BITS 16

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print:
; Prints string in si
; IN si: zero terminated string to print
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    mov ah, 0xE     ; Specify 'int 0x10' 'teletype output' function
                    ; [AL = Character, BH = Page Number, BL = Colour (in graphics mode)]
.printchar:
    lodsb           ; Load byte at address SI into AL, and increment SI
    test al, al
    jz .done        ; If the character is zero (NUL), stop writing the string
    int 0x10         ; Otherwise, print the character via 'int 0x10'
    jmp .printchar  ; Repeat for the next character
.done:
    ret

printCRLF:
    mov ah, 0xE
    mov al, 13
    int 0x10
    mov al, 10
    int 0x10
    ret

printSpace:
    mov ax, 0xE20
    int 0x10
    ret

printALChar:
    mov ah, 0xE
    mov bl, 0x02 ;color
    int 0x10
    ret

; Changing the screen the user i looking at. 
; value in 'al' is the screen we want to switch to. 
; starting from 0x00
os_change_screen: 
        ; Here the addresses for the screens stack 
        ; and base pointer has to be set. 
        mov ah, 0x05
        int 0x10
        ret

os_clear_screen: 
    pusha
    ; move cursor to the bootom 
    xor ax, ax
    xor bx, bx 
    mov ah, 0x02
    mov bh, 0x00 ; page number 0 
    mov dh, 0x18 ; row zero 
    mov dl, 0x00 ; coloumn zero
    int 0x10
    xor cx, cx
.loop:
    call printCRLF
    add cx, 1
    cmp cx, 0x19 ; until blank screen
    jne .loop
    ; Move curser back to the top of the screen 
    mov ah, 0x02
    mov bh, 0x00 ; page number 0 
    mov dh, 0x00 ; row zero 
    mov dl, 0x00 ; coloumn zero
    int 0x10
    
    popa
    ret
    
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print_reverse:
; Prints string in si
; IN bx: adress of string
; IN cx: length 
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    mov ah, 0xE     ; Specify 'int 0x10' 'teletype output' function
                    ; [AL = Character, BH = Page Number, BL = Colour (in graphics mode)]
    add bx, cx 
.printchar:
    mov BYTE al, [bx]           ; Load byte at address SI into AL, and increment SI
    test al, al
    jz .done        ; If the character is zero (NUL), stop writing the string
    cmp cx, 0
    jz .done
    int 0x10         ; Otherwise, print the character via 'int 0x10'
    dec bx
    dec cx
    jmp .printchar  ; Repeat for the next character
.done:
    ret