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MS-DOS 4.0: A Tale of Two Versions and a Legacy Unearthed

MS-DOS 4.0, released in 1988, holds a unique place in personal computer history. It wasn't a single, definitive release,

MS-DOS 4.0, released in 1988, holds a unique place in personal computer history. It wasn’t a single, definitive release, but rather a story of two distinct versions with a surprising twist.

The Multitasking Dream (Unreleased):

Microsoft originally envisioned MS-DOS 4.0 as a multitasking operating system, a significant leap forward from the single-tasking MS-DOS 3.3. This version aimed to allow users to run multiple programs concurrently, a feature highly sought after as computers became more powerful.

However, lack of interest from key partners, particularly IBM, led to this version being shelved.

The Unexpected Release (MS-DOS 4.01 and Later):

Meanwhile, IBM released its own PC DOS 4.0, which was essentially an enhanced version of DOS 3.3. Microsoft, in a surprising move, countered with its own MS-DOS 4.0, but this version lacked the ambitious multitasking features of the original concept. This MS-DOS 4.0 (and subsequent versions like 4.01) focused on improvements like expanded memory management and support for larger hard drives.

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A Legacy Unearthed:

The story takes another turn in 2024. Microsoft, in collaboration with IBM, released the source code for the original MS-DOS 4.0 (the unreleased multitasking version) under an open-source license. This unexpected move allows enthusiasts and historians to delve into the code and understand the evolution of MS-DOS.

The Significance of MS-DOS 4.0:

While the commercially available MS-DOS 4.0 wasn’t a revolutionary product, the story behind it highlights several crucial aspects of personal computing history:

  • The Rise of Multitasking: MS-DOS 4.0’s original concept foreshadowed the importance of multitasking in future operating systems like Windows.
  • The Influence of Partnerships: The collaboration and competition between Microsoft and IBM significantly shaped the early PC landscape.
  • Open Source and Historical Preservation: Releasing the source code allows for greater understanding and preservation of this important technology.

MS-DOS 4.0 Release of Source Code

A significant development occurred in 2024 when Microsoft and IBM released the source code for MS-DOS 4.0 on GitHub.

How the Code Came to Light

These codes came to light when researcher Connor Hyde (aka Starfrost) was documenting the relationship between DOS 4, MT-DOS (Multitasking DOS), and OS/2. He contacted Ray Ozzie, the former Microsoft Chief Technical Officer, who found the code in his collection of floppy disks. These disks, believed to be from 1984, contained the unreleased beta Multitasking DOS binaries and source code for the “” file.

Open-Sourcing the Code

Connor Hyde then contacted the Microsoft Open Source Programs Office (OSPO) to inquire about releasing the DOS 4 source code. Scott Hanselman, Microsoft’s VP for developers, along with Archivist Jeff Sponaugle, took the initiative to image the disks and scan the documentation.

While the OSPO team couldn’t find the full MT-DOS source code, they did find the MS-DOS source code, which they uploaded to GitHub under the MIT License.

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Enijes Shadap
Enijes Shadap
Enijes Shadap, an emerging professional content writer with an entrepreneur's enthusiasm, aims high in his goals.

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