Booting: how computers and electronic systems start up
Overview of booting: the sequence that starts a computer or device, key components (firmware, bootloader, kernel), types of boot, modern variations (UEFI, network boot, secure boot) and troubleshooting notes.
What is booting?
Booting is the sequence of operations that brings an electronic device from a powered-off or reset state to a condition where it can run user software. When a computer or other powered device initializes, built-in low-level firmware performs checks and hands control to a component that loads the main system. In everyday language people also say "boot up" or simply "boot" to mean starting such a device.
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Typical stages of a boot sequence
Although implementations vary, most boot processes follow a similar progression. Early steps are performed by permanent firmware; later steps involve loading code from a storage medium and starting the operating environment.
- Power-on and firmware initialization – Firmware stored in read-only memory runs first. Traditional systems use a BIOS; modern PCs commonly use UEFI or BIOS firmware which configures hardware and performs basic tests.
- Hardware detection – The firmware detects and configures peripheral devices such as disks, network interfaces, and input/output controllers (collectively called peripherals).
- Boot device selection – The firmware locates a boot device (local disk, removable media, or network) and loads a small program known as a bootloader or bootstrap loader.
- Bootloader and kernel load – The bootloader loads the operating system kernel and optional initial programs into memory and transfers execution to them. The kernel then initializes higher-level services and launches user-space operating system processes.
- Service start and user session – System services and graphical or command interfaces begin, culminating in a ready-to-use environment.
Types and terminology
People distinguish several kinds of boots. A cold boot or power-on boot begins after the device has been off; a warm boot or reboot restarts the system without removing power. The term "bootstrap" comes from the idiom "pull yourself up by your bootstraps," describing how a small initial program loads progressively larger components. The same word has been borrowed into popular culture to describe restarting a fictional franchise, often called a reboot.
Modern variations and technologies
Over time firmware and boot methods have evolved. UEFI provides richer services, faster startup, and support for secure boot features that verify signed software before execution. Systems can also boot from a network using protocols such as PXE, which is common in large deployments for automated installation. Embedded devices and mobile platforms may use simplified or specialized boot chains appropriate to their hardware. In many contexts, the boot sequence is tightly coupled with device firmware and low-level electronics.
Practical importance and examples
Understanding booting helps with troubleshooting, system administration, and security. Common administrative tasks include selecting the boot device order, entering firmware setup screens to change configuration, using recovery modes when the kernel or filesystem is damaged, and managing multiboot environments where several operating systems share a machine. Virtual machines and cloud instances perform similar boot steps, though some hardware-specific phases are emulated by host software. Firmware and bootloaders are also targets for updates and security hardening to prevent unauthorized code execution. Boot behavior influences how quickly a device becomes available after power-up and how it responds to failures or updates.
Notable details and distinctions
Some additional points to note: the tiny program that begins loading the OS is often called a bootstrap loader or bootloader and is conceptually distinct from the operating system itself. Tools and utilities that run during early boot are typically considered low-level software. Technicians sometimes refer to a desktop machine as a "box" and say they will "boot the box." The words "Internet" and "web" are unrelated jargon examples but illustrate how technical terms can enter everyday speech (Internet, web).
Overall, booting is a foundational process in computing: a sequence of firmware, hardware checks, bootloaders and kernel initialization that reliably moves a device from inert hardware to a functioning system capable of running applications.
Mainframe
On IBM mainframes, the boot process has traditionally been called Initial Program Load (IPL). However, this use of the term Initial Program Load is now obsolete, because one has switched to using this term for the first stage of the execution of a multi-stage boot load program.
Personal computer (x86 architecture)
When a PC boots, the processor starts processing the firmware stored at a specified memory address in ROM. In general, this performs a test of the connected devices (POST) and checks storage devices such as floppy disk drives, hard disks or CD/DVD drives etc. to see whether they are or contain bootable media. Depending on the concrete firmware implementation, the search sequence according to which these devices are accessed can be changed, e.g. via firmware setup or via boot menu.
On IBM PC-compatible computers up to the 2010s, this firmware is usually the BIOS. On PCs after about 2010, it was increasingly replaced by the (Unified) Extensible Firmware Interface, EFI or UEFI for short. Macs have also used an Apple-specific EFI since the switch to Intel processors in 2006.
A bootable medium requires a valid boot sector on a PC with BIOS, as introduced with the original IBM PC model 5150 from 1981. On larger data storage devices such as hard disks, this is usually the Master Boot Record (MBR), which also contains the partition table. The process of booting begins when the boot sector is loaded and executed by the BIOS. Since the boot sector is limited to a block size of 512 bytes, it usually contains a boot loader that loads further data, for example by searching for special files on the storage medium and then loading and executing them until, for example, the kernel and finally the entire operating system has been started. This process, in which one program loads the next, is also called chain loading. With boot managers that link into this chain early on, it is possible to change the boot process and, for example, to implement an additional boot menu for multi-boot systems.
On the BIOS successor Extensible Firmware Interface (EFI), which was introduced by Intel at the end of the 1990s and which has been further developed jointly by several PC manufacturers from the hardware and software sectors since 2005 as "Unified EFI" (UEFI), an EFI loader is loaded directly from a specified partition, the EFI System Partition (ESP). By specification, the ESP is a partition of arbitrary size formatted with the FAT32 file system and defined in a GUID partition table. The EFI loader is an executable program of the respective processor architecture, either found on a data memory found and checked by the firmware or specified directly by an entry in the NVRAM of the (U)EFI, possibly including a startup paramenter. As a successor to the BIOS, many (U)EFI implementations also have a compatibility module, the Compatibility Support Module (CSM), which emulates a BIOS and can thus continue to start IBM PC-compatible boot sectors. If this module is available and the function is configured accordingly (activated), UEFI automatically loads the CSM if a data carrier with a Master Boot Record (MBR) is found and is to be booted from it by default in the firmware setup or selection in the boot menu. If Secure Boot is enabled, the CSM is not available.
The EFI loader, like the boot code in a boot sector, is also called a boot loader. Boot loaders are computer programs whose task it is to advance the boot process. In a few cases, the boot loader is already the last stage in the boot process, for example in PC booters.
When installing an operating system, it is first booted from a bootable medium such as a CD or DVD in an optical drive or from a USB stick. However, the firmware must support this type of bootable medium. This usually contains a customized version of the operating system to be installed - if the boot process of this installation medium is successful, it automatically loads the installation program of the operating system. This program sets up a functioning boot configuration for the respective system on the selected installation target, a data storage device such as a hard disk, for example boot sectors and startup files on the corresponding partitions.
Intel has specified PXE as a method to boot PCs (IA-32) and Itanium computers (IA-64) over a computer network.
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AlegsaOnline.com Booting: how computers and electronic systems start up Leandro Alegsa
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