Is it possible to write an Operating System completely in C?

Question

Is it possible to write an Operating System completely in C?

By completely, I include everything up to the bootloader, but not the BIOS/UEFI.

Answer 1

The traditional answer to the question "can you write a program completely in C" is "no." The reason is that there is no way for a C program to set up its own stack.

Almost every processor has at least one register that points to the current address in the stack (usually called the frame pointer) and sometimes there's a second register (usually called the stack pointer) that needs to point to the next unused address in the stack. There is no way to use any expression or statement in the language itself to set either of the stack pointer or the frame pointer to an absolute value. (Procedure calls and returns can add and subtract from the stack pointer, but there's no way to initialize it to a known value.)

Posix defines a set of functions, setcontext, getcontext, makecontext and swapcontext (almost always written at least partly in assembler), that will enable you to read and write the stack and frame pointer.

Unfortunately, the setcontext family of functions is not widely implemented.