Microsoft has open-sourced one other little bit of computing historical past this week: The corporate teamed up with IBM to launch the supply code of 1988’s MS-DOS 4.00, a model higher identified for its unpopularity, bugginess, and convoluted growth historical past than its utility as a pc working system.
The MS-DOS 4.00 code is accessible on Microsoft’s MS-DOS GitHub web page together with variations 1.25 and a couple of.0, which Microsoft open-sourced in cooperation with the Pc Historical past Museum again in 2014. All open-source variations of DOS have been launched below the MIT License.
Initially, MS-DOS 4.00 was slated to incorporate new multitasking options that enable software program to run within the background. This launch of DOS, additionally generally referred to as “MT-DOS” or “Mutitasking MS-DOS” to tell apart it from different releases, was solely launched via just a few European PC OEMs and by no means as a standalone retail product.
The supply code Microsoft launched this week is not for that multitasking model of DOS 4.00, and Microsoft’s Open Supply Applications Workplace was “unable to search out the complete supply code” for MT-DOS when it went to look. Moderately, Microsoft and IBM have launched the supply code for a completely separate model of DOS 4.00, primarily developed by IBM so as to add extra options to the prevailing non-multitasking model of DOS that ran on most IBM PCs and PC clones of the day.
Microsoft by no means returned to its multitasking DOS thought in subsequent releases. Multitasking would develop into the purview of graphical working techniques like Home windows and OS/2, whereas MS-DOS variations 5.x and 6.x continued with the outdated one-app-at-a-time mannequin of earlier releases.
Microsoft has launched some documentation and binary recordsdata for MT-DOS and “might replace this launch if extra is found.” The corporate credit English researcher Connor “Starfrost” Hyde for shaking all of this supply code free as a part of an ongoing examination of MT-DOS that he’s documenting on his web site. Hyde has posted many screenshots of a 1984-era construct of MT-DOS, together with of the “session supervisor” that it used to trace and swap between working purposes.
The publicly launched model of MS-DOS 4.00 is understood much less for its new options than for its excessive reminiscence utilization; the 4.00 launch may eat as a lot as 92KB of RAM, method up from the roughly 56KB utilized by MS-DOS 3.31, and the 4.01 launch decreased this to about 86KB. The later MS-DOS 5.0 and 6.0 releases maxed out at 72 or 73KB, and even IBM’s PC DOS 2000 solely needed round 64KB.
These RAM numbers could be rounding errors on any fashionable laptop, however within the days when RAM was dear, techniques maxed out at 640KB, and digital reminiscence wasn’t a factor, such an enormous soar in system necessities was an enormous deal. As we speak’s retro-computing fanatics nonetheless are inclined to skip over MS-DOS 4.00, recommending both 3.31 for its decrease reminiscence utilization or later variations for his or her expanded function units.
Microsoft has open-sourced another legacy code through the years, together with these older MS-DOS variations, Phrase for Home windows 1.1a, 1983-era GW-BASIC, and the unique Home windows File Supervisor. Whereas most of those have been launched of their unique varieties with none updates or modifications, the Home windows File Supervisor is definitely actively maintained. It was initially simply modified sufficient to run natively on fashionable 64-bit and Arm PCs working Home windows 10 and 11, however it’s been up to date with new fixes and options as not too long ago as March 2024.
The discharge of the MS-DOS 4.0 code is not the one new factor that DOS historians have gotten their arms on this yr. One of many earliest identified variations of 86-DOS, the software program that Microsoft would purchase and switch into the working system for the unique IBM PC, was found and uploaded to the Web Archive in January. An early model of the deserted Microsoft-developed model of OS/2 was additionally unearthed in March.