OSDev.org

[BITS 16]  ; indique a nasm que l'on travaille en 16 bits
[ORG 0x0]

jmp start

%include "display.asm"

msgDebut: db "Chargement du kernel", 13, 10, 0

gdt:
  db 0, 0, 0, 0, 0, 0, 0, 0
gdt_cs:
  db 0xFF, 0xFF, 0x0, 0x0, 0x0, 10011011b, 11011111b, 0x0
gdt_ds:
  db 0xFF, 0xFF, 0x0, 0x0, 0x0, 10010011b, 11011111b, 0x0
gdtptr:
  dw 0  ; limite
  dd 0  ; base

start:

; initialisation des segments en 0x07C00
mov ax, 0x07C0
mov ds, ax
mov es, ax
mov ax, 0x8000
mov ss, ax
mov sp, 0xf000 ; stack de 0x8F000 -> 0x80000

; affiche un msg
mov si, msgDebut
call afficher

xor ah, ah
xor dl, dl
int 0x13

push es
mov ax, 0x100
mov es, ax
mov bx, 0

mov ah, 2
mov al, 50
mov ch, 0
mov cl, 2
mov dh, 0
mov dl, 0
int 0x13
pop es

mov ax, gdtptr
mov bx, gdt
sub ax, bx
mov word [gdtptr], ax

xor eax, eax
xor ebx, ebx
mov ax, ds
mov ecx, eax
shl ecx, 4
mov bx, gdt
add ecx, ebx
mov dword [gdtptr+2], ecx

cli
lgdt [gdtptr]

mov si, msgDebut
mov eax, cr0
or ax, 1
mov cr0, eax
mov si, msgDebut

jmp next
next:

mov ax, 0x10
mov ds, ax
mov fs, ax
mov gs, ax
mov es, ax
mov ss, ax
mov esp, 0x9F000

jmp dword 0x8:0x1000

times 510-($-$$) db 144
dw 0xAA55

[BITS 32]

EXTERN print

GLOBAL _start

_start:

jmp start

msg1 db "init kernel", 0

start:

mov byte [0xB8A00], 'H'
mov byte [0xB8A01], 0x56

mov eax, msg1
push eax
call print
pop eax

end:
jmp end

void print(char *string) {
  unsigned char *ptr = (unsigned char *) (0xB8A00);

  while (*string != 0) {
    *ptr = *string;
    *(ptr + 1) = 0x57;
    ++string;
    ptr += 2;
  }
}

nasm -f elf -o kernel.o kernel.asm
gcc -m32 -c screen.c
ld -m elf_i386 --oformat binary -Ttext 1000 kernel.o screen.o -o kernel

ndisasm -b32 -o0x1000 -a -s0x100e kernel
(...)
0000100E  C605008A0B0048    mov byte [dword 0xb8a00],0x48
00001015  C605018A0B0056    mov byte [dword 0xb8a01],0x56
0000101C  B802100000        mov eax,0x1002
00001021  50                push eax
00001022  E805000000        call dword 0x102c
00001027  58                pop eax
00001028  EBFE              jmp short 0x1028
(...)