Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsYou can build an operating system in assembly, but getting a small kernel to boot is a more realistic first goal than writing a complete OS. For a first milestone, choose one x86 boot route, use an existing bootloader unless building one is the point of the exercise, and test the handoff in an emulator.
What you are building: a boot path and a kernel
Firmware does not simply start running a kernel. It hands control to a boot path, which performs or relies on platform setup and eventually transfers control to the kernel. The sequence and the conditions at that handoff depend on the processor architecture, firmware, and boot protocol. See OSDev’s x86 System Initialization overview.
Those are separate projects. A boot sector or UEFI loader gets control from firmware and prepares a kernel handoff; the kernel then takes responsibility for the machine’s ongoing operation. A tiny assembly program that prints text or halts can demonstrate startup, but it is not a complete, usable operating system. Interrupt handling, memory management, drivers, storage, and user programs are substantial additional work.
Choose one x86 boot route
The distinction between legacy BIOS and UEFI changes the startup environment and the work your assembly must do. These are broad learning-path differences, not interchangeable code recipes. Follow the specification and boot protocol for your exact target; do not combine entry code, headers, or calling conventions from unrelated tutorials. OSDev’s UEFI guide and x86 initialization overview describe the broader startup context.
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| Route | What you learn | Initial work | Where it fits |
|---|---|---|---|
| Legacy BIOS / boot sector | Early x86 startup and explicit mode-transition work | Your boot code owns more low-level setup | A focused learning exercise or legacy target; BIOS is deprecated on newer machines, according to OSDev |
| UEFI application or loader | The firmware loader interface and a more prepared execution environment | You must understand EFI interfaces and the target environment even though firmware performs additional platform setup | More relevant to contemporary UEFI systems; specifics vary by CPU architecture |
| Existing bootloader with a kernel handoff | Kernel entry and development without first implementing a loader | Learn the chosen boot protocol and meet its kernel-entry requirements | A practical route to a first kernel milestone |
Do not assume old VGA text-mode examples describe modern graphics. OSDev notes that VGA text mode and BIOS are deprecated on newer machines, while UEFI uses pixel buffers. Display handling depends on the boot environment and interface you choose.
Use an existing bootloader or write your own?
If your goal is to learn kernel development, start with an existing bootloader. OSDev’s Bare Bones tutorial deliberately uses existing technology so learners can work on a kernel without first developing a bootloader, compiler, or programming language. The tutorial describes a 32-bit x86 route involving GRUB/Multiboot-style boot technology.
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Write a bootloader first only if the loader itself is your learning goal. Treat that as a separate milestone: firmware startup, loader behavior, and the kernel’s entry conditions are related, but they are not the same implementation problem.
Choose an assembler and a target-aware toolchain
Assembly is only one part of producing a bootable kernel image. An assembler translates instructions into object code; a linker combines and lays out object files. You also need to meet the selected boot protocol’s image and entry requirements.
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For its 32-bit x86 path, OSDev Bare Bones names GNU Assembler or NASM, GNU Linker, and GCC. The exact syntax, object format, linker layout, and compiler target depend on your chosen route. A compiler configured to generate ordinary Linux programs is not automatically suitable for a different, freestanding OS kernel. Use an OS-specific cross-compiler or otherwise ensure the compiler targets your intended kernel rather than Linux.
OSDev’s Getting Started guide covers prerequisites and development-environment concepts, and points to Limine Bare Bones for a 64-bit first kernel. The available documentation names different routes; choose one tutorial and follow its current requirements rather than mixing instructions.
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A practical sequence to reach a first kernel milestone
- Set the target. Decide that this first effort is for x86, and specify the intended mode and boot route. The cited starter material covers x86; it does not establish a portable recipe for ARM or RISC-V.
- Learn the required assembly and build basics. Understand the target CPU’s instruction syntax and machine model, plus the assembler, object format, and linker used by your selected tutorial.
- Pick one handoff protocol. For a first kernel, use an existing bootloader if you want to focus on kernel work. If writing a loader is the goal, make it its own initial project.
- Configure a suitable compiler target. Ensure any compiler-generated code is for your intended freestanding kernel, not an ordinary Linux executable.
- Write a minimal entry stub and kernel. Observe the exact entry conditions required by the chosen boot protocol. Aim for one observable milestone, such as a message through an output method supported by the environment, or a deliberate halt. Do not copy a boot header or calling convention from a different route.
- Boot it in an emulator. Iterate under QEMU before trying physical hardware. For UEFI, OSDev documents a QEMU setup using OVMF firmware in its UEFI guide.
- Add one subsystem at a time. Once the initial handoff works, tackle interrupts, memory management, drivers, storage, and user programs as separate learning and testing steps.
Test the real path, not just a shortcut
QEMU can help you test repeatedly without relying on a physical machine. For UEFI, pair QEMU with OVMF as described by OSDev. A separate QEMU direct Linux boot feature is a convenience path for Linux kernels; successfully using that shortcut does not show that your custom bootloader works. To validate your loader, boot through the firmware and loader path you intend to support.
Keep the test target narrow: confirm the selected loader reaches your kernel under the expected entry conditions, then add features incrementally. A successful boot milestone validates only that path and milestone, not a general-purpose operating system.
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Learning resources for the first steps
- OSDev Wiki: Bare Bones — a concrete 32-bit x86 kernel introduction using existing boot technology and naming GNU Assembler or NASM, GNU Linker, and GCC.
- OSDev Wiki: Getting Started — prerequisites, assembler examples, a GNU/Linux development environment, and a pointer to Limine Bare Bones for a 64-bit first kernel. Check the guide’s current links and version-specific requirements.
- OSDev Wiki: UEFI — BIOS/UEFI context and a QEMU/OVMF testing path.
- OSDev Wiki: System Initialization (x86) — an overview of the x86 startup sequence.
- OSDev Wiki: Tutorials — a directory of projects including MikeOS, a real-mode x86 assembly project. The directory marks some material as dated, so check a tutorial’s currency before relying on it.
These introductions help choose a direction, but they do not remove the need to verify exact assembler syntax, linker settings, boot-protocol requirements, and architecture-specific setup against the documentation for your chosen target.
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