Compile an asm bootloader with external c code

Question

I write a boot loader in asm and want to add some compiled C code in my project.

I created a test function here:

test.c

__as         


        

Answer 1

First i want to express how to link C compiled code with assembled file.

I put together some Q/A in SO and reach to this.

C code:

func.c

//__asm__(".code16gcc\n");when we use eax, 32 bit reg we cant use this as truncate 
//problem

#include 
int  x = 0;



int madd(int a, int b)
{
    return a + b;
}



void mexit(){
      __asm__ __volatile__("mov $0, %ebx\n");
    __asm__ __volatile__("mov $1, %eax \n");
    __asm__ __volatile__("int $0x80\n");
}


char* tmp;

///how to direct use of arguments in asm command
void print_str(int a,  char* s){

      x = a;
        __asm__("mov x, %edx\n");//  ;third argument: message length

        tmp = s;
        __asm__("mov tmp, %ecx\n");//  ;second argument: pointer to message to write

        __asm__("mov $1, %ebx\n");//first argument: file handle (stdout)
    __asm__("mov $4, %eax\n");//system call number (sys_write)


   __asm__ __volatile__("int $0x80\n");//call kernel

}


void mtest(){
    printf("%s\n", "Hi");
    //putchar('a');//why not work
}


///gcc -c func.c -o func

Assembly code:

hello.asm

extern  mtest
extern  printf
extern  putchar
extern  print_str
extern  mexit
extern  madd

section .text                   ;section declaration

                                ;we must export the entry point to the ELF linker or
    global  _start              ;loader. They conventionally recognize _start as their
                                      ;entry point. Use ld -e foo to override the default.

_start:
                                ;write our string to stdout         

    push msg
        push len
        call print_str;


        call  mtest                                 ;print "Hi"; call printf inside a void function



                                                                ; use add inside func.c

        push 5
        push 10                                                         
        call madd;              
                                                                ;direct call of  printf()
        push  eax
    push  format
    call  printf;                           ;printf(format, eax)




        call mexit;                                     ;exit to OS


section .data                   ;section declaration
format db "%d", 10, 0
msg db      "Hello, world!",0xa ;our dear string
len equ     $ - msg             ;length of our dear string



; nasm -f elf32 hello.asm -o hello

;Link two files 
;ld hello func -o hl -lc -I /lib/ld-linux.so.2
; ./hl run code

;chain to assemble, compile, Run
;;   gcc -c func.c -o func && nasm -f elf32 hello.asm -o hello && ld hello func -o hl -lc -I /lib/ld-linux.so.2 && echo &&./hl

Chain commands for assemble, compile and Run

gcc -c func.c -o func && nasm -f elf32 hello.asm -o hello && ld hello func -o hl -lc -I /lib/ld-linux.so.2 && echo && ./hl

Edit[toDO]

• Write boot loader code instead of this version
• Some explanation on how ld, gcc, nasm works.