Well it depends the various OS' tend to different languages for systems programming. PR1MOS used FORTRAN and PL1/G as a system programming language PICs will use PIC machine code?

Which systems are you interested in? You do understand what is meant by "systems" programming as opposed to general programming using a particular system?

And all systems programing will usually have some Assembler usage.

Ps for the younger readers PR1MOS is descended from the ur hackers OS ITS