[ORG 0x00]
 [BITS 16]

section .text

JMP 0x07C0:START ;

START:
    mov ax, cs
    mov ds, ax
    mov es, ax

    mov ax, 0x0000
    mov ss, ax
    mov ax, 0xffff
    mov sp, ax
    mov bp, ax

    mov si, 0xB800
    mov es, si
    mov bx, 0x0000
    mov cx, 80*25*2

CLEAR:          ;Clear screen
    mov byte[es:bx], 0x00
    inc bx
    loop CLEAR

READ:               ;Read disk
    mov si, 0x1000
    mov es, si
    mov bx, 0x0000  ; ES:BX, 0x1000:0000
    mov ah, 0x02

    mov al, 2
    mov ch, 0
    mov cl, 2
    mov dh, 0
    mov dl, 0
    int 0x13
    jnc A20_ENABLE  ; No Error -> jump to A20_ENABLE

READERROR:          ;If failed to read disk,
    jmp $

A20_ENABLE:         ;Enable A20 GATE with BIOS
    mov ax, 0x2401
    int 0x15
    jnc ENTRY

A20_FAIL:           ;If BIOS Interrupt is not working, use Control Port.
    mov al, 2   ;
    out 0x92, al

ENTRY:              ;Entry Point
    cli
    lgdt [GDTR]
    mov eax, cr0
    or eax, 1
    mov cr0, eax

    jmp $+2                         ; for pipeline
    nop
    nop

    jmp dword 0x08:0x10000           ; jump to second stage

GDTR:                                ; GDT
    dw GDTEND - GDT - 1
    dd GDT+0x7C00

GDT:
    NULL:
        dw 0x00     ;프로세서 예약구간
        dw 0x00
        db 0x00
        db 0x00
        db 0x00
        db 0x00

    CODE:
        dw 0xFFFF   ;LIMIT ADRESS
        dw 0x0000   ;BASE ADRESS
        db 0x00     ;BASE ADRESS
        db 0x9A     ;1001 1010 A code segment descriptor (for your kernel, it should have type=0x9A)
        db 0xCF     ;1100 1111
        db 0x00

    DATA:
        dw 0xFFFF
        dw 0x0000
        db 0x01
        db 0x92     ;1001 0010 A data segment descriptor (you cant write to a code segment, so add this with type=0x92)
        db 0xCF     ;1100 1111
        db 0x00

    VIDEO:
        dw 0xFFFF
        dw 0x8000
        db 0x0B
        db 0x92
        db 0x40
        db 0x00

GDTEND:

times 510 - ($-$$) db 0x00

db 0x55
db 0xAA

[ORG 0x10000]
[BITS 32]

PROTECTED:
    mov ax, 0x10
    mov gs, ax
    mov ss, ax
    mov ds, ax
    mov es, ax
    mov fs, ax

    xor esp, esp    ; make stack
    mov esp, 0xFFFE
    mov ebp, 0xFFFE

    mov si, 0x18     ; setting for print message
    mov es, si
    mov bx, 0
    lea si, [message]
    mov ah, 0x0f

MESSAGE:              ; print loop
    lodsb
    cmp al, 0
    je LOADKERNEL
    mov [es:bx], ax
    add bx, 2
    jmp MESSAGE

LOADKERNEL:
    jmp dword 0x08:0x10200   ; jump to C kernel

message: db 'Protected Mode Entered.', 0

times 512 - ($ - $$) db 0x00

void write_string();

void main(){
    write_string(0x09, "helloworld");
}

void write_string( int colour, const char *string )
{
    volatile char *video = (volatile char*)0xB8000;
    while( *string != 0 )
    {
        *video++ = *string++;
        *video++ = colour;
    }
}

gcc -c -m32 -ffreestanding kernel.c -o kernel.o

ld -melf_i386 -Ttext 0x10200 -nostdlib kernel.o -o kernel.tmp

objcopy -O binary kernel.tmp kernel.img

cat bootloader.img secondstage.img kernel.img > Disk