FRAMEBUFFER_ADDRESS equ 0xb8000

; Arguments:
;   [FURTHEST FROM EBP]
;     4. U32            Target Y-Position
;     3. U32            Target X-Position
;     2. U32            Length of Hexadecimal String
;     1. Ptr32          Hexadecimal String
;   [NEAREST TO EBP]
write_hexadecimal_string:
    push        dword                   esi
    sub         esp,                    64
    mov         esi,                    esp

    mov         [esi + (64 - 4)],       eax
    mov         [esi + (64 - 8)],       ebx
    mov         [esi + (64 - 12)],      ecx
    mov         [esi + (64 - 16)],      edx
    mov         [esi + (64 - 20)],      edi

    ; [esi]:    

.initialize_writer_loop:
    xor         ecx,                    ecx

.writer_loop:
    cmp ecx, [ebp - 8]
    jae .epilog

    ; Calculate the byte's start on-screen (A single byte from
    ; the input uses 6 bytes in the framebuffer as every single
    ; character uses 2 bytes and there are 3 characters per byte
    ; written; the upper nibble, the lower nibble and the padding.

    ; Multiply the row with 80, the number of cells per row
    mov eax, [ebp - 16]
    mov edx, 80
    mul edx

    ; Add the offset within the row ontop
    add eax, [ebp - 12]
    add eax, ecx

    ; Multiply with 2 to get the byte offset
    ; from the character index
    mov edx, 6
    mul edx

    ; Point into the framebuffer
    add eax, FRAMEBUFFER_ADDRESS

    ; Save the value
    mov [esi], eax

.write_upper_nibble:
    ; Lookup the hexadecimal character
    xor eax, eax
    mov ebx, [ebp - 4]
    add ebx, ecx
    mov al, [ebx]
    shr al, 4
    mov ebx, .hexadecimal_lookup
    add ebx, eax
    mov dl, [ebx]

    mov ebx, [esi]
    mov [ebx], dl
    ; Set the color code for leaving the background black (0x0?)
    ; and making the foreground be white (0x?f).
    mov [ebx + 1], byte 0x0f

.write_lower_nibble:
    ; Lookup the hexadecimal character
    xor eax, eax
    mov ebx, [ebp - 4]
    add ebx, ecx
    mov al, [ebx]
    and al, 0x0f
    mov ebx, .hexadecimal_lookup
    add ebx, eax
    mov dl, [ebx]

    mov ebx, [esi]
    mov [ebx + 2], dl
    ; Set the color code for leaving the background black (0x0?)
    ; and making the foreground be white (0x?f).
    mov [ebx + 3], byte 0x0f


    inc ecx
    jmp .writer_loop

.epilog:
    mov         eax,                    [esi + (64 - 4)]
    mov         ebx,                    [esi + (64 - 8)]
    mov         ecx,                    [esi + (64 - 12)]
    mov         edx,                    [esi + (64 - 16)]
    mov         edi,                    [esi + (64 - 20)]

    add         esp,                    64
    pop         dword esi
    ret

.hexadecimal_lookup:
    db "0123456789abcdef"