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PC-MOS-386 boot screen.jpg
A screen-print of the PC-MOS-386 startup screen
DeveloperThe Software Link
Written in80x86 assembly language c
Working stateDiscontinued
Source modelOpen-source
Initial release1987; 32 years ago (1987)
Latest release5.01
Available inEnglish
Platformsx86 architecture
Kernel typeMonolithic
Default user interfaceCommand-line interface
LicenseGNU GPL 3
Official websiteGithub

PC-MOS/386 was a proprietary multi-user, multitasking computer operating system produced by The Software Link (TSL), announced at COMDEX in November 1986 for February 1987 release.[1] PC-MOS/386, a successor to PC-MOS, can run many MS-DOS programs on the host machine or a terminal connected to it. Unlike MS-DOS, PC-MOS/386 is optimized for the Intel 80386 processor; however early versions will run on any x86 computer. It was released as open-source software in 2017.


The last version produced was v5.01, compatible with MS-DOS 5. It required a memory management unit (MMU) to support memory protection, so was not compatible with 8086 and 8088 processors.

MMU support for 286 class machines was provided using a proprietary hardware shim inserted between the processor and its socket. 386 machines did not require any special hardware.

Multi-user operation suffered from the limitations of the day including the inability of the processor to schedule and partition running processes. Typically swapping from a foreground to a background process on the same terminal used the keyboard to generate an interrupt and then swap the processes. The cost of RAM (over US$500/Mb in 1987) and the slow and expensive hard disks of the day limited performance.

PC-MOS terminals could be x86 computers running terminal emulation software communicating at 9600 or 19200 baud, connected via serial cables. Speeds above this required specialized hardware boards which increased cost, but the speed was not a serious limitation for interacting with text-based programs.

PC-MOS also figured prominently in the lawsuit Arizona Retail Systems, Inc. v. The Software Link, Inc., where Arizona Retail Systems claimed The Software Link violated implied warranties on PC-MOS. The case is notable because The Software Link argued that it had disclaimed the implied warranties via a license agreement on the software's shrinkwrap licensing. The result of the case, which Arizona Retail Systems won, helped to establish US legal precedent regarding the enforceability of shrinkwrap licenses.[2]

There was a year 2000 problem-like issue in this operating system, first manifesting on 1 August 2012 rather than 1 January 2000: files created on the system from this date on would no longer work.

On 21 July 2017 PCMOS/386 was relicensed under GPL v3 and its source code uploaded to GitHub,[3] with the "year 2012" issue corrected.[4]


Commands supported by PC-MOS Version 4 are:[5]

See also[edit]


  1. ^ Multiuser Operating System to Use 386 Microprocessor's Virtual Modes, InfoWorld, Nov 17, 1986
  2. ^ Bennett, Oliver. "Arizona Retail Systems". cyber.law.harvard.edu.
  3. ^ Jansen, Roeland (8 February 2018). "pcmos386v501: PC-MOS/386 v5.01 final release including cdrom driver sources" – via GitHub.
  4. ^ Sprinkle, James (30 January 2019). "Date Bug" – via GitHub.
  5. ^ PC-MOS User Guide