Microsoft, IBM release source code of 1988's MS-DOS 4.00

Microsoft, IBM release source code of 1988's MS-DOS 4.00
27 Apr 2024

Microsoft and IBM have publicly released the source code of the infamous 1988 MS-DOS 4.00.

This version, notorious for its bugs and complex development history, is now accessible on Microsoft's MS-DOS GitHub page.

It joins versions 1.25 and 2.0 which were made public in a similar fashion in 2014, through a partnership with the Computer History Museum.

All open-source DOS versions have been released under the MIT License.

MS-DOS 4.00 was supposed to be a multitasking OS
Historical context

The original plan for MS-DOS 4.00 was to introduce new multitasking features, enabling software to operate in the background.

This version, also known as "MT-DOS" or "Multitasking MS-DOS," was distributed exclusively through select European PC manufacturers, and never as a standalone retail product.

However, the source code released this week does not pertain to this multitasking version of DOS 4.00.

Microsoft and IBM have shared the source code for DOS
Lost code

Microsoft's Open Source Programs Office attempted to locate the complete source code for MT-DOS, but their efforts were unsuccessful.

Instead, Microsoft and IBM have shared the source code for a distinct version of DOS 4.00, mainly developed by IBM.

This version was designed to enhance the existing non-multitasking version of DOS, that operated on most IBM PCs as well as PC clones at that time.

MS-DOS 4.00 was a memory hog
Memory consumption

The publicly accessible version of MS-DOS 4.00 is more recognized for its high memory usage than its new features.

It could consume up to 92KB of RAM, a significant increase from the approximately 56KB used by MS-DOS 3.31.

Subsequent releases like MS-DOS 5.0 and 6.0 peaked at around 72KB or 73KB, while even IBM's PC DOS 2000 only required around 64KB.

Microsoft acknowledges researcher for his role in discovery
Code discovery

Microsoft has shared some documentation, as well as binary files for MT-DOS and may update this release if more is discovered.

The company acknowledges English researcher Connor "Starfrost" Hyde, for his role in uncovering this source code.

Hyde's ongoing study of MT-DOS, which he is documenting on his website, played a crucial part in the discovery of the MS-DOS 4.00 source code.