PC-MOS/386

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PC-MOS/386

A screen-print of the PC-MOS-386 startup screen
Developer	The Software Link
Written in	80x86 assembly language, C
OS family	DOS
Working state	Active
Source model	Open source
Initial release	1987; 37 years ago (1987)
Latest release	5.01
Repository	github.com/roelandjansen/pcmos386v501
Available in	English
Platforms	x86 architecture
Kernel type	Monolithic
Influenced by	MS-DOS
Default user interface	Command-line interface (COMMAND.COM)
License	GPL-3.0-only
Official website	Github

PC-MOS/386 is a 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. PC-MOS/386 used to be proprietary, but it was released as open-source software in 2017.

The last commercial 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. However, the greatest benefit was reached when using standard, "dumb" terminals which shared the resources of the then central 386-based processor. 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]

• DoubleDOS
• Multiuser DOS - Digital Research's unrelated multi-user operating system
• VM/386 - unrelated multi-tasking DOS environment
• Virtual DOS machine
• Multiuser DOS Federation
• FreeDOS
• Timeline of operating systems