C compiler is not generating valid assembly output

Question

I am writing a simple kernel in C and ASM(to call assembly instructions generated by C compile). I am following this excellent articles from osdev.org. After read an kernel written in assembly and compiled and executed, it was worked fine, I see the kernel.bin running on qemu, fantastic. But I want really to write it in C. I found an example in same web page.

I compiled it and tried run the kernel.bin on qemu but I get "boot failed" from all devices: hard disk, floopd and CD-rom. then I disassembled the kernel.bin generated by ks.o (kernel.asm assembled using nasm) and kernel.o (kernel.c assembled using gcc) assembled to bin file using the ld. The kernel.bin:

http://pastebin.com/Y0pLFvij I can't see my string or any instruction to assembly write my string on video.

The C code that generate it:

#define WHITE_TXT 0x07 // white on black text
unsigned int k_printf(char *message, unsigned int line);
void k_clear_screen();

k_main() 
{
    k_clear_screen();
    k_printf("Hi!\nHow's this for a starter OS?", 0);
};

unsigned int k_printf(char *message, unsigned int line) // the message and then the line #
{
    char *vidmem = (char *) 0xb8000;
    unsigned int i=0;

    i=(line*80*2);

    while(*message!=0)
    {
        if(*message=='\n') // check for a new line
        {
            line++;
            i=(line*80*2);
            *message++;
        } else {
            vidmem[i]=*message;
            *message++;
            i++;
            vidmem[i]=WHITE_TXT;
            i++;
        };
    };

    return(1);
};

void k_clear_screen() // clear the entire text screen
{
    char *vidmem = (char *) 0xb8000;
    unsigned int i=0;
    while(i < (80*25*2))
    {
        vidmem[i]=' ';
        i++;
        vidmem[i]=WHITE_TXT;
        i++;
    };

and the assembly that call it:

[bits 32]

[global start]
[extern k_main] ; this is in the c file

start:
  call k_main

  cli  ; stop interrupts
  hlt ; halt the CPU

the link.ld

OUTPUT_FORMAT("binary")

ENTRY(start)

SECTIONS

{

  .text  0x100000 : {

    code = .; _code = .; __code = .;

    *(.text)

    . = ALIGN(4096);

  }

  .data  : {

    data = .; _data = .; __data = .;

    *(.data)

    . = ALIGN(4096);

  }

  .bss  :

  {

    bss = .; _bss = .; __bss = .;

    *(.bss)

    . = ALIGN(4096);

  }

  end = .; _end = .; __end = .;

}

and the makefile:

bin:
    nasm -f aout -o ks.o kernelbase.asm 
    gcc -Wall -c -o kernel.o kernel.c    
    ld -T link.ld -o kernel.bin ks.o kernel.o 
run1:
    qemu kernel.bin

clear:
    rm -f *.o

cbin:
    rm -f *.bin

and the kernel in pure assembly, disassembled: http://pastebin.com/Bkvkq3YQ that worked fine for me, and I can see the instrutions to write on video.

My OS: Ubuntu 10.4
Compiler: gcc
Assembler : NASM

Where am I wrong?

There is code missing. The C code cannot possibly compile in that form. — Niklas B.
– Niklas B., Commented Mar 4, 2012 at 15:48
No, the C code is does not compile too well. Also, there are warnings. — Niklas B.
– Niklas B., Commented Mar 4, 2012 at 15:52
What do you think the difference is between C converted to asm then assembled and "really write it in C". Certainly gcc does this, you compile a c program it compiles to asm, then calls the assembler and linker for you. No difference (so long as you use the same compiler/assembler/linker, compile options, and linker script. — old_timer
– old_timer, Commented Mar 4, 2012 at 16:09
@dwelch:I know that there no difference, all is assembly. When I say "Written in C" is uses C syntax and C compiler to make assembly. — Jack
– Jack, Commented Mar 4, 2012 at 16:34

qwertz · Accepted Answer · 2012-03-09 15:57:05Z

1

I tried to do that tutorial and the code failed too. Finally I did this:

Compile both files for 32 bit:

nasm -f elf32 ks.asm -o kernelbase.o
gcc -m32 -c kernel.c -o kernel.o

And link:

gcc -m32 kernelbase.o kernel.o -o Kernel.bin

After a while I found out that I don't get any errors by using gcc for linking.

Hope I helped :-)

edited Mar 9, 2012 at 15:57

answered Mar 9, 2012 at 14:20

qwertz

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2 Comments

Jack Over a year ago

what is -c flag on nasm options? I removed it and was compiled successfully and I can see valid assembly output. another question: have you tried run kernel.bin? I tried run it with qemu kernel.bin but I get something like "boot failed" cannot found any device, then I liked it using ld: ld -T link.ld -o kernel.bin kernelbase.o kernel.o , looks like a progress, it know the and show "Booting from hard disk" but never start the kernel. Thanks very much, @XCoder.

qwertz Over a year ago

Sorry "-c" was my mistake I compiled it without the "-c". So far I haven't test it. (no time) ;)

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