Historically,
#C was not always the default language for writing operating systems.
There is a rich tradition of operating systems written in
#Pascal, with the probably best-known example being classical Mac OS (not to be confused with macOS, that is written in Objective C). Up until System 7, Mac OS was nearly entirely written in Pascal.
Other operating systems written in Pascal include
#LisaOS and the
#Lillith system software.
The operating system for the famous
#XeroxStar was written in Mesa, which is a modular programming language of the Pascal family.
ALT Classic Mac OS