Bootloader in C/C++?

Question

Is it possible to create a bootloader in C or C++ without using some type of Assembler (and preferably without using __asm)? I'm writing an Operating System and would like it to be completely written in C and C++.

Answer 1

That's pretty system dependent. In most cases, the answer is going to be no - you will need to write some custom assembly to set up the C runtime before you start running your C code. There are some exceptions, however. The ARM Cortex-M0, for example, can run C code straight out of reset.

Presumably, though, you're not using an M0, so you're going to need to write some assembly. Again, it's system/chip dependent, but you might be able to get away with something as simple as:

reset_vector:     mov  sp, SOME_KNOWN_GOOD_STACK_ADDRESS     call c_entry_point 

which simply initializes the stack pointer and calls your C program's entry point. Of course, this simple a setup depends on your chip having a reset vector/vector table that supports it, RAM (or something like RAM) being initialized before the reset vector gets called, and so on. There tend to be a lot of "gotchas" in early system initialization.

Prepare to get pretty friendly with your compiler, assembler, and linker documentation - generating a flat binary that you can flash down as a first stage bootloader is often a big pain in and of itself.

Good luck!

Answer 2

Assuming this is for x86, you can probably get something running in 16bit mode if you have the right compiler options and manage to get the layout of your bootsector correct with the right linker magic.

But you won't get far with plain C (or C++): you'll need to mask interrupts real fast, and there is no C function for that. Assembly is required.

This is probably the same for most of the other architectures: C and C++ simply don't have those features built in (some compiler extensions might help you though).

A great resource for what you are attempting to do: OSDev.