Computer Turns on Then Off Then on Again
When yous power on a computer, it goes through a "boot up" process– a term that comes from the word "bootstrap." Hither's what's happening in the background—whether you're using a Windows PC, Mac, or Linux organization.
The Hardware Powers On
When you lot press the power button, the computer supplies power to its components—the motherboard, CPU, hard disks, solid country drives, graphics processors, and everything else in the reckoner.
The piece of hardware that supplies power is known as the "power supply." Inside a typical desktop PC, it looks similar a box at the corner of the case (the yellowish thing in the moving picture above), and information technology's where you connect the AC power string.
The CPU Loads the UEFI or BIOS
Now that it has electricity, the CPU initializes itself and looks for a small plan that is typically stored in a flake on the motherboard.
In the past, the PC loaded something called a BIOS (Basic Input/Output System.) On modern PCs, the CPU loads UEFI (Unified Extensible Firmware Interface) firmware instead. This is a modern replacement for the erstwhile-style BIOS. But, to brand it extra confusing, some PC manufacturers still call their UEFI software "BIOS" anyway.
RELATED: What Is UEFI, and How Is It Dissimilar from BIOS?
The UEFI or BIOS Tests and Initializes Hardware
The BIOS or UEFI firmware loads configuration settings from a special identify on the motherboard—traditionally, this was in memory backed up by a CMOS bombardment. If yous alter some low-level settings in your BIOS or UEFI settings screen, this is where your custom settings are stored.
The CPU runs the UEFI or BIOS, which tests and initializes your system'southward hardware—including the CPU itself. For example, if your reckoner doesn't take whatsoever RAM, information technology will beep and prove y'all an error, stopping the boot process. This is known as the Mail (Power On Cocky Test) procedure.
You may run into the PC manufacturer's logo appear on your screen during this procedure, and yous can often press a button to access your BIOS or UEFI settings screen from hither. However, many modernistic PCs fly through this process so fast that they don't bother displaying a logo and require accessing their UEFI setting screen from the Windows Boot Options menu.
UEFI tin can do a lot more than than merely initialize hardware; it's really a tiny operating system. For example, Intel CPUs have the Intel Management Engine. This provides a variety of features, including powering Intel's Active Management Technology, which allows for remote management of business PCs.
The UEFI or BIOS Easily Off to a Kicking Device
Afterward it'southward done testing and initializing your hardware, the UEFI or BIOS will paw off responsibility for booting your PC to your operating system'south boot loader.
The UEFI or BIOS looks for a "boot device" to boot your operating system from. This is ordinarily your calculator's hard deejay or solid-state drive, but may also be a CD, DVD, USB bulldoze, or network location. The kick device is configurable from within the UEFI or BIOS setup screen. If yous accept multiple kicking devices, the UEFI or BIOS attempts to hand off the startup process to them in the gild they're listed. So, for example, if you have a bootable DVD in your optical drive, the organisation might attempt starting from that before it tries starting from your hard bulldoze.
Traditionally, a BIOS looked at the MBR (master boot tape), a special kick sector at the beginning of a disk. The MBR contains lawmaking that loads the rest of the operating system, known as a "bootloader." The BIOS executes the bootloader, which takes it from there and begins booting the actual operating organisation—Windows or Linux, for example.
Computers with UEFI can even so use this old-style MBR kick method to kicking an operating arrangement, only they normally use something called an EFI executable instead. These don't have to be stored at the beginning of a disk. Instead, they're stored on something called an "EFI system segmentation."
Either style, the principle is the aforementioned—the BIOS or UEFI examines a storage device on your organization to wait for a small program, either in the MBR or on an EFI system partition, and runs it. If there's no bootable boot device, the bootup process fails, and y'all'll see an mistake message proverb so on your brandish.
On mod PCs, the UEFI firmware is generally configured for "Secure Kick." This ensures the operating system that it starts hasn't been tampered with and won't load low-level malware. If Secure Boot is enabled, the UEFI checks whether the bootloader is properly signed before starting it.
The Bootloader Loads the Full Bone
The bootloader is a modest program that has the large chore of booting the remainder of the operating system. Windows uses a bootloader named Windows Boot Manager (Bootmgr.exe), about Linux systems use GRUB, and Macs use something called kick.efi.
If there'south a problem with the bootloader—for example, if its files are corrupted on disk—y'all'll see a bootloader error message, and the kick process will stop.
The bootloader is but one small program, and it doesn't handle the kick process on its own. On Windows, the Windows Kicking Manager finds and starts the Windows OS Loader. The OS loader loads essential hardware drivers that are required to run the kernel—the core part of the Windows operating system—and so launches the kernel. The kernel and so loads the system Registry into memory and as well loads any boosted hardware drivers that are marked with "BOOT_START," which ways they should be loaded at kicking. The Windows kernel then launches the session manager process (Smss.exe), which starts the system session and loads boosted drivers. This process continues, and Windows loads background services equally well as the welcome screen, which lets you lot sign in.
On Linux, the GRUB boot loader loads the Linux kernel. The kernel also starts the init system—that's systemd on most modern Linux distributions. The init system handles starting services and other user processes that lead all the fashion to a login prompt.
This involved process is merely a way of making everything load correctly past doing things in the correct order.
By the way, and then-chosen "startup programs" actually load when you sign into your user business relationship, not when the system boots. But some background services (on Windows) or daemons (on Linux and macOS) are started in the groundwork when your arrangement boots.
The shutdown process is pretty involved, too. Here's exactly what happens when you shut downwards or sign out of a Windows PC.
Image Credit: Suwan Waenlor/Shutterstock.com, DR-images/Shutterstock.com,
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Source: https://www.howtogeek.com/398493/what-exactly-happens-when-you-turn-on-your-computer/
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