How to open a .asm file (Windows, Mac, Linux)

Q: Why can't I open my .asm file?

Usually your computer lacks an app that understands Assembly. Install one of the apps listed here, or convert the file to a more common format.

A .asm file is a Assembly Language Source File. If double-clicking does nothing, your computer simply does not have an app that reads Assembly yet. Here is how to fix that on each operating system, using free software wherever possible.

Open a .asm file step by step

Confirm the file really is a .asm. Drop it into the WhatFileType file identifier, which reads its signature without uploading anything.
On Windows, open it with VS Code or NASM/MASM (to assemble).
On Mac, use VS Code or clang/as (to assemble).
On Linux, try VS Code or nasm/as (to assemble).
Still stuck? Convert the .asm to o (object file), which opens more widely.

Best apps to open .asm files

Windows

VS Code
NASM/MASM (to assemble)
any text editor

macOS

VS Code
clang/as (to assemble)
any text editor

Linux

VS Code
nasm/as (to assemble)
any text editor

Before you open it: Caution. An .asm file is source code; the text itself is harmless, but assembling and running it executes low-level code. Review before building.

Frequently asked questions

Why can't I open my .asm file?

Almost always because no installed app understands Assembly. Install one of the apps above, or convert the file to a format your existing software already reads.

How do I open an .asm file?

Any text editor opens it, since assembly is plain text. To turn it into a program you assemble it with a tool such as NASM, MASM or GNU as.

What is the difference between .asm and .s?

Both hold assembly source. .asm is common on Windows and with NASM/MASM; .s is the Unix convention, and a capital .S usually means it runs through the C preprocessor first.