🚀 Building a Bootable "Hello World" OS in Assembly

"Most people write Hello World in C. I wrote mine in Assembly… and booted it."

Yes. It's simple. But it's booted with love. ❤️

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🧠 What Is It?

A tiny, minimal operating system that boots directly from a .img file, shows a "Hello, World!" message using BIOS interrupts, and displays a cute signature line — all in 16-bit real mode Assembly.

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🔧 Tools Used:

• NASM – The Netwide Assembler (to write and compile x86 Assembly code)
• QEMU – Emulator to run and test your bootable image
• BIOS Interrupts – Like int 0x10 for controlling screen output
• Hex and Boot Signatures – 0x7C00 offset and 0xAA55 at the end for boot sector

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🛠 How It Works:

1. Assembly Language – This OS is written in pure 16-bit x86 Assembly (real mode).
2. BIOS Boot – When you boot from the image, BIOS loads the first 512 bytes (the boot sector) from the drive into memory at 0x7C00.
3. Video Interrupts – We use int 0x10 (BIOS video service) to print text to the screen.
4. No OS dependencies – It’s raw metal. Your code runs directly on the hardware.
5. Boot Signature – The last two bytes are 0xAA55, which tells the BIOS it's a valid boot sector.

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🌈 Features:

• 🖥️ Bootable image (.img) that runs on any x86 machine/emulator.
• ✍️ Prints "Hello, World!" and a custom line: "Booted with love ;)"
• ⚙️ Works both on 32-bit and 64-bit emulators
• 📜 Pure assembly, no C, no OS, no clutter — minimalism at its best
• 👨‍💻 A beginner-friendly intro to low-level system programming

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▶️ How to Run:

1. Clone the Repo:

git clone https://github.com/Sreeju7733/helloworld-os.git
cd helloworld-os

2. Assemble the Boot Sector:

nasm -f bin main.asm -o helloworld.img

3. Run with QEMU:

qemu-system-x86_64 -drive format=raw,file=helloworld.img

🪄 And boom! You’ll see "Hello, World!" + your lovely message.

P.S. Even though it’s small, it’s technically a real operating system. 😉