Why some part of an os has to be written in assembly?

The scheduler of my mini os is written in assembly and I wonder why. I found out that the instruction eret can't be generated by the C compiler, is this somthing that can be generalized to other platforms than Nios and also x86 and/or MIPS architechture? Since I believe that part of os is always written in assembly and I'm searching for why a systems programmer must know assembly to write an operating system. Is is the case that there are builtin limitations of the C compiler that can't generate certain assembly instructions like the eret that returns the program to what is was doing after an interrupt?


  • The generic answer is for one of three reasons:

    1. Because that particular type of code can't be written in C. I think eret is a "return from exception" instruction, so there is no C equivalent to this (because hardware exceptions such as page faults, divide by zero or similar are not C/C++ style exceptions). Another example may be saving the registers onto the stack when task-switching, and saving the stack pointer into the task-control block. The C code can't do that, because there is no direct access to the stack pointer.

    2. Because the compiler won't produce as good code as someone clever writing assembler. Some specialized operations can be hard to write in C - the compiler may not generate very good code, or the code gets very convoluted to achieve something that is simple in assembler.

    3. The startup of C code needs to be written in assembler, because a C program needs certain things set up before you can run actual C code. For example configuring the stack-pointer, and some other registers.