Is it possible to implement a small Disk OS in C or C++?

Question

I am not trying to do any such thing, but I was wondering out of curiosity whether one could implement an "entire OS" (not necessarily something big like Linux or Microsoft Windows, but more like a small DOS-like operating system) in C and/or C++ using no or little assembly.

By implementing an OS , I mean making an OS from scratch starting the boot-loader and the kernel to the graphics drivers (and optionally GUI) in C or C++. I have seen a few low-level things done in C++ by accessing low-level features through the compiler. Can this be done for an entire OS?

I am not asking whether it is a good idea, I am just asking whether it is even remotely possible?

Answer 1

Obligatory link to the OSDev wiki, which describes most of the steps needed to create an OS as described on x86/x64.

To answer your question, it is gonna be extremely difficult/unpleasant to create the boot loader and start protected mode without resorting to at least some assembly, though it can be kept to a minimum (especially if you're not really counting stuff like using __asm__ ( "lidt %0\n" : : "m" (*idt) ); as 'assembly').

A big hurdle (again on x86) is that the processor starts in 16-bit real mode, so you need some 16-bit code. According to this discussion you can have GCC generate 16-bit code, but you would still need some way to setup memory, load code from some storage media and so on, all of which requires interfacing with the hardware in ways that standard C just has no concept of (interrupts, IO ports etc.).

For architectures which communicate with hardware solely through memory mapped IO you could probably get away with writing everything except the C start-up code (that sets up the stack, initializes variables and so on) in pure C, though specific requirements of interrupt routines / exception or syscall gates etc. may be difficult to impossible to implement (as you have to access special CPU registers).

Answer 2

I assume that you have an OS for x86 in mind. In that case you need at least a few pages of assembler to set up protected mode and stuff like that, and besides that a lot of knowledge of all the stuff like paging, call gates, rings, exceptions, etc. If you are going to use a form of system calls you'll also need some lines of assembly code to switch between kernel and userspace mode.

Besides those things the rest of an OS can easily be programmed in C. For C++ you'll need a runtime environment to support things like virtual members and exceptions, but as far as I know that can all be programmed in C.

Just take a look at Linux kernel source, the most important assembler code (for x86) can be found in arch/x86/boot, but you'll notice that even in that directory most files are written in C. Furthermore you'll find a few assembly lines in the arch/x86/kernel directory for handling system calls and stuff like that.

Outside the arch directory there is hardly any assembler used (because assembler is machine specific, that machine specific code belongs in the arch directory). Even graphic drivers don't use assembler (e.g. nvidia driver in drivers/gpu/drm/nouveau).

Answer 3

A boot loader? You might want to skip that bit. For instance, Linux is quite often started by non-Linux boot loaders such as UBoot. After all, once the system is running, the OS will be present but not the boot loader, that's just there to get the OS proper into memory.

And once you've selected a decent existing bootloader, the remainder is pretty much all straightforward. Well, you have to deal with memory and files yourself; you can't rely on fopen obviously. But even a C++ compiler has little problem generating code that can run without OS support.