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Add file descriptor and boot process details
Apr 17, 2025
d830fd7 · Apr 17, 2025

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boot.md

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The Boot Process

Table of Contents

Introduction

Booting the the process of loading and starting an Operating System.

The OS Boot Process

  • Hit the power button.
    • Triggers a 'power good' signal.
      • Electric pulse sent to reset pin of the CPU. (Power On Reset)
      • CPU is in 'Reset' mode, i.e., it is not executing any instructions.
    • All devices get power and initialize themselves.
    • Memory layout and mapping is done.
    • Every register is set to zero, except Code Segment (CS) and Instruction Pointer (IP), which are set to 0xf000 and 0xfff0 respectively.
      • Thus, the physical address = (CS << 4) + IP = (0xf000 << 4) + 0xfff0 = 0xf0000 + 0xfff0 = 0xffff0
        • This physical address is the place where the CPU starts executing instructions.
    • The CPU is activated in Real Mode and it starts executing from 0xffff0 (or ffff0h), which is a memory location in the BIOS chip and not in the RAM.
      • Real Mode
        • Only 1 MB of RAM addressability in the range 0x0 to 0x100000.
          • This is because there are 20 physical address bus lines available. (2^20 = 1048576 = 1 MB)
        • Available registers (Eg: AX) are of size 16 bits, so two registers are combined to give the physical address.
          • Logical address (LBA) = segment:offset
          • Physical address = (segment << 4) + offset
            • The segments are segments/parts of the addressable 1 MB of RAM and the offsets are offsets into that segment.
            • Eg: If the segment is 0xf000 and the offset is 0xfff0, then the physical address = (CS << 4) + IP = (0xf000 << 4) + 0xfff0 = 0xf0000 + 0xfff0 = 0xffff0
  • Basic Input/Output System (BIOS) takes over.
    • Placed in Flash/EPROM Non-Volatile Memory.
    • In a multi-processor environment, one processor is a Boot Processor (BSP) which executes all instructions and the others are Application Processors (APs).
    • Conducts a Power-On Self-Test (POST).
      • Performs system inventory.
      • Checks and registers all devices connected.
      • Plug-and-Play (PnP) device detection.
    • Finds Master Boot Record (MBR) in the first sector of a device (the hard disk, SSD, USB, etc.) that is usually 512 bytes in size, loads it into RAM at position 0x7c00, jumps to that location and starts executing.
      • MBR is a 512 byte sector that's logically split into three sections.
        • The first 446 bytes is reserved for a program, which is usually a Bootloader. (Eg: GRUB)
        • The next 64 bytes (16x4 bytes) are for a partition table with four partitions in it.
        • The last two bytes are for the Boot Signature bytes 0x55 (or 55h) and 0xAA(or AAh) in order, that identify that a particular sector is the MBR.
          • If this signature is not found in the first sector, then the next device is searched.
    • The BIOS loads the Bootloader into memory.
      • This might be the first stage of the Bootloader, which loads the second stage of the Bootloader into memory, as 446 bytes are not sufficient to store all the complex logic required to load an OS.
      • Bootloader might give an option to load a particular Operating System.
      • Sets up the GDT/IVT for the Operating System.
      • Switches from Real Mode to Protected Mode.
        • Memory addressability goes from 1 MB to the entire range of available RAM.
  • The Bootloader starts executing and checking the partition table for an active/bootable partition table.
    • On finding the bootable partition, the Bootloader loads the first sector of that partition (called the Boot Record) from the hard disk to the RAM.
  • The Boot Record loads the operating system into memory.
  • Timers, devices, hard disks, etc. are initialized by the Operating System in the Kernel Space.
  • In Linux, the init process is the first process in User Space that initializes the OS processes, daemons and displays login prompt.

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