SM-1800 was Intel based, not PDP-11 based. 1800 was based on Russian variant of 8080, and 1810 had both 8080 and 8086 as I remember. https://ru-m-wikipedia-org.translate.goog/wiki/%D0%A1%D0%9C_... will give overview of the SM series.
Regarding OS - PDP-based were initially on RSX, later on Soviet variants of unix. Intel based had either custom OS for 8080, or ms-dos like for x86 - Wikipedia article covers it well.
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