Multitasking MS-DOS 4

Not to be confused with the IBM-produced MS-DOS 4.
Multitasking MS-DOS 4
Version of MS-DOS
Multitasking MS-DOS 4 First Boot.png
Version4.00
Release date1986
Replaces
MS-DOS 3

Multitasking MS-DOS 4 (sometimes also referred to as European MS-DOS 4) refers to a dead-end branch of MS-DOS that included support for multitasking, which was only offered by certain European OEMs for the most part. Unlike its predecessors and even contemporary versions of Microsoft Windows, it implements proper preemptive multitasking with a time-sliced scheduler. Documents indicate many features, such as threads, were also planned but removed during development.

The project was intended to be the successor to MS-DOS 3, although it does not share much amount of code with it. Most executables are New Executable (NE) files (excluding the DOS BIOS), with a large amount of the operating system being rewritten in C, including the entire Command Interpreter (COMMAND.COM) and a large percentage of the DOS BIOS. The default SHELL entry in CONFIG.SYS points to the Session Manager (SM.EXE), which is a small shell application that provides a UI for application switching. The real shell is provided as an argument to the Session Manager.

Most components of this version of DOS were compiled on 26 November 1985, shortly after the completion of Windows 1.0. Internally, it identifies as "internal revision 6.7" a la OS/2, although the string has been patched to not display, which indicates that this may be a pre-release that was shoved out to OEMs. Goupil released an OEM version of Multitasking MS-DOS 4.0 in May 1986, and is mostly the same as the non-OEM version. An updated version, MS-DOS 4.10, was developed for the ICL DRS Professional Workstation and is known to exist in private collections.[1][2]

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