MS-DOS 4.00 source code is now published on GitHub under MIT license. There is quite a complex and fascinating history behind the DOS 4.0 versions, as Microsoft collaborated with IBM on some of the code, but also created a branch of DOS called Multitasking DOS , which was not widely adopted.
Ten years ago, Microsoft provided the source code for MS-DOS 1.25 and MS-DOS 2.0 to the Computer History Museum in California, and later republished it for reference purposes in the public domain on GitHub. This code holds a significant place in history and provides fascinating insight into an operating system that was written entirely in assembly language for the Intel 8086 nearly 45 years ago.
The release of MS-DOS 4.0 is notable for the ability to use a graphical interface and mouse, support for disk partitions larger than 32 MB (up to 2 GB), the addition of the DOSSHELL file manager, support for EMS (Expanded Memory Specification), FASTOPEN and FASTSEEK commands.
In the Microsoft Open Source blog, the post "Open sourcing MS-DOS 4.0" reveals that a researcher made the open-source release of this MS-DOS version possible by contacting Microsoft about discovering unreleased beta binaries of DOS 4.0 on old floppy disks from their time at Lotus. After imaging and scanning the disks, it was revealed that they contained earlier versions of the operating system.
According to Microsoft, the release code successfully runs on IBM PC XT, a more modern Pentium, and within open source PCem and 86box emulators. The GitHub repo is here.
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