads essential hard disk drivers required to run the kernel – the core part of the Windows operating system – and then launches the kernel. The kernel then loads the system registry into memory and also loads additional hard disk drivers that are marked with "BOOT_START", which means that they must be loaded at startup. The Windows kernel then starts the session management process (Smss.exe), which starts the system period and loads additional drivers. This process continues, and Windows loads background services and the welcome screen, which lets you sign in.
On Linux, the GRUB bootloader loads the Linux kernel. The kernel also starts the init system – it is systemd on most modern Linux distributions. The Init system handles startup services and other user processes that lead all the way to a login prompt.
This involved process is just one way to make everything loaded properly by doing things in the right order.
By the way, so-called "startup programs" are actually loaded when you log in to your user account, not when the system starts. But some background services (on Windows) or daemon (on Linux and MacOS) are started in the background when the system starts.
The shutdown process is also quite involved. This is exactly what happens when you turn off or log off from a Windows computer.
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