An emscripten-compiled version of the dasm macro assembler

Usage no npm install needed!

<script type="module">
  import dasm from 'https://cdn.skypack.dev/dasm';


The dasm macro assembler (for JavaScript)

npm Dependency Status

This is an emscripten-compiled version of the dasm macro assembler.

The dasm macro assembler transforms assembly code into 6502-compatible executable binary code. Since this is a JavaScript port of dasm, it allows that compilation process from JavaScript programs; more especifically, it can be used to create ROMs for Atari VCS 2600 and Fairchild Channel F (and others!) from a string containing dasm-compatible assembly source code.

In other words, it turns something like this:

; Pick the correct processor type
        processor 6502
; Basic includes
        include "vcs.h"
        include "macro.h"
; Start address
        org $f000
; Actual instructions
start   SEI
        LDX  #$FF
        LDA  #$00

...into its equivalent byte code:

f000 78
f001 d8
f002 a2 ff
f004 9a
f005 a9 00

Among other features, dasm sports:

  • fast assembly
  • several binary output formats available
  • expressions using [] for parenthesis
  • complex pseudo ops, repeat loops, macros, etc

On top of that, this JavaScript port also offers:

  • More high-level parsing of symbols and labels' information (including declaration origin, type, etc)
  • A direct library interface for assembling (rather than a command-line interface)

This port of dasm was created so I could have dasm compiling working in vscode-dasm, my Visual Studio Code extension that aims to allow Atari development and debugging from within Visual Studio Code.

Technical information

This package uses version 2.20.11 of dasm. It supports the following processor architectures:

  • 6502 (and 6507)
  • 68705
  • 6803
  • HD6303 (extension of 6803)
  • 68HC11

This specific port was built on Linux (err, Windows 10 bash) from the dasm source using emscripten 1.37.0. Check the dasm folder for the script that was used to compile dasm.js, including its pre/post-JS includes to wrap the code in a module function and return its results in a more usable way.


Install with NPM:

npm install dasm --save

Or with Yarn:

yarn add dasm

Import as a module:

import dasm from "dasm"; // ES6
var dasm = require("dasm").default; // ES5

Finally, convert code to a binary data ROM. Instead of forcing developers to use a command line-like interface, the function that wraps the emscripten module provides a modern interface to dasm:

// Read utf-8 assembly source
const src = "...";

// Run with the source
const result = dasm(src);

// Read the output as a binary (Uint8Array array)
const ROM = result.data;

Advanced usage

Advanced options can be passed to the dasm call via an options parameter. For example:

// Create a rom using the typical Atari VCS 4096-byte format
dasm(src, { format: 3 });

// Just create a rom without exporting symbols or lists
dasm(src, { quick: true });

// Typical assembly to create an Atari VCS ROM with built-in .h includes
dasm(src, { format: 3, quick: true, machine: "atari2600" });

// Pass original command-line parameters
dasm(src, { parameters: "-f3 -p2 -v4 -DVER=5" });

These are all the options currently parsed:

  • format: binary output format. Dictates the size and arrangement of the generated ROM.
    • 1 (default): output includes a 2-byte origin header.
    • 2: random access segment format. Output is made of chuncks that include a 4-byte origin and length header.
    • 3: raw format. Just the data, no headers.
  • quick: boolean. If set to true, don't export any symbol and pass list as part of its returned data. Defaults to false.
  • parameters: string. List of switches passed to dasm as if it was being called from the command line.
  • include: key-value object, or IIncludeInfo[]. This is a list of files that should be made available for the source code to include. If an objct, the key contains the complete file path, and the value contains its content, either as a string or an Uint8Array for binary files. It can also be an array of include file info, as returned by resolveIncludes() (more on that below).
  • machine: target machine as a string. Similarly to dasm's -I switch, this picks a list of (embedded) files to make available to the include command.
    • "atari2600": includes dasm's own atari2600/macro.h and atari2600/vcs.h files.
    • "channel-f": includes dasm's own channel-f/macro.h and channel-f/ves.h files.

For more examples, check the test files on reference implementations of typical compilations.

Also check the dasm documentation for a list of all command-line switches available, for more information on binary formats, and for a list of all macros available.

