Run c code from a bootloader program

Question

So far i have an assembly script that lets you boot from it and write hello world to the screen. now i want to be able to write some c code and be able to have my assembly script run it somehow. Im using mingw gcc for my compiler. This is my bootloader in assembly:

 org 7C00h
 jmp short Start  
 Msg:    db "Hello, world"  EndMsg:  Start:  mov bx, 000Fh   
 mov cx, 1       
 xor dx, dx  
 mov ds, dx
 cld               Print:  mov si, Msg   

          Char:   mov ah, 2      
 int 10h
 lodsb


 mov ah, 9   
 int 10h
 inc dl      
 cmp dl, 80      
 jne Skip
 xor dl, dl
 inc dh
 cmp dh, 25    
 jne Skip
 xor dh, dh  
 Skip:   cmp si, EndMsg
 jne Char        
 jmp Print        times 0200h - 2 - ($ - $$)  db 0    
 dw 0AA55h

My question is how to compile the c code so that the assembly script will execute it? also how would i edit my current boot loader so that would be possible?

Answer 1

First, you should select a compiler to use and study its specifics. C programs can be compiled for "hosted" environment which supposes standard initialization, main() as entry point and standard library, and "freestanding" environment where such guarantees aren't provided. To call C code you shall do appropriate environment setup (stack, data segments, etc.) in the bootloader. Details for this are compiler-specific. Converting of object code to form suitable for your bootloader can also be problem. For 16-bit code, there are at least Turbo C and Watcom C available for free use. Watcom is more progressive and modern and is open source now. Note that standard library is unavailable in such setup, so C code shall be specially written for your environment; this includes such underwater stones as e.g. multiplication of 32-bit longs was usually implemented in library, despite you don't write library call.

[In general, I'm confused with your request to use precisely C; as soon as bootloader unlikely needs execution speed, it's more proper to use something more applicable for such context, as Forth. There are ready to use implementations (see FreeBSD bootloader).]