The Pascal and C calling conventions were different in the way the stack got cleaned after a function call, and Pascal turned out to be ever so slightly faster.

The Pascal and C calling conventions were different in the way the stack got cleaned after a function call, and Pascal turned out to be ever so slightly faster. So the OS functions used that as the standard. Microsoft had a keyword that could be applied to any function in C to make it compatible. Assembly sounds very believable for early OS work.