Writing a Bootloader Part 3
FreeExtend your bootloader beyond 512 bytes with C++ and BIOS disk services.
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About Writing a Bootloader Part 3
A detailed tutorial on writing a bootloader that extends beyond the traditional 512-byte limit. Part 3 of a series covers loading additional sectors using BIOS interrupt 0x13, transitioning to C++ by compiling and linking a simple C++ function into the bootloader, and setting up a cross-compiler for i386-elf targets. Includes step-by-step assembly and C++ code examples, linker script configuration, and instructions for testing with QEMU.
Key Features
Loading additional disk sectors using int 0x13 (AH=0x02)
Calling C++ functions from bootloader assembly
Cross-compiler setup for 32-bit i386-elf target (macOS via Homebrew tap)
Linker script configuration for raw binary output
Memory-mapped VGA text output in C++
Practical example with assembly and C++ code integration
Pros & Cons
Pros
- Clear, hands-on explanation with complete code samples
- Addresses the common limitation of 512-byte boot sector size
- Provides a cross-compiler installation guide for macOS
- Shows real integration of assembly and C++ in a bootloader context
- Includes debug tips and QEMU testing instructions
Cons
- Assumes familiarity with previous parts of the series
- Cross-compiler setup is limited to macOS (Homebrew)
- No discussion of error handling or disk read failures
- Stops before implementing more complex kernel features
Best For
Learning x86 bootloader and OS development fundamentalsUnderstanding BIOS disk services and memory layoutPrototyping a simple bootable program with C++ componentsEducational projects in low-level system programming
FAQ
How do I load more than 512 bytes in a bootloader?
Use BIOS interrupt 0x13 with AH=0x02 to read additional sectors from disk into memory. The bootloader must specify cylinder, head, sector, and target address.
How can I call C++ code from a bootloader?
Compile the C++ function with extern 'C' linkage to prevent name mangling, then link it with your assembly using a linker script that places code at 0x7C00 and beyond.
What cross-compiler is needed?
An i386-elf GCC cross-compiler (e.g., i386-elf-gcc) that generates 32-bit code. The tutorial provides a Homebrew tap for macOS installation.
What is the purpose of the linker script?
The linker script ensures the boot sector starts at offset 0x7C00 and outputs raw binary format, placing the boot code and C++ code sequentially in the final image.