Something rather unexpected happened over the weekend: disk images of the near-mythical multitasking DOS 4 suddenly popped up.
This is “MS-DOS Version 4.00”—from 1985. It looks almost exactly like MS-DOS 3.0, with COMMAND.COM, FORMAT, SYS, FDISK, JOIN, SUBST, ATTRIB, and so on.
The big difference is that this version of DOS includes an optional “session manager” (SM.EXE) which allows starting additional programs and switching between them:
The files are from late November 1985; it is unknown if and how different this version is from the actual multitasking DOS 4.0 release shipped to OEMs. The package contains both classic MZ-format executables as well as segmented NE-format modules more similar to Windows and OS/2. The session manager and hard-error pop-up are rather reminiscent of OS/2, while the .PIF files resemble Windows. The NE format was of course shared by all three.
A deeper dive into multitasking DOS 4.0 will follow sometime soon…