;=----------------------------------------------------------------------------=;
;                          GNU GPL OS/K                                        ;
;                                                                              ;
;   Authors: spectral`                                                         ;
;            NeoX                                                              ;
;                                                                              ;
;   Desc:   Bootsector for OS/K INCLUDED FUNCTIONS                             ;
;                   (x86_64 architecture only)                                 ;
;=----------------------------------------------------------------------------=;
[BITS 16]

read_clusters:
;---------------------------------------------------;
; Read file clusters, starting at the given cluster,;
; expects FAT to be loaded into the disk buffer.    ;
; Please note that this may allocate up to 128KB    ;
; of ram.                                           ;
;                                                   ;
; Expects: AX    = Starting cluster                 ;
;          ES:DI = Location to load clusters        ;
;                                                   ;
; Returns: None                                     ;
;                                                   ;
;---------------------------------------------------;
    pusha
    push es
  .cluster_loop:
    xor bh, bh
    xor dx, dx
    push ax                                     ; Get the cluster start = (cluster - 2) * sectorsPerCluster + UserData
    sub ax, 2                                   ; Subtract 2
    mov bl, byte [sectorsPerCluster]            ; Sectors per cluster is a byte value
    mul bx                                      ;   multiply (cluster - 2) * sectorsPerCluster
    add ax, word [UserData]                     ;   add the UserData
    xor ch, ch
    mov cl, byte [sectorsPerCluster]            ; Sectors to read
    call read_sectors                           ; Read the sectors
    pop ax                                      ; Current cluster number
    xor dx, dx
    ;; Calculate next sector for FAT16 (cluster * 2)
    mov bx, 2                                   ; Multiply the cluster by two (cluster is in ax)
    mul bx
    ;; Load sector in RAM
    push ds
    push si
    mov si, BUFFER_SEG
    mov ds, si                                  ; Temporarly set ds:si to the FAT buffer
    mov si, BUFFER_OFF
    add si, ax                                  ; Point to the next cluster in the FAT entry
    mov ax, word [ds:si]                        ; Load ax to the next cluster in FAT
    pop si
    pop ds
    ;; Next
    cmp ax, 0xfff8                              ; Check if we are at the end of the file?
    jae .done
    add di, 512                                 ; Add to the pointer offset
    jnc .cluster_loop 
    ;; Correct the buffer because an error will occur if the buffer in memory
    mov dx, es                                  ;   overlaps a 64k page boundry, when di overflows
    add dh, 0x10                                ;   it will trigger the carry flag, so correct
    mov es, dx                                  ;   extra segment by 0x1000
    jmp .cluster_loop                           ; Load the next file cluster
  .done:
    pop es
    popa
    ret


read_sectors:
;---------------------------------------------------;
; Read sectors starting at a given sector by        ;
; the given times and load into a buffer. Please    ;
; note that this may allocate up to 128KB of ram.   ;
;                                                   ;
; Expects: AX    = Starting sector                  ;
;          CX    = Number of sectors to read        ;
;          ES:DI = Location to load sectors         ;
;                                                   ;
; Returns: None                                     ;
;                                                   ;
;---------------------------------------------------;
    pusha
    push es
    mov bx, di                                  ; Convert es:di to es:bx for int 13h
  .sector_loop:
    push ax
    push cx
    xor dx, dx
    div word [sectorsPerTrack]                  ; Divide the lba (value in ax) by sectorsPerTrack
    mov cx, dx                                  ; Save the absolute sector value 
    inc cx
    xor dx, dx                                  ; Divide by the number of heads
    div word [heads]                            ;   to get absolute head and track values
    mov dh, dl                                  ; Move the absolute head into dh
    mov ch, al                                  ; Low 8 bits of absolute track
    shl ah, 1                                   ; High 2 bits of absolute track
    shl ah, 1
    shl ah, 1
    shl ah, 1
    shl ah, 1
    shl ah, 1
    or cl, ah                                   ; Now cx is set with respective track and sector numbers
    mov dl, byte [Bootdrv]                      ; Set correct Bootdrv for int 13h
    mov di, 21                                  ; Try five times to read the sector because i love 21
  .attempt_read:
    mov ax, 0x0201                              ; Read Sectors func of int 13h, read one sector
    int 0x13                                    ; Call int 13h (BIOS disk I/O)
    jnc .read_ok                                 ; If no carry set, the sector has been read
    xor ah, ah                                  ; Reset Bootdrv func of int 13h
    int 0x13                                    ; Call int 13h (BIOS disk I/O)
    dec di                                      ; Decrease read attempt counter
    jnz .attempt_read                            ; Try to read the sector again
    mov si, DiskError                           ; Error reading the disk :/
    call print
    jmp reboot
  .read_ok:
    pop cx
    pop ax
    inc ax                                      ; Increase the next sector to read
    add bx, word [bytesPerSector]               ; Add to the buffer address for the next sector
    jnc .next_sector 
    ;; Fixing buffer because an error will occur if the buffer in memory
    mov dx, es                                  ; overlaps a 64k page boundry, when bx overflows
    add dh, 0x10                                ; it will trigger the carry flag, so correct
    mov es, dx                                  ; es segment by 0x1000
  .next_sector:
    loop .sector_loop
    pop es
    popa
    ret

print:
;---------------------------------------------------;
; Print out a simple string.                        ;
;                                                   ;
; Expects: DS:SI = String to print                  ;
;                                                   ;
; Returns: None                                     ;
;                                                   ;
;---------------------------------------------------;
    lodsb                                       ; Load byte from ds:si to al
    or al, al                                   ; If al is empty stop looping
    jz .done                                    ; Done looping and return
    mov ah, 0x0e                                ; Teletype output
    int 0x10                                    ; Video interupt
    jmp print                                   ; Loop untill string is null
  .done:
    ret
    
reboot:
    xor ax, ax
    int 0x16                                    ; Get a single keypress
    mov ah, 0x0e                                ; Teletype output
    mov al, 0x0d                                ; Carriage return
    int 0x10                                    ; Video interupt
    mov al, 0x0a                                ; Line feed
    int 0x10                                    ; Video interupt
    mov al, 0x0a                                ; Line feed
    int 0x10                                    ; Video interupt
    xor ax, ax
    int 0x19                                    ; Reboot the system