Call C kernel from assembly bootloader

Question

EDIT: plz jump to my second post below...

I'm looking for a minimalist way to enter to my kernel from my bootloader. Do you have any workable example to do so ?

Here is the bootloader which enters protected mode : "boot.asm"

[org 0x7C00]

mov bp , 0x9000
mov sp , bp

cli
lgdt [gdt_descriptor] 

; Enter PM
mov eax, cr0
or  eax, 0x1
mov cr0, eax

jmp 0x8:init_pm 


[bits 32]
init_pm :
    mov ax, 0x10
    mov ds, ax
    mov ss, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    ;Tried to call my C function from here 
    call 0x8000      ; Kernel entry point       

    jmp $


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
[bits 16]

GDT:
;null : 
    dd 0x0 
    dd 0x0

;code : 
    dw 0xffff       ;Limit
    dw 0x0          ;Base
    db 0x0          ;Base
    db 0b10011010   ;1st flag, Type flag
    db 0b11001111   ;2nd flag, Limit
    db 0x0          ;Base

;data : 
    dw 0xffff       
    dw 0x0          
    db 0x0
    db 0b10010010 
    db 0b11001111 
    db 0x0

gdt_descriptor :
    dw $ - GDT - 1      ;16-bit size
    dd GDT              ;32-bit start address


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Bootsector padding
times 510-($-$$) db 0
dw 0xaa55

Here my kernel entry point I would like to call from the bootloader : "kernel.c"

void main() {
    char * vga = (char *) 0xb8000 ;
    *vga = "X";
}

Thanks for your help,

So far, this is what I tried to do, but it doesn't work : I just put the kernel to 0x8000 with ld and concatenate with cat, then jump from the bootloader to 0x8000

nasm boot.asm -f bin -o boot.bin
gcc -ffreestanding -c kernel.c -o kernel.o
ld -o kernel.bin -Ttext 0x8000 kernel.o --oformat binary
cat boot.bin kernel.bin > os-image

Answer 1

Oh, this definitely works. It just seems to be your knowledge of C, really.

I changed around your C kernel a bit and got this:

void main (void) 
{
   unsigned char* vga = (unsigned char*) 0xb8000;
   vga[0] = 'X'; //need to make sure that this is a character
   vga[1] = 0x09; //append the attribute byte
   for(;;); //make sure our kernel never stops, with an infinite loop
}

This seems to have solved the problem for me! Everything is in working order now, just write a shell in that file and then BAM! there you go!

Answer 2

Here is what I successed to do but with a kernel written in asm :

boot.asm :

[org 0x7c00]

KERNEL_OFFSET equ 0x1000

call load_kernel

;Switch PM
cli
lgdt [gdt_descriptor]

mov eax, cr0
or eax, 0x1
mov cr0, eax

jmp 0x8:init_pm

[bits 32]
init_pm :
   mov ax, 0x10
   mov ds, ax
   mov ss, ax
   mov es, ax
   mov fs, ax
   mov gs, ax

   mov ebp, 0x90000
   mov esp, ebp

   call KERNEL_OFFSET
   jmp $

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
[bits 16]

load_kernel:
   mov bx, KERNEL_OFFSET
   mov dh, 15
   mov dl, 0

   mov ah, 0x02
   mov al, dh
   mov ch, 0x00
   mov dh, 0x00
   mov cl, 0x02
   int 0x13
   ret

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
[bits 16]

GDT:
;null :
   dd 0x0
   dd 0x0

;code :
   dw 0xffff      ;Limit
   dw 0x0         ;Base
   db 0x0         ;Base
   db 10011010b    ;1st flag, Type flag
   db 11001111b    ;2nd flag, Limit
   db 0x0         ;Base

;data :
   dw 0xffff      
   dw 0x0         
   db 0x0
   db 10010010b
   db 11001111b
   db 0x0

gdt_descriptor :
   dw $ - GDT - 1       ;16-bit size
   dd GDT            ;32-bit start address

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
times 510 -($-$$) db 0
dw 0xaa55

kernel.asm:

[bits 32]

mov al, 'K'
mov ah, 3 ; cyan
mov edx, 0xb8000
mov [edx], ax
jmp $

I build the floppy image so that:

nasm boot.asm -o boot.bin
dd if=/dev/zero of=image.bin bs=512 count=2880
dd if=boot.bin of=image.bin conv=notrunc

nasm kernel.asm -o kernel.bin
dd if=kernel.bin of=image.bin conv=notrunc bs=512 seek=1

qemu -fda image.bin -boot a

This way it works ! that's almost that... But now, I replace kernel.asm by this kernel.c

kernel.c

void main () {
   char * vga = (char *) 0xb8000 ;
   *vga = "X";
}

And the following script:

nasm boot.asm -o boot.bin
dd if=/dev/zero of=image.bin bs=512 count=2880
dd if=boot.bin of=image.bin conv=notrunc

gcc -ffreestanding -m32 -c kernel32.c -o kernel.bin
dd if=kernel.bin of=image.bin conv=notrunc bs=512 seek=1

qemu -fda image.bin -boot a

Note that I'm running a 64bit Linux distro, so I use the '-m32' gcc option to compile to 32bits But it doesn't work...... please help !