| Operating system | Developer | First release | License |
|---|---|---|---|
| PEARL Operating System | Brown, Boveri & Cie | — | — |
| RT-11 | Digital Equipment Corporation | — | proprietary license |
| TSX-Plus | — | — | — |
| UCSD p-System | — | — | — |
| Xenix | Microsoft, Santa Cruz Operation, AT&T Corporation | — | proprietary license |