Small kernel code problem

[aloktiagi]

Hi,

I started writing a small kernel code that prints a character when it is loaded by my bootloader wriiten in assembly. Everything worked fine till i was printing a character. But when i tried putting function calls in my c code the computer reboots, no idea why!!. I'm using tftpboot to load the binary.
Following are my code with the disassembled c code

bootloader.asm

bits 16
org 7c00h

cli
xor ax,ax
mov ds,ax
mov es,ax
lgdt [gdt_desc]
mov si,7e00h ; if your kernel has been placed at offset 200h, like you did
mov di,1000h ; the address where the kernel should be
mov cx,512 ; whatever the size of the kernel is
rep movsb
mov eax,cr0
or al,1
mov cr0,eax
jmp 08h:clear_pipe
bits 32
clear_pipe:
mov ax,10h
mov ds,eax
mov es,eax
mov ss,eax
mov esp,90000h
jmp 08h:00001000h
;mov byte [ds:0B8000h], 'P'      ; Move the ASCII-code of 'P' into first video memory
;mov byte [ds:0B8001h], 1Bh      ; Assign a color code

jmp $
gdt dd 0ffffh,0cf9a00h,0ffffh,0cf9200h
gdt_desc dw 23
dd gdt-8

main.c

#define WHITE_TXT 0x07 // white on black text

int k_clear_screen();
int k_printf(char *message, unsigned int line);


int main() // like main in a normal C program
{
        //for(;;);
        k_clear_screen();
        k_printf("Hi!\nHow's this for a starter OS?", 0);
        for(;;);
}

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

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 0;
}

compilation steps
nasm -f bin bootloader.asm -o bootloader.bin
gcc -ffreestanding -c main.c -o main.o
ld -i -e _main -Ttext 0x1000 -o kernel.o main.o
objcopy -R .note -R .comment -S -O binary kernel.o kernel.bin
dd if=bootsect.bin of=os1.img bs=512 count=1
dd if=kernel.bin of=os1.img bs=512 seek=1

disassembled main.o

00001000 <main>:
    1000:       55                      push   %ebp
    1001:       89 e5                   mov    %esp,%ebp
    1003:       83 ec 08                sub    $0x8,%esp
    1006:       83 e4 f0                and    $0xfffffff0,%esp
    1009:       b8 00 00 00 00          mov    $0x0,%eax
    100e:       83 c0 0f                add    $0xf,%eax
    1011:       83 c0 0f                add    $0xf,%eax
    1014:       c1 e8 04                shr    $0x4,%eax
    1017:       c1 e0 04                shl    $0x4,%eax
    101a:       29 c4                   sub    %eax,%esp
    101c:       c7 45 fc 00 80 0b 00    movl   $0xb8000,0xfffffffc(%ebp)
    1023:       8b 45 fc                mov    0xfffffffc(%ebp),%eax
    1026:       c6 00 41                movb   $0x41,(%eax)
    1029:       8b 45 fc                mov    0xfffffffc(%ebp),%eax
    102c:       40                      inc    %eax
    102d:       c6 00 07                movb   $0x7,(%eax)
    1030:       e8 fc ff ff ff          call   1031 <main+0x31>  this is the jump to clear screen(should this be 1049 instead of 1031)
    1035:       83 ec 08                sub    $0x8,%esp
    1038:       6a 00                   push   $0x0
    103a:       68 00 00 00 00          push   $0x0
    103f:       e8 fc ff ff ff          call   1040 <main+0x40>
    1044:       83 c4 10                add    $0x10,%esp
    1047:       eb fe                   jmp    1047 <main+0x47>

00001049 <k_clear_screen>:
    1049:       55                      push   %ebp
    104a:       89 e5                   mov    %esp,%ebp
    104c:       83 ec 08                sub    $0x8,%esp
    104f:       eb fe                   jmp    104f <k_clear_screen+0x6>

Thanks
Alok