MS-DOS v1.25, v2.0, v4.0 Source Code

This repo contains the original source-code and compiled binaries for MS-DOS v1.25 and MS-DOS v2.0, plus the source-code for MS-DOS v4.00 jointly developed by IBM and Microsoft.

The MS-DOS v1.25 and v2.0 files were originally shared at the Computer History Museum on March 25th, 2014 and are being (re)published in this repo to make them easier to find, reference-to in external writing and works, and to allow exploration and experimentation for those interested in early PC Operating Systems.

About this fork

This fork adds a working build system and CI pipeline for the MS-DOS 4.0 source code. It builds the original 8086 assembly and C sources into a complete, bootable operating system -- both the OEM MS-DOS and IBM PC-DOS flavors.

The build produces ready-to-use disk images (64MB hard disk and period-correct floppies from 360KB to 1.44MB) that boot in QEMU, VirtualBox, bochs, dosemu, 86Box, PCem, or on real vintage hardware. Pre-built images are available on the Releases page.

This work benefits greatly from prior work done by others to get the MS-DOS 4.0 source code building by cleaning up some minor issues in the released source tree (wrong file paths and mangled character encodings), including, but not limited to, ecm and hharte.

Try it

Download msdos4.img from the latest release and boot it:

qemu-system-i386 -hda msdos4.img

Or with DOSBox, start DOSBox as normal and then boot the image (omit -l C if using a floppy image):

BOOT msdos4.img -l C

Or with dosemu (download msdos4-dosemu.img, which has a special header for dosemu2):

dosemu -f <(echo '$_hdimage = "msdos4-dosemu.img"')

License

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

For historical reference

NOTE: This section is preserved from Microsoft's original README.md accompanying the source code release. The build scripts and tooling in this repository are maintained separately from the historical source code. Pull requests for improvements to the build system are welcome.

The source files in this repo are for historical reference and will be kept static, so please don’t send Pull Requests suggesting any modifications to the source files, but feel free to fork this repo and experiment 😊.

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

Building MS-DOS 4.0

Pre-built disk images are available on the Releases page. To build from source:

Requirements

• dosemu2
• mtools
• mkfatimage16 (from dosemu2)

Important

comcom64 0.4-0~202602051302 or later is required. comcom64 is the command.com replacement used by dosemu2. Some earlier versions have a bug in the COPY command that breaks file concatenation (copy /b a+b dest) when invoked via COMMAND /C, which nmake uses to execute makefile commands. This causes the IO.SYS build to fail silently.

Check your version with dpkg -s comcom64 | grep Version (Debian/Ubuntu) or check your package manager. If you're on an older version:

• Recommended: Update comcom64 to 0.4-0~202602051302 or later
• Workaround: Build comcom64 from source: git clone https://github.com/dosemu2/comcom64 && cd comcom64 && make && sudo make install (ensure make install installs over top of your existing command.efi)

Quick Start

cd v4.0
./mak.sh              # Build DOS 4
./mkhdimg.sh          # Create 64MB hard disk image
./mkhdimg.sh --floppy # Create 1.44MB boot floppy

Build Flavors

The source code supports multiple build configurations. Use the --flavor flag:

./mak.sh                    # Build MS-DOS (default)
./mak.sh --flavor=pcdos     # Build IBM PC-DOS

Flavor	System Files	Description
msdos	IO.SYS, MSDOS.SYS	OEM MS-DOS for IBM-compatible PCs (default, recommended)
pcdos	IBMBIO.COM, IBMDOS.COM	IBM PC-DOS (for historical accuracy)

Both flavors include IBM PC hardware-specific code (INT 10H video BIOS, 8259 PIC, PCjr ROM cartridge support).

Important: The msdos flavor contains more bug fixes than pcdos. Presumably, Microsoft could push fixes to OEM MS-DOS faster than IBM's approval process allowed for PC-DOS. It's known that IBM essentially took over DOS development temporarily circa DOS 3.3 through DOS 4.0, so one possible theory is that PC-DOS was essentially considered frozen when the code was handed back to Microsoft, who found a number of bugs that they fixed in OEM MS-DOS without touching PC-DOS.

Notable differences:

• DOS kernel INT 24 (critical error) handling fix
• FDISK integer overflow protection for large disks
• Larger EMS buffers in FASTOPEN
• Better input validation in EXE2BIN

The default msdos flavor is what OEMs like Compaq, Dell, and HP shipped as "MS-DOS" on their IBM-compatible PCs. This source release actually appears to derive from the OAK (OEM Adaptation Kit) -- the code Microsoft provided to OEMs to allow them to customize MS-DOS for their hardware.

Disk Image Options

# Hard disk images (produces msdos4.img + msdos4-dosemu.img)
./mkhdimg.sh                    # 64MB FAT16 image
./mkhdimg.sh --size 32          # 32MB image

# Floppy images (all sizes)
./mkhdimg.sh --floppy           # 1.44MB minimal (system files only)
./mkhdimg.sh --floppy=360 --floppy-full   # 360KB with utilities
./mkhdimg.sh --floppy=720 --floppy-full   # 720KB with utilities
./mkhdimg.sh --floppy=1200 --floppy-full  # 1.2MB with utilities
./mkhdimg.sh --floppy=1440 --floppy-full  # 1.44MB with utilities

Hard disk images are produced in two formats: msdos4.img is a raw disk image that works with QEMU, VirtualBox, bochs, and most emulators. msdos4-dosemu.img is the dosemu hdimage format (128-byte header). Floppy images are raw and work everywhere.

Known Limitations

• DOS Shell not included: The DOS Shell (DOSSHELL) source code was not open-sourced. SELECT.EXE (the installer) shares some code with DOSSHELL and cannot be built.
• PC-DOS branding incomplete: The --flavor=pcdos build uses IBM system file names (IBMBIO.COM, IBMDOS.COM) and some PC-DOS-specific code paths, but still displays "MS-DOS" in VER and the startup banner. This is because IBM's message file (usa-ibm.msg or similar) was not open-sourced—only the Microsoft-branded usa-ms.msg was released.
• Non-IBM-compatible build: A third configuration (IBMVER=FALSE) exists in the source for non-IBM-compatible hardware, but it doesn't build successfully. It appears that this configuration is for some of the short-lived non-IBM-compatible x86 PCs from the early 1980s -- it specifically dodges certain IBM-compatible-specific code paths dealing with hardware like the BIOS, PIC, PIT, and so on. Further investigation is necessarily, but it's likely that this source tree is incomplete and missing the hardware-specific code (IO.SYS stuff) that would need to be provided to implement DOS services on such hardware.

Trademarks

This project may contain trademarks or logos for projects, products, or services. Authorized use of Microsoft trademarks or logos is subject to and must follow Microsoft's Trademark & Brand Guidelines. Use of Microsoft trademarks or logos in modified versions of this project must not cause confusion or imply Microsoft sponsorship. Any use of third-party trademarks or logos are subject to those third-party's policies.