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;==========================================NASM HEADERS=============================================

%define KERNEL_SEG 0x0000 ; kernel segment
%define KERNEL_OFF 0x7E00 ; kernel offset
%define KERNEL_SIZ 40     ; kernel size (into sectors of 512 bytes)
%define KERNEL_SEC 2      ; kernel sector
%define KERNEL_CYL 0      ; kernel cylinder
%define KERNEL_HEA 0      ; kernel head

%define STACK_ESP 0xCDFF 
%define STACK_EBP 0XCDFF

;==========================================NASM HEADERS=============================================

[BITS 16]

org 0x7C00                    ; ip = 0x7C00
jmp start                     ; cs = 0x0000

start:
    mov [drive],dl            ; store current drive number
    xor ax, ax 
    mov ds, ax                ; ds = 0x0000

;set up video mode 
    mov     ah, 00h           ; subfunction: set video mode
    mov     al, 03h           ; 0x03 = 80x25, 16 colors
    int     10h               ; interrupt: video

; Reset the Floppy Drive
reset:            
    mov ax, 0x00              ; subfunction: reset
    mov dl, [drive]           ; drive number
    int 0x13                  ; interrupt: drive
    jc reset

; Load Kernel at es:bx

    mov bx, KERNEL_SEG
    mov es, bx
    mov bx, KERNEL_OFF
    mov ah, 0x02              ; subfunction: read
    mov al, KERNEL_SIZ        ; number of sectors that will be read
    mov cl, KERNEL_SEC        ; sector
    mov ch, KERNEL_CYL        ; cylinder
    mov dh, KERNEL_HEA        ; head
    mov dl, [drive]           ; drive

; HERE IS THE PROBLEM
read:

    int 13h                   ; interrupt: drive
    jc read

lgdt [gdtr]                   ; load gdtr. ATENTION DS MUST BE EQUALS TO 0x0000

cli                           ; disable BIOS interrups

; Set Protected Mode
mov eax, cr0         
or  eax, 1   
mov cr0, eax

jmp dword codesel:pmode       ; Enter Protected Mode

[BITS 32]

; Load Segment registers with 32 bit Selectors
pmode:                                                      
    mov eax,datasel
    mov ss,eax
    mov ds,eax
    mov es,eax
    mov fs,eax
    mov gs,eax 
    mov ax,0x10
    mov ss,ax
    mov esp,STACK_ESP         ; maxmimum
    mov ebp,STACK_EBP         ; maxmimum
    mov dl,[drive]            ; we want to know what drive in
    
    push dx

jmp dword codesel:KERNEL_OFF  ; jump to kernel  

;============================================VARIABLES==============================================


; Global Descriptor Table Register
gdtr:
    dw gdt_end-1
    dd gdt

; Global Descriptor Table Descriptors
gdt:
    nullsel equ $-gdt   ; null descriptor
        dd 0
        dd 0

    codesel equ $-gdt   ; kernel code descriptor
        dw 0xFFFF
        dw 0x0000
        db 0x00
        db 10011010b    ; access byte
        db 11001111b    ; flag and hi limit
        db 0x00

    datasel equ $-gdt   ; kernel data descriptor
        dw 0xFFFF
        dw 0x0000
        db 0x00
        db 10010010b    ; access byte
        db 11001111b    ; flag and hi limit
        db 0x00
gdt_end:

; Drive Number
drive db 0                           

;============================================VARIABLES==============================================

;=========================================NASM DIRECTIVES===========================================

; fill the end of sector with signature's boot
   times 510-($-$$) db 0
                    dw 0xAA55

;=========================================NASM DIRECTIVES===========================================