PC-MOS/386

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PC-MOS/386 is a DOS-like operating system produced by The Software Link, incorporated. Unlike DOS, it is a multi-user/multi-tasking system with support for terminals. PC-MOS/386 is optimized for the 80386 processor, however it will run on any x86 computer. It is binary-compatible with some other DOS programs.

The latest version ever made was v5.01 and was dos v5 compatible. In order to have memory protection working, you need a MMU. Therefore it was almost not usable for any x86.

This screenshot shows the boot screen of release 2.20 from 1987.

MMU support for 286 class machines was provided using a proprietary hardware shim where the processor was lifted from its socket, the shim (with mmu) inserted and the processor re-attached. Often this resulted in damage to the processor or its socket.

Multi-user operation suffered from the limitations of the day including the inability of the processor to schedule/partition running processes. Typically it used the keyboard to generate an interrupt and then swap other processes. The other limitations included memory (RAM was > $500/meg in 1987) and slow hard disks - typically MFM (or RLL) based drives in PC hardware. Some of these drives has seek times of > 150ms - compare this to 8ms for modern drives.

PC/MOS workstations were often x86 computers connected via serial cables. Typically they ran at 19600 baud as the serial chips in late 1980s PCs were not advanced buffered UARTs. Speeds above this required specialist hardware boards which increased the overall cost of the solution.

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 holding in the case, which was for Arizona Retail Systems, helped to establish legal precedent about the enforceability of shrinkwrap licenses.