Arm is not open source, but parts of its instruction set are publicly documented

Arm is a company that designs processor instruction sets — the basic commands that tell a chip what to do. Arm does not make chips themselves. Instead, they license their designs to companies like Apple, Qualcomm, and Samsung, who then manufacture the actual processors you find in phones, tablets, and computers.

Arm's business model is licensing, not open source. You cannot freely read Arm's full design files, modify them, and redistribute them the way you can with truly open source projects like Linux. However, Arm does publish parts of its instruction set architecture publicly, which creates confusion about whether it counts as "open source."

The distinction matters because it affects what you can actually do with Arm technology. If you are a chip manufacturer, you pay Arm for a license. If you are a software developer, you can write code that runs on Arm processors without paying Arm directly — but the chip maker who licensed from Arm has already paid.

Key Takeaways

  • Arm licenses its processor designs to manufacturers for a fee; it does not release designs under an open source license.
  • Arm publishes its instruction set architecture (the commands processors understand) publicly so software developers can write code for Arm chips.
  • Publishing the instruction set is not the same as open source — you cannot modify Arm's core designs or build competing processors without a license.
  • Software written for Arm processors is often open source, but the Arm architecture itself is proprietary.

What Arm publishes and what it keeps private

Arm publishes its instruction set architecture — the reference manual that describes what commands an Arm processor can execute. This document is freely available and tells programmers exactly how to write code that will run on any Arm chip. This is why Linux, Windows, and countless other operating systems can run on Arm processors from different manufacturers.

What Arm does not publish is the actual design files — the detailed blueprints, circuit layouts, and implementation details that a manufacturer needs to build a physical chip. Those files are proprietary and available only to companies that have paid Arm for a license. The license agreement typically includes restrictions on what the manufacturer can do: they cannot share the designs with competitors, they cannot use them to build chips for certain markets, and they cannot modify the core architecture without Arm's permission.

This is why you see companies like Qualcomm and Apple paying Arm billions of dollars in licensing fees over the years. They are paying for access to those private design files and the right to manufacture chips based on them.

Why publishing the instruction set is not the same as open source

Open source means you can see the source code, modify it, and redistribute your changes — usually under a license like GPL or MIT that protects those rights. Arm publishes documentation, not source code. The instruction set manual tells you what a processor can do, but it does not give you the tools or permission to build your own competing processor.

Think of it like publishing a recipe versus publishing the ingredients. Arm publishes the recipe (the instruction set) so anyone can cook with it. But Arm does not publish how to grow the ingredients or build the kitchen (the actual chip design). Only licensed manufacturers get that information.

Some Arm-related projects are genuinely open source — for example, RISC-V is an open source instruction set architecture that anyone can use to design processors without paying licensing fees. But Arm itself remains proprietary, even though the instruction set is public.

What this means for software developers

If you write software, you do not need an Arm license. You can write code that runs on Arm processors using free tools like GCC or LLVM, and you can publish that code under any license you choose — open source or proprietary. The instruction set documentation is public, so you have everything you need to understand how your code will execute.

Many open source projects run on Arm processors. Linux, Apache, Python, and thousands of other projects work on Arm chips without paying Arm anything. The software is open source; the underlying processor architecture is not.

Where you might encounter Arm's licensing restrictions is if you are developing firmware or bootloader code that needs to interact directly with Arm's proprietary extensions or security features. Some of those features are only documented to licensed manufacturers, not to the general public.

What this means for chip manufacturers

If you want to design and manufacture an Arm-based processor, you must license from Arm. The license fee varies depending on the scope — whether you are building a single chip design or a family of products, whether you need support for Arm's proprietary security features, and how many chips you plan to sell.

Once you have a license, you can modify Arm's designs to some extent — adding your own features, optimizing for your own use case, or integrating Arm cores with your own custom logic. But you cannot strip away the Arm branding, sell the design to a competitor, or use it in ways that violate your license agreement.

This is why companies like Apple and Qualcomm have different processor designs even though they both use Arm cores. They each licensed Arm's architecture and then customized it for their own products.

How Arm's model compares to truly open source alternatives

RISC-V is an open source instruction set architecture. Anyone can design a processor based on RISC-V without paying licensing fees or asking permission. The designs, documentation, and tools are all freely available. This makes RISC-V attractive for research, education, and companies that want to avoid licensing costs.

The trade-off is that RISC-V is newer and has far fewer commercial chips available compared to Arm. Most phones, tablets, and embedded devices use Arm because Arm has been around longer and has more mature tools, libraries, and support.

Arm's licensing model has allowed the company to invest heavily in architecture improvements and to maintain tight control over quality and compatibility. RISC-V's open approach makes it easier to experiment and innovate without permission, but it also means less centralized coordination.

Why the confusion exists

The confusion around Arm and open source comes from mixing two different things: the instruction set (which is public) and the implementation (which is proprietary). When someone says "Arm is open source," they usually mean the instruction set is documented publicly. When someone says "Arm is proprietary," they mean you cannot freely build Arm chips without a license.

Both statements are true in different contexts. For a software developer, Arm's public instruction set feels open. For a chip manufacturer, Arm's licensing requirements feel proprietary. The reality is that Arm sits in the middle — more open than some proprietary architectures, but far less open than RISC-V or other truly open source projects.

Frequently Asked Questions

Can I read Arm's chip designs for free?

No. Arm publishes the instruction set architecture (the reference manual), but not the actual design files. Those files are proprietary and available only to companies that have paid Arm for a license. The instruction set documentation is free and public.

If I write software for Arm processors, do I have to pay Arm?

No. Software developers do not pay Arm licensing fees. You can write code for Arm processors using free tools and publish it under any license you choose. The chip manufacturer who licensed from Arm has already paid for the right to make the processor.

Is Linux on Arm open source?

Yes. Linux itself is open source regardless of what processor it runs on. The Arm architecture that Linux runs on is proprietary, but the Linux operating system and its source code are freely available under the GPL license.

What is the difference between Arm and RISC-V?

Both are instruction set architectures, but Arm requires a paid license to manufacture chips, while RISC-V is open source and free to use. RISC-V is newer with fewer commercial chips available. Arm has more mature tools and wider industry support.

Can I modify Arm's designs if I have a license?

You can modify Arm's designs to some extent within the terms of your license agreement, but you cannot redistribute them or use them in ways that violate the license. The exact restrictions depend on what type of license you have purchased.