Microsoft and IBM Release MS-DOS 4.0’s Source Code under the MIT License | Windows

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Microsoft and IBM have open-sourced on git the 1988 operating system MS-DOS 4.0 under the MIT License. In addition to the source code for MS-DOS 4, the public git repository contains unreleased beta Multitasking DOS binaries, the source, and the scanned PDFs of the Multitasking MS-DOS 4.0 (MT-DOS) documentation.

MS-DOS 4.0 was notable for its support of FAT16 hard disk partitions greater than 32 MB and the addition of the MS-DOS Shell. MS-DOS 4.0 however was originally supposed to include multi-tasking capabilities. As its product specification mentions:

“MS-DOS 4.0 is a multitasking operating system, developed from and downwardly compatible with MS-DOS 3.0. It supports true multitasking as well as multiple current screen image facility which gives the user the illusion of and benefits from many independent computers. Further, MS-DOS 4.0 allows most existing MS-DOS 2.0 applications to run without changing the MS-DOS 4.0 multitasking environment.”

To ease the transition from 8086/8088 line of processors to the (then) new 286 processor without disrupting the installed base, Multitasking MS-DOS targeted two-way compatibility:

Microsoft resolves this…


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