<b>HC900-CAOS</b>, standard operating system of the East German home computer <a href="https://en.wikipedia.org/wiki/KC_85" class="extiw" title="en:KC 85"><b>HC 900</b> (later renamed into <b>KC 85/2</b>)</a>, manufactured by the <a href="https://en.wikipedia.org/wiki/Kombinat_Mikroelektronik_Erfurt" class="extiw" title="en:Kombinat Mikroelektronik Erfurt">VEB Mikroelektronik</a> "Wilhelm Pieck" Mühlhausen, 1984. However, it already runs on a KC 85/2. It is displayed on a <a href="https://en.wikipedia.org/wiki/Colormat" class="extiw" title="en:Colormat">RFT Colormat</a> television set.
Guido Grohmann, Zerbst/Anhalt (Germany)
Attribution
The author sent the photo to me.