the boot process
Booting is the chicken-and-egg trick of getting an operating system up and running on a machine that, the instant you press power, knows almost nothing — no OS in memory, no idea where the OS even is. The name comes from the phrase pulling yourself up by your own bootstraps, because the computer must, step by step, load progressively more capable software until the full OS is running. Each small program's job is mainly to find and start the next, slightly bigger one.
Here is the chain in plain steps. (1) Power on: the CPU starts executing fixed firmware burned into a chip on the motherboard (the BIOS or, on modern machines, UEFI). The firmware runs a basic self-check of the hardware. (2) The firmware locates a small program called the bootloader on a storage device and loads it into memory. (3) The bootloader's job is to find the operating system kernel on disk, load it into memory, and hand control to it (sometimes via an intermediate temporary file system that holds early drivers). (4) The kernel initializes itself — setting up memory management, starting device drivers, mounting the real file system. (5) Finally the kernel starts the very first user-space process (historically called init, or systemd on many Linux systems), which in turn launches all the services and, eventually, the login screen or desktop you see. From near-total ignorance to a usable system, in a handful of handoffs.
Understanding the boot chain demystifies a lot of practical experience: a flashing firmware screen, a boot menu, the moment the disk light flickers as the kernel loads. It also clarifies why boot is a security-sensitive moment — if an attacker can slip malicious code in early in the chain, it runs before any defenses are up, which is exactly why secure boot exists to cryptographically check each stage. The honest simplification here is that real boot has many more details (multiple firmware phases, recovery paths, encrypted disks), but the essential shape — firmware, then bootloader, then kernel, then first process — is the durable mental model.
Press power on a Linux laptop: UEFI firmware self-tests and loads a bootloader; the bootloader loads the Linux kernel (plus a small initramfs of early drivers); the kernel sets itself up and mounts the disk; then it starts process 1 (systemd), which brings up services and the login screen.
Firmware to bootloader to kernel to first process — each stage starts the next.
This is an orientation-level sketch; real boot has many extra phases. The security angle is real: code that runs early runs before defenses exist, which is why secure boot verifies each stage before trusting it.