works with aout not with elf why

[kiran]

hello

i was studying various tutorials and other articles and books regarding os development. closely following mr xsism's xosdev chapter1 and 2 and some others i built a boot loader and a hello world kernel. the bootloader sets a20, pm,gdt copies kernel to 0x100000 and jumps to it.

after writing the programs i used the following commands

nasm -f aout kernel_start.asm -o ks.o
gcc -c kernel.c -o kernel.o
ld -T link.ld -o kernel.bin ks.o kernel.o
nasm -f bin bootloader.asm -o bootloader.bin
cat bootloader.bin kernel.bin > os.img
dd if=os.img of=/dev/fd0
bochs -q

but it i gave command
nasm -f elf kernel_start.asm -o ks.o
then when i run ld the output is a big file.
and the os doesnt work

whats the reason?

when i referred to a tutorial written by joachin bock and KJ on Making a simple kernel with basic printf
he had also used nasm -f aout.

so whats the reason why not elf.

Kiran

[Pype.Clicker]

i fail to see a link.ld file in those tutorial .

If you could post the output of "objdump -x kernel_start.o", it could help me pointing at what could go wrong.

[slash]

aout format is only useful if you want to mix 16 and 32 bit code else its obsolete.

so whats the reason why not elf

.Look at http://my.execpc.com/~geezer/osd/exec/ for that.

[slash]

if you do the same compilation proccess on windows with mingw you will get error "PE operations on non PE files"
although it has nothing to do with PE binary.

[Pype.Clicker]

then when i run ld the output is a big file.
and the os doesnt work

how 'big' is 'a big file' ?

- creating an ELF asm makes nasm add its .comment to the thing, AOUT has no .comment section ...
- more padding is added at the end of .text in ELF mode

That being said, with ld version 2.13.90.0.18, gcc (GCC) 3.3 and NASM version 0.98.34, the files generated with a 'aout' or an 'elf' startup asm are byte-to-byte identical as soon as i use --oformat binary link statement ...

k_init.asm

Code: Select all

[bits 32]        ; 32bit code here
[global start]   ; start is a global function
[extern _k_main] ; this is the kernel function
start:     

call _k_main     ; jump to k_main() in kernel.c

hlt              ; halt the cpu

kernel.c

Code: Select all

void _k_main()
{
  int num;
  char ch;

  char *text_video = (char*)0xB8000;
  char attrib = 0x07;
  char *str="Kernel Loaded";

  while(*str!=0)
  {
    *text_video = *str;
    *text_video++;
    *text_video = attrib;
    *text_video++;
    *str++;
  }
  return;
}

commands:

Code: Select all

> nasm k_init.asm -f aout -o k_init_aout.o
> gcc kernel.c -c
> ld --oformat binary -Ttext 0x100000 k_init_aout.o kernel.o -o kaout.bin
> l kaout.bin 
-rwxr-xr-x    1 martin   run            93 2004-08-05 15:28 kaout.bin

[kiran]

hello

here are my k_init.asm and kernel.c and linker.ld

k_init.asm
----------
GLOBAL __start
EXTERN _k_main
[BITS 32]
__start:
   call _k_main
jmp $   

kernel.c
--------
#define COLUMNS 80
#define LINES 24
#define ATTRIBUTE 7
#define VIDEO 0xB8000

static int xpos;
static int ypos;
static volatile unsigned char *text_video;

void _k_main ();
void _clear_screen (void);
static void itoa (char *buf, int base, int d);
static void putchar (int c);
void printk (const char *format, ...);

void
_k_main ()
{
_clear_screen ();
printk ("\n \n Welcome to the Ekalavya Operating System \n \n");
}

void
_clear_screen ()
{
int i;
text_video = (unsigned char *) VIDEO;

for (i = 0; i < COLUMNS * LINES * 2; i++)
*(text_video + i) = 0;

xpos = 0;
ypos = 0;
}

static void
itoa (char *buf, int base, int d)
{
char *p = buf;
char *p1, *p2;
unsigned long ud = d;
int divisor = 10;

if (base == 'd' && d < 0)
{
*p++ = '-';
buf++;
ud = -d;
}

else if (base == 'x')
divisor = 16;

do
{
int remainder = ud % divisor;
*p++ = (remainder < 10) ? remainder + '0' : remainder + 'a' - 10;
}
while (ud /= divisor);

*p = 0;
p1 = buf;
p2 = p - 1;
while (p1 < p2)
{
char tmp = *p1;
*p1 = *p2;
*p2 = tmp;
p1++;
p2--;
}
}

static void
putchar (int c)
{
if (c == '\n' || c == '\r')
{
newline:
xpos = 0;
ypos++;
if (ypos >= LINES)
   ypos = 0;
return;
}
*(text_video + (xpos + ypos * COLUMNS) * 2) = c & 0xFF;
*(text_video + (xpos + ypos * COLUMNS) * 2 + 1) = ATTRIBUTE;
xpos++;
if (xpos >= COLUMNS)
goto newline;
}

void
printk (const char *format, ...)
{
char **arg = (char **) &format;
int c;
char buf[20];
arg++;

while ((c = *format++) != 0)
{
if (c != '%')
   putchar (c);
else
   {
    char *p;
    c = *format++;
    switch (c)
    {
    case 'd':
    case 'u':
    case 'x':
    itoa (buf, c, *((int *) arg++));
    p = buf;
    goto string;
    break;

    case 's':
    p = *arg++;
    if (!p)
      p = "(null)";

    string:
    while (*p)
      putchar (*p++);
    break;

    default:
    putchar (*((int *) arg++));
    }
   }
}
}

linker.ld
-------

OUTPUT_FORMAT("binary")
ENTRY(__start)

SECTIONS
{
   .text 0x100000 : {
   *(.text)
   }
   .data : {
   *(.data)
   }
   .bss : {
   *(.bss)
   }
}

[kiran]

hello

i used the following commands with the above files and got different results for aout and elf

For aout
gcc -c kernel.c -o kernel.o
nasm -f aout -o k_init.o k_init.asm
ld -T linker.ld -o kernel.bin k_init.o kernel.o

ll -h
kernel.bin 824
k_init.o 92
kernel.o 1.9K

For elf

gcc -c kernel.c -o kernel.o
nasm -f elf -o k_init.o k_init.asm
ld -T linker.ld -o kernel.bin k_init.o kernel.o

ll -h
kernel.bin 1.1M
k_init.o 608
kernel.o 1.9K

THat kernel is 1.1 M. big right ???

should i give an objdump also?

I use linux for all these

Kiran

[Legend]

Your linker thinks that the file will be loaded to 0.
And as the code starts at the 1MB mark, it adds a 1MB padding, so that when it gets loaded to 0, the code is at 1MB ...

[Pype.Clicker]

Your linker thinks that the file will be loaded to 0.
And as the code starts at the 1MB mark, it adds a 1MB padding, so that when it gets loaded to 0, the code is at 1MB ...

I second that. Really, we could better help you if we could see your linker script ...

[kiran]

hello

I already gave my linker script. Here is it.

linker.ld
-------

OUTPUT_FORMAT("binary")
ENTRY(__start)

SECTIONS
{
.text 0x100000 : {
*(.text)
}
.data : {
*(.data)
}
.bss : {
*(.bss)
}
}

Then how did it work with aout?

Kiran

[kiran]

hello

Yes i found that there is a lot of padding happening.

So what should be the change in my linker script.

kiran

[Pype.Clicker]

you're missing .rodata* in your linker script.
Probably the linker default them at 00000000 while the rest starts at 0x100000, hence the padding...

[kiran]

hello

it worked. thank you.

Kiran