Booting & Memory Hierarchy
Understanding the startup sequence, CPU architectures, and the multi-layered storage hierarchy that powers computation.
Phase 1: Hardware & Firmware
When you press the power button, the CPU is essentially "brain dead." It needs a tiny, built-in program to tell it how to find the Operating System.
The BIOS / UEFI
The Basic Input/Output System is stored on the motherboard in a non-volatile ROM chip. Modern PCs use UEFI (Unified Extensible Firmware Interface) which is faster and supports larger disks.
POST (The Health Check)
Before doing anything else, the BIOS performs a Power-On Self-Test. It checks if the RAM, Keyboard, and CPU are functioning. If POST fails, the system stops and emits a "beep" code.
The Booting Pipeline
Power On
Electricity flows to CPU
BIOS / UEFI Load
CPU reads ROM chip
MBR / GPT Search
Look for Master Boot Record
Bootloader (GRUB)
Initialize OS Kernel
Phase 2: The Handover
Once BIOS finds a bootable disk, its job is over. It hands the "baton" to a small program sitting in the very first sector of the disk (MBR).
Locate Kernel
Bootloader finds where the OS Kernel is stored on the disk and loads it into RAM.
Initialize Hardware
Kernel takes over, detects hardware, and initializes device drivers.
First Process (Init)
Kernel starts the very first user-space process (called init or systemd in Linux).
[ 0.000000] Linux version 5.15.0-gener...
[ 0.124501] CPU0: Intel(R) Core(TM) i9...
[ 0.456012] Memory: 16384K/32768K available
[ 0.892110] Freeing unused kernel memory...
[ 1.250110] Run /sbin/init as process 1