Bootloaders in Computer

Bootloaders: High-Level Overview

1. What is a Bootloader?

• A bootloader is a small program that runs before an operating system (OS) starts.
• Its main job: initialize the system and load the main OS or application code into memory.
• Found in PCs, embedded systems, smartphones, microcontrollers, routers, and more.

2. Why Do We Need a Bootloader?

• Hardware is “dumb” at power-on—it needs instructions on what to do next.
• The OS is usually stored on disk/flash; it cannot run directly from reset.
• Bootloader bridges the gap: sets up hardware, locates OS image, loads it into RAM, and jumps to it.
• Enables:
  • OS/app updates and recovery (flashing new images)
  • Secure boot (verify code signature before running)
  • Selection between multiple OSes (dual-boot)
  • Diagnostics or factory tests before main software

3. High-Level Bootloader Workflow

1. System Reset / Power-On

   • CPU fetches the very first instructions from a fixed memory location (e.g., ROM, Flash, or reset vector).

2. Stage 1: Minimal Hardware Initialization

   • Set up CPU registers, clock, and stack pointer.
   • Initialize memory controller, basic RAM access.
   • (Embedded) Configure essential peripherals (UART for debug, LEDs).

3. Stage 2: Bootloader Code Execution

   • Bootloader code runs (in ROM/Flash or copied to RAM).
   • May present a menu or delay for user input (e.g., “Press F12 for boot menu”).

4. Stage 3: Image Selection and Verification

   • Locate OS/application image (disk, flash, SD card, network).
   • (Secure systems) Verify image signature (cryptographic check, anti-tamper).

5. Stage 4: Load OS/Application into Memory

   • Copy or map OS kernel (or app) into RAM.
   • Set up environment: memory map, device tree, arguments.

6. Stage 5: Jump to Entry Point

   • Bootloader sets program counter to the OS/application entry point.
   • Transfers control—OS/application starts executing.

4. Real-World Bootloader Examples

4.1 PC Bootloaders

• BIOS (legacy): First-stage bootloader in PC, initializes hardware, loads MBR from disk.
• UEFI (modern): More complex, can load large executables, supports secure boot.
• GRUB: Common Linux bootloader, presents boot menu, loads chosen OS kernel.

4.2 Embedded Bootloaders

• MCU Boot ROM: Minimal bootloader built into chip, checks for valid user app, jumps to it or enters programming mode.
• U-Boot: Widely used embedded Linux bootloader, supports many platforms, can boot from flash, network, or USB.
• Microcontroller Bootloaders: Simple code to allow firmware update (UART, USB, I2C, CAN, etc.).

4.3 Smartphone Bootloaders

• Android devices have multi-stage bootloaders (SPL, secondary, fastboot/recovery).
• Secure boot enforces cryptographic signature checks.

5. Practical Example: Embedded Boot Sequence (MCU, e.g., STM32)

1. Power-On Reset: CPU executes from Boot ROM.
2. Boot ROM: Checks BOOT pins or flags; may jump to:
   • User Flash (user application)
   • System Memory (factory bootloader for programming)
   • SRAM (for debugging/testing)
3. User Bootloader (optional): Receives firmware over UART/USB, writes to Flash, or jumps to main app.
4. Main Application: Executes after bootloader finishes.

6. Bootloader Features (Advanced)

• Fail-safe/Recovery: If the main app is corrupt, bootloader can enter update mode.
• Dual-bank/dual-image: Two images for A/B updates (always bootable, even after failed update).
• Secure boot: Checks digital signatures or hashes before running any code (prevents malware).
• Chainloading: Bootloader loads another bootloader (e.g., UEFI loads GRUB, which loads Linux).

7. Summary Table