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Writing a Simple Bootloader

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In this section, we will explore how to write a simple bootloader. A bootloader is a small program that runs when a computer starts up. It is responsible for loading the operating system into memory and starting it. Writing a bootloader is a great way to understand low-level programming and how computers start up.

Objectives

  • Understand the role of a bootloader.
  • Learn the basic structure of a bootloader.
  • Write a simple bootloader in Assembly language.
  • Test the bootloader using an emulator.

Prerequisites

  • Basic understanding of Assembly language.
  • Familiarity with hexadecimal and binary number systems.
  • A development environment set up for Assembly programming.

Key Concepts

What is a Bootloader?

  • Definition: A bootloader is a small piece of code that initializes the hardware and loads the operating system into memory.
  • Function: It runs immediately after the computer is powered on or reset.
  • Location: Typically resides in the first sector of the bootable storage device (e.g., hard drive, USB stick).

Bootloader Structure

  • Boot Sector: The first 512 bytes of the storage device.
  • Magic Number: The last two bytes of the boot sector must be 0x55 and 0xAA to be recognized as a bootable device by the BIOS.

Development Environment

  • Assembler: NASM (Netwide Assembler) is commonly used.
  • Emulator: QEMU can be used to test the bootloader.

Writing the Bootloader

Step 1: Setting Up the Environment

Ensure you have NASM and QEMU installed. You can install them using the following commands:

sudo apt-get install nasm qemu

Step 2: Writing the Bootloader Code

Create a file named bootloader.asm and write the following code:

; bootloader.asm - A simple bootloader

BITS 16                 ; We are working in 16-bit real mode
ORG 0x7C00              ; The origin, where the BIOS loads the bootloader

start:
    mov ax, 0x07C0      ; Set up the segment registers
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7C00

    ; Print a message
    mov si, msg
print_char:
    lodsb               ; Load byte at [SI] into AL
    cmp al, 0           ; Check if end of string (null terminator)
    je done
    mov ah, 0x0E        ; BIOS teletype function
    int 0x10            ; Call BIOS interrupt
    jmp print_char

done:
    cli                 ; Disable interrupts
    hlt                 ; Halt the CPU

msg db 'Hello, World!', 0

times 510-($-$$) db 0   ; Fill the rest of the boot sector with zeros
dw 0xAA55               ; Boot sector signature

Step 3: Assembling the Bootloader

Assemble the bootloader using NASM:

nasm -f bin bootloader.asm -o bootloader.bin

Step 4: Testing the Bootloader

Run the bootloader using QEMU:

qemu-system-x86_64 -drive format=raw,file=bootloader.bin

You should see the message "Hello, World!" displayed on the screen.

Practical Exercise

Exercise: Modify the Bootloader

Modify the bootloader to display a different message and change the background color of the text.

Steps:

  1. Change the message in the msg variable.
  2. Modify the mov ah, 0x0E instruction to change the text color.

Solution:

; bootloader.asm - A simple bootloader with modified message and color

BITS 16
ORG 0x7C00

start:
    mov ax, 0x07C0
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7C00

    ; Print a message
    mov si, msg
print_char:
    lodsb
    cmp al, 0
    je done
    mov ah, 0x0E
    mov bx, 0x1E00  ; Set text color (red background, yellow text)
    int 0x10
    jmp print_char

done:
    cli
    hlt

msg db 'Welcome to Assembly!', 0

times 510-($-$$) db 0
dw 0xAA55

Assemble and test the modified bootloader as described in the previous steps.

Common Mistakes and Tips

  • Magic Number: Ensure the last two bytes of the boot sector are 0x55 and 0xAA.
  • Segment Registers: Properly set up the segment registers to avoid segmentation faults.
  • Message Null Terminator: Ensure the message string is null-terminated.

Conclusion

In this section, we learned how to write a simple bootloader in Assembly language. We covered the basic structure of a bootloader, wrote a simple program to display a message, and tested it using an emulator. This foundational knowledge prepares you for more advanced topics in bootloader and operating system development.

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