; User-accessible part of FCB (12 bytes)
%define fcb_drv   0 ; 1 byte
%define fcb_fn    1 ; 8 bytes
%define fcb_ext   9 ; 3 bytes

; Drive & FS data (4 bytes)
%define fcb_spt   12 ; byte    sectors per track
%define fcb_nos   13 ; byte    number of sides/heads
%define fcb_ss    14 ; byte    sector size: 2^ss
%define fcb_cs    15 ; byte    cluster size: 2^(ss+cs)

; Read/Write pointer (12 bytes):
; Will be overwritten by new filename for CL=17
%define fcb_fsize 16 ; dword   file size
%define fcb_fptr  20 ; dword   handle position
%define fcb_clus  24 ; word    current cluster fptr points in
%define fcb_co    26 ; word    start sector for theoretical cluster 0

; Link to directory item (3 bytes)
%define fcb_ds    28 ; word    directory sector
%define fcb_do    30 ; byte    directory offset

fcb_open_rootdir:
  cmp byte [bx], 0
  jne .drivedone
  mov al, default_drive
  mov [bx], al
.drivedone:
  mov dl, [bx]
  dec dl

  xor ax, ax
  xor dx, dx
  ; set directory sector to 0 (=rootdir)
  mov [bx+fcb_ds], ax
  ; load first sector
  call drive_read

  ; transfer sector size
  mov ax, [diskbuf + 0x0B]
  call log2
  mov byte [bx+fcb_ss], al

  ; transfer cluster size
  mov al, [diskbuf + 0x0D]
  call log2
  mov byte [bx+fcb_cs], al

  ; transfer sectors per track
  mov al, [diskbuf + 0x18]
  mov byte [bx+fcb_spt], al

  ; transfer number of heads/sides
  mov al, [diskbuf + 0x1A]
  mov byte [bx+fcb_nos], al

  ret