Is this written in assembly? I thought Unix was written in C and you had to only write a C compiler to port it to other platforms. I remember that was the story I was told.
Likewise there were other high level systems programming languages being used since 1961.
The lost notable one being ESPOL, replaced a couple of years later by NEWP, both Algol derivatives for systems programming. The OS was called Burroughs B5000, used compiler intrisics with zero Assembly and is still being sold nowadays by Unisys, as ClearPath MCP.
There are other notable examples available to discover there was a decade old of other OSes and systems languages.