Building Your Own Boot Loader from Scratch with Assembly

A project plan for creating a BIOS-based boot loader in assembly language, with step-by-step guidance and troubleshooting.

Building Your Own Boot Loader from Scratch with Assembly

See Also:

• The actual implementation code for this boot loader is available in the QuantumVault project (SentinelDOS directory) on GitHub. If you want to preview the code or try it out before the blog series is complete, check it out there.
• For a broader introduction to the QuantumVault project, see the QuantumVault project article. This new blog post will provide a detailed deep dive into the custom boot loader component of that project.

Motivation

Understanding how computers start up at the lowest level is both fascinating and empowering. Writing your own boot loader from scratch is a classic project for anyone interested in operating systems, reverse engineering, or low-level programming.

Technical Background

• BIOS Boot Process: The BIOS loads the first 512 bytes (MBR) from the boot device into memory and executes it.
• MBR (Master Boot Record): Contains the boot loader code and partition table.
• Assembly Language: Required for precise control over hardware and memory at boot time.

Project Plan

1. Write a Minimal Boot Sector

• Create a 512-byte boot sector in x86 assembly.
• Display a message or perform a simple action to verify execution.

2. Load and Execute Additional Code

• Implement routines to load more code from disk (e.g., a second-stage loader or kernel).
• Use BIOS interrupts (INT 13h, INT 10h) for disk and screen operations.

3. Handle BIOS Services and Hardware

• Read keyboard input, display text, and manage memory using BIOS interrupts.
• Address real-mode memory limitations and segment:offset addressing.

4. Troubleshooting and Common Pitfalls

• Debugging boot sector code with emulators (e.g., QEMU, Bochs, VirtualBox).
• Handling alignment, boot signature (0xAA55), and disk geometry issues.

Future Blog Updates

This post is the start of a hands-on series. Future articles will provide:

• Complete annotated assembly code examples
• In-depth explanations of BIOS services
• Step-by-step debugging and troubleshooting guides
• Advanced topics: protected mode, file system loading, custom kernels

Stay tuned for code walkthroughs and real hardware experiments!