section .text



initialize_video_mode:
    
.prolog:
    push ax
    
    ; set the video mode correctly
    xor ah, ah          ; bios-function (set video mode)
    mov al, 3           ; video mode with 80x25 characters
    int 0x10
    
.epilog:
    pop ax
    ret

; cx: len_buffer
; si: buffer
; dl: start_x       NOTE: Will contain the end of the text on-screen
; dh: start_y       NOTE: Will contain the end of the text on-screen
print_string:
    
.prolog:
    push di
    push cx
    push bx
    push ax
    
    xor di, di          ; character index
    
.printing_loop:
    
.set_cursor_position:
    
    mov ah, 0x02
    mov bh, 0
    int 0x10
    
.write_character:
    
    push cx             ; save len_buffer
    
    ; get the current character
    mov bx, si
    add bx, di
    mov al, [bx]
    ; al now contains the current character
    
    ; set the other needed interrupt values
    mov ah, 0x0a        ; wanted function (write character at cursor position)
    mov bh, 0           ; page number
    mov cx, 1           ; repetitions of character
    int 0x10            ; call bios basic draw functions
    
    pop cx              ; restore len_buffer
    
    
    
    inc dl              ; go to next column
    
    ; go to a new line if needed
    cmp dl, 80
    jb .no_new_line_needed
    xor dl, dl
    inc dh              ; increase column
    
.no_new_line_needed:
    
    inc di              ; go to next character
    
    ; go further if the string-printing hasn't been finished
    cmp di, cx
    jb .printing_loop
    
.epilog:
    pop ax
    pop bx
    pop cx
    pop di
    
    ret



; ds: source
; es: destination
; dx: len_to_copy
mem_copy:
    push ax
    push bx
    push cx
    push dx
    xor cx, cx
    mov di, dx
    
    ; ax: first region's value
    ; bx: address at index
    ; cx: byte_index
    ; dx: second region's value
.loop:
    
    ; retrieve the source byte at the current index
    mov bx, cx
    mov dx, [bx]
    
    ; swap the two segments
    ; here, ax and bx are used differently.
    mov ax, es
    mov bx, ds
    mov ds, ax
    mov es, bx
    
    mov [bx], cx
    
    ; swap the two segments back
    mov ax, es
    mov bx, ds
    mov ds, ax
    mov es, bx
    
    inc cx
    
    ; if the end has NOT been reached
    cmp cx, di
    jb .loop        ; => continue
    
    ret



; ds: region_1
; es: region_2
; dx: len_regions
; ax: result
mem_equals:
    
    push bx
    push cx
    push dx
    push di
    xor cx, cx
    mov di, dx
    
    ; ax: first region's value
    ; bx: address at index
    ; cx: byte_index
    ; dx: second region's value
.loop:
    mov bx, cx
    mov dx, [bx]
    
    ; swap the two segments
    ; here, ax and bx are used differently.
    mov ax, es
    mov bx, ds
    mov ds, ax
    mov es, bx
    
    mov bx, cx
    mov ax, [bx]
    
    cmp ax, dx
    
    ; swap the two segments back
    mov ax, es
    mov bx, ds
    mov ds, ax
    mov es, bx
    
    ; jump accordingly to the comparison above
    jne .not_equal
    inc cx
    
    cmp cx, di          ; if the loop needs to continue
    jb .loop            ; (if the end is NOT reached yet)
    
    ; it goes here if the two regions are completely equal
    pop di
    pop dx
    pop cx
    pop bx
    
    mov ax, 1
    ret
    
.not_equal:
    
    pop di
    pop dx
    pop cx
    pop bx
    
    mov ax, 0
    ret



; ax: start_sector (upper)
; bl: start_sector (lower)
; cx: wanted_sector_count 
; dh: drive
; si: buffer_address (upper)
; di: buffer_address (lower)
load_sectors:
    
    ; INNER WORKING:
    ;
    ; This function first checks if it needs to load sectors going over
    ; a cylinder boundary and loads to that cylinder boundary first continuing
    ; in 63-sector - parts after that until the last, < 63-sectors part is read.
    ;
    ; This is done because some systems don't like reading over the boundaries
    ; between the cylinders. 
    ;
    ; The actual part loading the data is in .read_from_disk, it's no different
    ; from the first read to the other reads. It just gets different arguments
    ; from 1).load_first_sectors, and 2) .load_further_sectors
    ;
    
.prolog:
    push ax
    push bx
    push cx
    push dx
    push si
    xor si, si          ; current sector index
    
.load_first_sectors:
    
    
    
    
    
; ah:           EMPTY
; al:           num_sectors_to_read
; bx:           EMPTY
; ch:           cylinder
; cl:           sector
; dh:           head
; dl:           drive
.read_from_disk:
    
    ; set es:bx to the address in bytes to which the output should be written.
    
    mov bx, si
    shr bx, 9
    
    
    
    
    mov ah, 0x02
    int 0x13
    
    
    
.load_further_sectors:
    
    
    
    
    
.epilog: