It's worth pointing out that the original high-level programming language for the Primos operating system (and its tools, I believe) was FORTRAN. Yes! Really! Naturally there was assembler involved as well, but much of the O/S was written in FORTRAN.
Later on code was added using the PL/P variant of PL/1. For later tools/utilities, the SPL language - another PL/1 variant, very similar to PL/P but with library dependencies which prevented its use in the kernel - was used. Modula-2 was also used a little, I believe.
Primos had architectural "challenges" which meant that C was a difficult fit, though there were C compilers available for later releases. These challenges included a segmented memory architecture where addresses naturally wrapped at segment boundarys, 16-bit native I/O, null pointers which weren't all zeroes and so much more! I believe writing the C compiler(s) was quite a task, which IIRC was out-sourced. To the best of my knowledge, C was never used as a systems programming language internally to Prime.
(I worked for Prime for a while in the 1980s)