Document the Full Power-On Sequence: From PSU to OS

Power-On to Login

Pressing the power button starts a fixed chain: PSU stabilization, CPU reset at the firmware entry point, POST and boot-device selection, bootloader work, then kernel init. Each stage has distinct responsibilities and failure modes. For example, a hang with no video often means POST never finished, while a GRUB prompt means firmware succeeded but the kernel path failed.

The five boot stages

1. PSU: Converts AC to +12V, +5V, +3.3V and asserts Power Good (~100 to 500 ms).
2. CPU reset: Fetches the first instruction near 0xFFFFFFF0 with CS=0xF000, IP=0xFFF0 (effective 0xFFFF0).
3. BIOS/UEFI POST: Tests CPU/RAM, enumerates hardware, picks a boot device, loads sector 1 to 0x7C00.
4. Bootloader: Parses partitions/filesystems, loads the kernel, may enable protected mode and paging.
5. Kernel: Sets up MMU, drivers, mounts root, starts PID 1.

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• Cold boot to desktop login: often 10 to 60 seconds
• BIOS POST alone: 2 to 10 seconds on older hardware
• UEFI can skip full RAM walk but still runs device init

Why for this exercise

You will document each stage from PSU through OS init with addresses, timing, and an ASCII flow diagram. This exercise requires explaining why the reset vector sits at 0xFFFFFFF0 and how BIOS differs from UEFI before any bootloader code runs.