Returned object

The object returned by the dasm function has more than just a binary ROM. This is what's available:

  • data: Uint8Array. The exported ROM, as a list of integers.
  • output: string[]. All data written by dasm to stdout.
  • list: ILine[]. A list of all lines available in the source code, and their parsed info (address, bytecode, comments, command, etc).
  • listRaw: string. The raw output of the list file (equivalent to the -L switch).
  • symbols: ISymbol[]. A parsed list of all symbols (labels and constants) defined by the source code.
  • symbolsRaw: string. The raw output of the symbols file (equivalent to the -s switch).
  • exitStatus: number. A code indicating the exit status of the assembler module. Normally 0 if exited without problems, or 1 if aborted prematurely.
  • success: boolean. true if successfully compiled, false if otherwise.

Of specially note are the list and symbols objects. Those include parsed information about the source code, including line-specific error messages.

Convenience functions

For convenience, this library also exposes one additional function that can be useful when processing assembly sources, resolveIncludes. This function parses all include, incbin and incdir pseudo-ops from the source, and resolves them to their respective file URIs (in POSIX format) and content. Use it like so:

import { resolveIncludes } from "./../lib/index";

const includes = resolveIncludes(source, getFile = undefined, baseFolder = "");

These are its parameters:

  • source: string containing assembly source code. The same source that is passed to the dasm() call.

  • getFile: a function that takes two parameters: sourceEntryRelativeUri (relative location of a file as string) and isBinary (whether it's a binary include, as boolean) and returns contents of that file (as either a string for text files, an Uint8Array for binary files, or undefined for files that were not found).

    This function is optional and should be used as a convenience function to allow resolveIncludes to parse file contents and children includes (includes of includes). It ommitted, resolveIncludes returns a list of includes for the original source file without their contents (but with their best guess at the file's actual uri).

    When using dasm inside node, a typical implementation of getFile that just gets file contents from the file system (fs) is as such:

    function bufferToArray(b) {
        const arr = new Uint8Array(b.length);
        return arr.map((v, i) => b[i]);
    function getFile(sourceEntryRelativeUri, isBinary) {
        const fullUri = path(__dirname, sourceEntryRelativeUri);
        if (fs.existsSync(fullUri)) {
            if (isBinary) {
                return bufferToArray(fs.readFileSync(fullUri));
            } else {
                return fs.readFileSync(fullUri, "utf8");

    It's important to check for a file existence because getFile might get called with uris that do not exist. This inevitable if the incdir pseudo-op is used in the code.

  • baseFolder: uri string to be used when generating possible include file uris.

  • recursive: boolean on whether the resolution should be recursive or not. The default is true, in which case every included file will also be parsed for includes of its own.

This function returns an array of IIncludeInfo, each containing:

  • line: number. Line in the source file where this resource filename is.
  • column: number. Column in the source file where this resource filename is.
  • entryRelativeUri: string. The uri of this include, relative to the original entry source (and its baseFolder, if any).
  • parentRelativeUri: string. The uri actually used when including this file.
  • isBinary: boolean. Whether it's a binary file (used with incbin) or a text file (used with include).
  • includes: IIncludeInfo[]. Child dependencies of this file.
  • contents: string|Uint8Array|undefined. Contents of this file: a string if text, Uint8Array if binary, or undefined if not found or if getFile was not passed.

More information

TypeScript definitions are included with this distribution, so TypeScript projects can use the module and get type checking and completion for all dasm calls. Non-TypeScript JavaScript developers using Visual Studio Code will also benefit from auto-completion without any change thanks to VSC's Automatic Type Acquisition.

TypeScript projects also have the benefit of being able to import the interfaces themselves, if needed:

// dasm() options
import { IOptions } from "dasm";

// dasm() return object
import { IDasmResult } from "dasm";

// Other interfaces used by the IDasmResult object
import { ISymbol, ILine } from "dasm";

// The include info returned by resolveIncludes(), or passed to dasm() for inclusion
import { IIncludeInfo } from "dasm";

Todo and ideas

  • Allow asynchronous assembly (run as a worker?)
  • Allow asynchronous resolveIncludes?
  • Command-line package? (dasm-cli)
    • Allow direct FS use?

Contributions are welcome.


Check the release list for a list of what has changed in every new version.


The dasm macro assembler was created by by Matthew Dillon. It was further augmented by Olaf "Rhialto" Seibert, Andrew Davie, and Peter H. Froehlich.


This follows dasm itself and uses the GNU Public License v2.0.