Simple x86 Assembly Bootloader Examples

This is a basic x86 bootloader project that demonstrates the fundamentals of OS development. The bootloader prints "Hello, World!" to the screen using BIOS interrupts.

Project Structure

• hello_world.asm - A bootloader that prints "Hello, World!" to the screen
• hello_world_func.asm - Same as hello_world.asm but using a function call to print
• single_char.asm - A minimal bootloader that prints a single character
• single_char_min.asm - An even more minimal version of single_char.asm
• bochs_config.bxrc - Configuration file for the Bochs emulator

Prerequisites

To build and run this project, you'll need:

• FASM (Flat Assembler) for compiling the assembly code
• Bochs x86 emulator for running the bootloader

Installing FASM

1. Download FASM from the official website
2. For Windows:
   • Download "flat assembler for Windows" package
   • Extract the zip file
   • The package includes an integrated editor with syntax highlighting
   • Documentation is provided in PDF format

Installing Bochs

1. Download the latest Bochs release (currently 2.8) from the official Bochs releases page
2. For Windows:
   • Download the .exe installer
   • Either run the installer directly
   • Or extract the files using 7-Zip if you prefer a portable installation

Building

To compile the bootloaders, run either:

fasm hello_world.asm hello_world.bin

or

fasm single_char.asm single_char.bin

Running

To run a bootloader in Bochs, first update the bochs_config.bxrc file to point to the desired .bin file, then:

bochs -f bochs_config.bxrc

Technical Details

The bootloader implements these key features:

• Loads at memory address 0x7C00 (a fixed address chosen by IBM in 1981)
• Runs in 16-bit real mode
• Uses BIOS interrupt 0x10 with function 0x0E for teletype output, which automatically advances the cursor after printing
• Uses lodsb instruction to efficiently load and process string bytes into AL register while auto-incrementing SI
• Stores string characters using db (Define Byte) directive with a null terminator (0)
• Uses null-terminated strings for text output processing
• Implements an infinite loop after printing to prevent executing past our code
• Fits within the required 512 bytes for boot sectors:
  • Main code and data in first 510 bytes
  • Automatically pads unused space with zeros
  • Includes standard boot signature (0x55, 0xAA) in final 2 bytes, required by BIOS to mark the sector as bootable

References

Learning Resources

• Single Character Bootloader Tutorial - Tutorial for the single character bootloader implementation
• Writing a Simple Operating System — from Scratch - Tutorial that inspired our Hello World bootloader implementation
• Concise Hello World Bootloader - A streamlined implementation of the Hello World bootloader
• OS Tutorial - Boot Sector Printing - Detailed guide on implementing boot sector text printing, directly relevant to our Hello World implementation
• Writing a Simple Operating System from Scratch - The original source material that inspired the OS Tutorial series by cfenollosa

Assembly and FASM Documentation

• FASM Documentation - Official FASM documentation and manual

BIOS and Hardware References

• INT 10h - BIOS Video Services - General documentation for BIOS video services
• INT 10,E - Write Text in Teletype Mode - Documentation for the BIOS interrupt used for text output

Tools and Software

• Bochs Releases - Official Bochs emulator downloads

License

This project is open source and available under the MIT License.