🧠 Simple x86 Bootloader with BIOS Sector Read

This project is a simple 16-bit bootloader written in x86 Assembly that demonstrates how to read a specific disk sector using BIOS interrupt INT 13h. It is designed for floppy disk emulation (e.g., in QEMU) and uses BIOS services to print messages and load data from the disk.

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📁 Project Structure

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├── asm/
│   ├── disk\_read.asm        # Reads one disk sector using BIOS INT 13h
│   ├── print.asm            # BIOS-based print functions (char, string)
│   └── stage1\_bootloader.asm # Main bootloader (runs at 0x7C00)
├── build/
│   └── bootloader.bin       # Compiled bootloader binary
├── README.md                # Project documentation

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🧾 Features

• Reads sector 2 from a floppy disk using BIOS interrupt INT 13h.
• Loads the sector into memory at 0x0000:0500.
• Displays custom messages using BIOS interrupt INT 10h.
• Demonstrates modular assembly structure with reusable print and disk functions.

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🚀 Getting Started

🔧 Prerequisites

Ensure the following tools are installed:

• NASM – Assembler for x86 architecture
  Install with: sudo apt install nasm (Linux) or from https://www.nasm.us/
• QEMU – Emulator to run the bootloader
  Install with: sudo apt install qemu-system-x86 or from https://www.qemu.org/
• Unix-like terminal (Linux, macOS, WSL, or Git Bash on Windows)

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⚙️ Build Instructions

# 1. Assemble the bootloader
nasm -f bin asm/stage1_bootloader.asm -o build/bootloader.bin

# 2. Create a 1.44MB floppy disk image
dd if=/dev/zero of=build/os_image.img bs=512 count=2880

# 3. Write the bootloader to sector 1 (boot sector)
dd if=build/bootloader.bin of=build/os_image.img conv=notrunc

# 4. Write test data to sector 2 (512-byte sector)
echo -n ">> Sector 2 loaded!" | dd of=build/os_image.img bs=512 seek=1 conv=notrunc

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🧪 Run with QEMU

qemu-system-x86_64 -boot a -fda build/os_image.img

You should see the following output in the QEMU window:

Reading sector 2...
>> Sector 2 loaded!

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🧠 How It Works

Bootloader Flow:

1. BIOS loads boot sector (512 bytes) to 0x7C00 and jumps to it.
2. Bootloader initializes segment registers (DS, ES).
3. Displays message "Reading sector 2..."
4. Calls read_sector, which uses BIOS INT 13h to read sector 2
5. If read succeeds, it prints the content of memory at 0x0500

Key BIOS Calls:

• INT 10h / 0Eh: Print character in text mode
• INT 13h / 02h: Read sectors from disk

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🛠️ Modular Assembly

• print.asm: Provides reusable routines to print characters and strings.
• disk_read.asm: Abstracts BIOS sector read into a callable subroutine.
• stage1_bootloader.asm: Main bootloader logic, includes the other modules.

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📌 Notes

• This project assumes the boot device is a floppy (DL = 0x00). Change DL to 0x80 if using hard disk emulation.
• The sector read logic is minimal and may fail silently if BIOS returns an error.
• BIOS limits restrict this to CHS access; advanced support (e.g., LBA) is not included here.

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📚 References

• Ralf Brown's Interrupt List
• Intel 8086 Reference Manual
• Writing a Simple Bootloader

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📄 License

This project is open-source and licensed under the MIT License.

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👨‍💻 Author

Aayush Gid Electronics & Communication Engineering Driven by systems programming, embedded systems, and OS dev.