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Microsoft Open-Sources MS-DOS 4.0 Source Code
Microsoft has recently made headlines by open-sourcing yet another piece of computing history. In collaboration with IBM, the company has released the source code of MS-DOS 4.00, a version that is widely recognized for its infamy rather than its success as a computer operating system. This move adds to Microsoft’s ongoing efforts to preserve and share seminal software.
Background and Context
MS-DOS 4.00, released in 1988, was initially envisioned to introduce new multitasking capabilities that would allow software to run concurrently in the background. Dubbed “MT-DOS” or “Multitasking MS-DOS,” this version never gained widespread acceptance and was only distributed through select European PC original equipment manufacturers (OEMs). Despite the ambitious objectives, the multitasking features of MS-DOS 4.00 never materialized as intended.
While the multitasking version of DOS 4.00, known as MT-DOS, remains elusive, Microsoft and IBM have instead shared the source code of a distinct iteration of DOS 4.00. This version, primarily developed by IBM, aimed to enhance the existing non-multitasking variant that was prevalent on IBM PCs and PC clones during that era.
It is worth noting that Microsoft diverged from the multitasking DOS concept in subsequent releases. Instead, multitasking capabilities found their way into graphical operating systems like Windows and OS/2. Meanwhile, MS-DOS 5.x and 6.x continued the tradition of single-application operation.
Technical Insights
The released version of MS-DOS 4.00, while lacking the multitasking functionalities, is recognized for its high memory usage. Compared to its predecessor, MS-DOS 3.31, which consumed approximately 56KB of RAM, MS-DOS 4.00 could eat up as much as 92KB. Subsequently, release 4.01 managed to reduce this figure to around 86KB. In contrast, MS-DOS 5.0 and 6.0 capped memory requirements at 72 or 73KB, with IBM’s PC DOS 2000 requesting about 64KB.
During a time when memory was a precious resource, these differences were significant. With systems limited to 640KB of memory and lacking virtual memory capabilities, the spike in memory usage presented challenges. Today, enthusiasts exploring retro-computing generally bypass MS-DOS 4.00, opting for earlier versions due to lower memory consumption or later releases offering enhanced functionality.
Contributions and Future Possibilities
Microsoft’s decision to open-source legacy software aligns with its commitment to fostering technical education and historical preservation. In addition to MS-DOS versions, Microsoft has previously released pioneering software like Word for Windows 1.1a, 1983-era GW-BASIC, and the original Windows File Manager. Notably, the Windows File Manager has received updates to ensure compatibility with modern systems.
This release of the MS-DOS 4.0 source code represents a continuation of the journey for DOS historians. In recent months, discoveries such as early versions of 86-DOS, a precursor to PC operating systems, and the unearthing of abandoned Microsoft-developed OS/2 versions have enriched the archival landscape for enthusiasts and scholars alike.
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