How an Operating System Starts: From Power‑On to the Boot Loader
This article explains how a computer powers on, the CPU fetches initial instructions from firmware such as BIOS or UEFI, performs hardware checks, reads the MBR, runs first‑stage and second‑stage boot loaders, and finally loads the operating system kernel into memory.
In this article, the author explains that an operating system, like any program, is stored on disk and must be loaded into memory to run.
When the power button is pressed, the CPU cannot fetch instructions from empty RAM, so it begins executing a small set of instructions stored in non‑volatile memory such as ROM, BIOS, or the newer UEFI firmware.
The firmware performs a power‑on self‑test (POST) to verify hardware, then searches for a boot device (usually a disk) and reads the first 512 bytes—the Master Boot Record (MBR).
The MBR contains a tiny boot loader program and a partition table; the boot loader copies itself into memory and, based on the partition information, loads a larger second‑stage boot loader.
The second‑stage loader can present a menu for multiple operating systems and finally loads the selected OS kernel into memory, after which the OS performs its own initialization and creates user‑mode processes.
Modern systems using UEFI may skip the MBR stage entirely, but the overall principle of firmware handing control to a boot loader remains the same.
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