Thread: Program without standard library?

Originally Posted by Sly

I am actually trying to write a function that will work at bootup. I have a loader already, and what I want to do is create a function like printf, and then also be able to jump from asm to C code. Oh, and I don't know how to make a compiler spit out asm.

gcc spits out assembler with the option "-S". Other compilers use other options. Look in the documentation for the compiler itself.

To call printf from assembler, in x86, you would push the arguments in backwards order...

Code:

     .code
     push   4711
     push   offset hellow_world
     push   offset format
     call      _printf
     add    esp, 12      // remove 3 args of 4 bytes. 

     .data
format:  db "%s the number is %d\n",0
hello_world: db "Hello, World"

The syntax of this may not be correct for nasm, but I think you get the idea.

--
Mats

Thanks for the -s switch, but the asm is not what I meant. I wanted to code my own simple puts function in C. And learn how to jump to a C executable.
I don't have the standard library. Where does gcc put the asm file?

Yes, and I think I've given you the basic ingredients:
1. how to write a function that writes to the screen from C.
2. How to parse a printf-style format string.
3. How to deal with the arguments.
4. How to call such a function.

So here's the final installment, and I'm off to be:

Code:

// assuming we have the code from a couple of posts ago to set up screen. 
int x = 0, y = 0;
char attr = 0x07;   // standard attribute. 
void puts(const char *str)
{
    char c;
    while((c = *str++))   // Extra brackets to prevent gcc from moaning about assignment in condition. 
    {
       if (y >= rows-1) 
       {
              int i; 
              for(i = rows-1; i > 1; --i)
              {
                  memcpy(screen[i-1], screen[i], sizeof(screen[0]));
               }
               memset(screen[rows-1], 0, sizeof(screen[0]));
        }
        if (c == '\n')
        {
             x = 0; 
             y++;
        }
        else
        {
            screen[y][x].ch = c;
            screen[y][x].attr = attr;
            x++;
            if (x == cols)
               y++;
        }
   }
}

If you actually want a moving cursor as well, then you need to mess about with the video controller (VGA) - I think this page describes it sufficiently to be able to do something with it: http://www.stanford.edu/class/cs140/...ga/textcur.htm

--
Mats

Originally Posted by Sly

Thanks for the -s switch, but the asm is not what I meant. I wanted to code my own simple puts function in C. And learn how to jump to a C executable.
I don't have the standard library. Where does gcc put the asm file?

Note that it's an -S not -s (-s means something different - can't remember what, but it's "nothing useful"). It puts the file in your working directory, I believe - but it may be the directory of the source (or where you point it with -o, I expect). Note that you have to choose between -S and -c, so you can't generate object file and assembler at the same time.

You can also use objdump -S when disassembling object files, and it will use debug (-g in gcc) info to determine what source code belongs to which line(s) of assembler code.

--
Mats

Originally Posted by Sly

Thanks tons. It's some strange asm, but it's asm.

Use -masm=intel, and gcc should output something much closer to nasm's syntax - it may not be exactly right, but at least registers are in the same order.

--
Mats