This category is for operating systems (OSs) which main purpose is research into OSs and into larger, more general system issues. Mostly the links in this category point to other OS categories and OSs therein. The only OSs listed exclusively herein are those for which no better or clearer category seems suitable. On this page, OSs are arranged in two groups and levels: 1) Top group: types or classes of OSs, where research is more commonly done, experimental OSs are more likely to be found. 2) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.