The first system that was not written in assembly language was probably
the Master Control Program (MCP) for Burroughs computers. MCP was written
in a variant of ALGOL. MULTICS, developed at MIT, was written mainly in
the system programming language PL/1. The Linux and Windows operating
system kernels are written mostly in C, although there are some small sections
of assembly code for device drivers and for saving and restoring the state of
registers.