; Utilities to read data from the disk. Used to load the kernel
; into main memory


load_disk:
    ; Loads 'dh' sectors from drive 'dl' (this
    ; register is set by the BIOS before calling
    ; the bootloader) into es:bx
    pusha
    push dx
    mov ah, 0x2   ; Perform a read operation
    mov al, dh    ; Number of sectors
    mov cl, 0x2   ; 0x1 is us, so 0x2 is the first sector we want
    mov ch, 0x0   ; Cylinder 0
    mov dh, 0x0   ; Head position is 0 too
    int 0x13      ; Once this returns, the data will be in es:bx
    jc disk_error ; If an error occurs, the carry bit is set
    pop dx
    ; The BIOS tells us how many sectors have been
    ; actually read. If the desired and actual value 
    ; don't match, an error occurred
    cmp al, dh
    jne sectors_error
    popa 
    ret


disk_error:
    mov si, disk_read_error_msg
    call bios_print
    mov dh, ah     ; Error code is in ah
    call bios_printh
    call bios_newline
    jmp disk_loop


sectors_error:
    mov si, disk_sectors_error_msg
    call bios_println


disk_loop:
    jmp $


disk_read_info: db "Booting from disk type ", 0
disk_read_error_msg: db "Read error: ", 0
disk_sectors_error_msg: db "Read error (wrong number of sectors)", 0