Building a Bootloader from Scratch: Pure x86 Assembly
This video demonstrates the creation of a hand-written Stage-1 bootloader in raw x86 assembly - no GRUB, no operating system, no standard library. Just pure bare-metal programming that boots directly from BIOS memory.
What Makes This Different from GRUB?
Unlike GRUB (Grand Unified Bootloader) which is a sophisticated, feature-rich bootloader, this implementation shows the fundamental mechanics of how bootloaders actually work at the hardware level:
- Direct BIOS interaction at memory address 0x7C00
- Manual MBR (Master Boot Record) parsing
- Raw CHS (Cylinder-Head-Sector) geometry extraction
- Custom boot parameter handling
- Stage 2 loading via BIOS INT 13h interrupts
Technical Deep Dive
BIOS Boot Process
The video covers the complete boot sequence from power-on to Stage 2 handoff:
- POST (Power-On Self-Test) completes
- BIOS loads first 512 bytes from boot device to 0x7C00
- Boot signature verification (0x55AA magic bytes)
- Control transfer to our custom assembly code
MBR Structure Analysis
Our bootloader manually parses the Master Boot Record:
- Partition table entries (4 × 16 bytes)
- Boot signature validation
- Active partition identification
- CHS geometry calculation for legacy BIOS compatibility
INT 13h Disk Operations
Low-level BIOS interrupt calls for disk I/O:
mov ah, 0x02 ; Read sectors function
mov al, 0x01 ; Number of sectors to read
mov ch, 0x00 ; Cylinder number
mov cl, 0x02 ; Sector number
mov dh, 0x00 ; Head number
int 0x13 ; BIOS disk interrupt
Why This Matters for Security Professionals
Reverse Engineering Applications
- Malware analysis: Understanding how boot-level persistence works
- Firmware analysis: Reverse engineering UEFI and legacy BIOS implementations
- Rootkit detection: Identifying boot-level compromise indicators
Operating System Development
- Kernel developers need to understand the boot handoff process
- Hypervisor development requires deep boot sequence knowledge
- Embedded systems often use custom bootloaders
Cybersecurity Research
- Boot process security: Understanding attack vectors at the firmware level
- Secure boot implementation: How cryptographic verification works
- Boot-time forensics: Analyzing boot artifacts and persistence mechanisms
Connection to Our Research
This bootloader work connects directly to our broader security research:
- QuantumVault Project: Post-quantum cryptography implementations often require custom boot environments
- Secured VLAN Implementation: Network security starts with trusted boot processes
- Buffer Overflow Research: Understanding memory layout from boot time helps with exploit development
Practical Applications
For OS Developers
- Custom operating systems: Replace GRUB with optimized, minimal bootloaders
- Embedded systems: Resource-constrained environments need lightweight boot code
- Real-time systems: Deterministic boot timing requirements
For Security Researchers
- Boot forensics: Understanding how boot sectors can be modified
- Persistence mechanisms: How malware achieves boot-level persistence
- Recovery tools: Building custom boot environments for incident response
Code Availability
The complete source code is available on GitHub, including:
- Commented assembly source with detailed explanations
- Build instructions for cross-platform development
🔗 GitHub Repository 📖 Detailed Blog Post
Technical Prerequisites
To follow along with this implementation:
- x86 assembly knowledge: Understanding of registers, memory addressing, and interrupts
- BIOS familiarity: How legacy BIOS systems work vs UEFI
- Development tools: NASM assembler, QEMU for testing, hex editors for debugging
Conclusion
Building a bootloader from scratch provides invaluable insight into:
- Computer architecture fundamentals
- Operating system initialization
- Low-level security mechanisms
- Bare-metal programming techniques
Perfect for reverse engineers, OS developers, and anyone who wants to understand what happens between pressing the power button and seeing the desktop.