[BITS 16]
[ORG 7c00h]

init:
  mov cx, 0xB800
  mov gs, cx

start:
  call clr_vga
  mov si, HELLO
  call write_str
  call disk_read
  call verify_read
  mov si, READ_NERR
  call write_str
  jmp word [0x7dfc]
  jmp hang

read_err:
  mov si, READ_ERR
  call write_str
hang:
  jmp hang

verify_read:
  mov bx, word [0x7dfc]
  mov al, 0x66
  cmp al, byte [bx]
  jne read_err
  mov al, 0x55
  cmp al, byte [bx+1]
  jne read_err
  mov al, 0x66
  cmp al, byte [bx+2]
  jne read_err
  mov al, 0x89
  cmp al, byte [bx+3]
  jne read_err
  ret

write_str:
  push ax
  push si
  xor ax, ax
write_str_each_char:
  mov al, byte [si]
  cmp al, 0
  je write_str_done
  inc si
  call write_char_default_attr
  jmp write_str_each_char
write_str_done:
  pop si
  pop ax
  ret

write_char_default_attr:
  push bx
  mov bl, 0x07
  jmp write_char_inner
write_char:
  push bx
write_char_inner:
  push di
  mov di, [CURSOR_LOC]
  mov byte [gs:di], al
  inc di
  mov byte [gs:di], bl
  inc di
  mov [CURSOR_LOC], di
  pop di
  pop bx
  ret

clr_vga:
  mov di, 4000
clr_vga_loop:
  dec di
  mov byte [gs:di], 0x00
  cmp di, 0
  jne clr_vga_loop
  mov word [CURSOR_LOC], 0
  ret

disk_read:
  ; ax = disk number to read from
  ; reference: http://www.delorie.com/djgpp/doc/rbinter/id/14/7.html
  mov dx, 0x80
  or ax, dx
  mov si, LBA_DISK_READ_PACKET
  mov ah, 0x42
  int 0x13
  jc read_err
  ret

READ_ERR:
  db 'Read failed: ', 0
READ_NERR:
  db 'No err reading, continuing: ', 0
HELLO:
  db 'Bootloader init: ', 0
CURSOR_LOC:
  dw 0

 ; reference: http://www.delorie.com/djgpp/doc/rbinter/it/72/2.html
LBA_DISK_READ_PACKET:
  db 0x10
  db 0
  dw 32     ; number of blocks to read, reset to # actually read when done
  dw 0x7dfc ; read destination address
  dw 0x0000
  dd 1      ; lba to read from
  dd 0      ; for extended lba adresses (not really used here)

;  times 510-($-$$) db 0
;  db 0x55
;  db 0xAA