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Microsoft GW-BASIC Interpreter Source Code

(GitHub pipeline status) · (GitLab pipeline status)

This repo contains the original source-code for Microsoft's GW-BASIC interpreter, as of 1983, adjusted for assembling with JWasm or available versions of the Microsoft Macro Assembler, plus additional "OEM" modules needed to complete the interpreter.

There is also a still-experimental subproject (GWBASICA.EXE) to re-introduce features to GW-BASIC which were only present in post-1983 BASIC interpreters.

Announcement blog (from Microsoft)



All files can now be assembled with Microsoft MASM 5.1A. This is the version that currently seems to match the code most closely.

It is now also possible to cross-assemble the source files ― with some preprocessing ― using JWasm and JWlink.

Implementation ― GWBASIC.EXE

The interpreter is semi-working, but some parts of the platform-specific support code are still missing or incomplete.

Specifically, Diomidis Spinellis had observed that several OEM-specific functions were missing from the original source code release, and have to be added. Most of these have been implemented in the new modules OEM.ASM, OEMEV.ASM, and OEMSND.ASM, mainly from scratch.


  • Some routines are still missing, and are currently stubs: INICOM, RECCOM, SNDCOM, STACOM, TRMCOM. These are meant to implement serial port I/O.
  • Some routines need testing with the appropriate hardware: POLLEV, RDPEN, RDSTIK, RDTRIG, SNDLPT. These currently implement general event polling, light pen input, joystick input, and printer output.

(Many of the needed OEM routines, such as SETC and MAPXYC, turn out to be present in BASICA.COM from Microsoft's earlier MS-DOS v1.25 code release. However, BASICA.COM is only released in binary form, so some analysis is needed to extract the routines.)

Implementation ― GWBASICA.EXE

GWBASICA adds the following post-1983 features:

  • The TIMER function, for getting the time of the day in floating-point form.
    • The semantics of the RANDOMIZE statement have also been altered, so that the command RANDOMIZE TIMER works (mostly) as expected.
  • The MKDIR, CHDIR, and RMDIR statements.
    • However, other file-oriented commands, such as OPEN and LOAD, still do not work with subdirectory paths. These should be fixed in due course.

Building instructions

With JWasm and JWlink

You need a system with

  • AWK and GNU Make, and
  • either
    • installed binaries for JWasm and JWlink, or
    • Git and GCC, for downloading and building JWasm and JWlink.

Run make. This should build two executable files, GWBASIC.EXE and GWBASICA.EXE.

With MASM 5.10A

Using DOSBox mount a directory containing:

  • This code
  • The Microsoft Macro Assembler (MASM) version 5.1A (MASM.EXE).
  • The Microsoft MAKE and LINK programs that come with MASM (MAKE.EXE, LINK.EXE).

Run make Makefile.mas to assemble the files. This currently just builds GWBASIC.EXE. Note the tools may leave behind partly-built executables or object files. If you want to rebuild them without changing the source code, you need to delete these files by hand.

You can fetch MASM 5.1A from this site as follows.

  • From the pull-down menu select MS Macro Assembler 5.10A (Update)
  • Press the Load button to load the disk image into the emulator
  • Press the Save button to save the disk image to your computer
  • Copy the saved disk image to a Linux computer
  • Mount the image using the command sudo mount MASM51A-UPDATE.img /mnt
  • Copy the files from /mnt to your development directory

You can fetch MAKE.EXE and LINK.EXE from the same site, under MS Macro Assembler 5.00 (Disk 1).


All files within this repo are released under the MIT (OSI) License as per the LICENSE file stored in the root of this repo.


Pull requests addressing problems in getting GW-BASIC to build and run are welcomed.

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact with any additional questions or comments.