section .text
org 0x7c00
start:
    
    mov ax, 0
    mov ds, ax
    mov es, ax
    mov gs, ax
    mov fs, ax
    
    mov ax, 0x7e00
    mov ss, ax
    
    mov sp, 256
    mov bp, 256
    
    mov si, dx              ; save the origin disk number
    
    mov ax, 0x7c00
    add ax, 446
    ; ax now points to the start of the partition table
    
    xor di, di
    
    call initialize_video_mode
    
.search_partition:
    
    mov bx, ax              ; get the offset of the current entry
    add bx, 4               ; go to the offset of the partition type
    mov cl, [bx]
    cmp cl, 0x9d
    je .load_partition
    
    inc di
    
    ; leave and print debug message if the last partition entry
    ; did not provide the wanted bootloader - partition
    cmp di, 4
    je .no_boot_partition
    
    add ax, 16
    jmp .search_partition
    
    
    
.load_partition:
    ; ax is the start of the bootloader's partition entry in the mbr
    
    ; read the chs-entry and push it onto the stack
    
    ; head
    xor dx, dx
    mov bx, ax
    add bx, 1
    mov dl, [bx]
    push dx
    
    ; cylinder
    xor dx, dx
    mov bx, ax
    add bx, 3
    mov dh, [bx]
    mov bx, ax
    add bx, 2
    mov dl, [bx]
    shr dx, 6
    push dx
    
    ; sector
    xor dx, dx
    mov bx, ax
    add bx, 2
    mov dl, [bx]
    and dl, 0b00111111
    push dx
    
    
    
    ; push the partition's length
    
    mov bx, ax
    add bx, 12
    mov dx, [bx]
    push dx             ; lower part
    
    mov bx, ax
    add bx, 14
    mov dx, [bx]
    push dx             ; higher part
    
    
    
    ; load it all and read from disk
    
    mov bx, sp
    add bx, 4
    
    mov al, [bx]        ; sector-count
    
    add bx, 2
    mov cl, [bx]        ; start-sector
    
    add bx, 2
    mov ch, [bx]        ; start-cylinder
    
    mov dx, si          ; drive
    mov bx, 0x0800
    mov es, bx          ; output segment
    
    mov bx, sp
    add bx, 8
    mov dh, [bx]        ; start-head
    
    
    
    call .write_hex
    
    
    
    mov cl, 2
    mov ah, 0x02
    mov al, 1
    mov bx, 0x0000      ; output address in es - segment
    int 13h
    
    
    
    mov cx, 8
    mov si, 0x8000
    mov dh, 12
    mov dl, 32
    call print_string
    jmp .keepalive
    
.no_boot_partition:
    
    mov cx, LEN_TEXT_NO_BOOT_PARTITION
    mov si, error_texts.no_boot_partition
    mov dh, 12
    mov dl, 32
    call print_string
    
.keepalive:
    jmp .keepalive
    
.write_hex:
    push bx
    push cx
    
    mov bx, error_texts.hextab
    and cx, 0x0f
    add bx, cx
    mov cl, [bx]
    mov bx, error_texts.hexbuf
    inc bx
    mov [bx], cl
    
    mov bx, error_texts.hextab
    and cx, 0xf0
    shr cx, 4
    add bx, cx
    mov cl, [bx]
    mov bx, error_texts.hexbuf
    mov [bx], cl
    
    mov cx, 2
    mov si, error_texts.hexbuf
    mov dh, 12
    mov dl, 32
    call print_string
    
    pop cx
    pop bx
    
    ret

; %include "exfat.asm"
%include "gdt.asm"
%include "utility.asm"

LEN_TEXT_NO_BOOT_PARTITION equ 17

error_texts:
    
.no_boot_partition:
    ;db "NO_BOOT_PARTITION"

.hextab:
    db "0123456789abcdef"

.hexbuf:
    resb 4


times 446 - ($-$$) db 0
db 0x80
db 0, 2, 0
db 0x9d
db 0, 3, 0
dd 1
dd 1

times 510 - ($-$$) db 0

db 0x55
db 0xAA

db "################"