INCOMP.txt

Areas where 2.0 is not compatible with  previous  versions  of
the DOS.

+ Direct calls to the BIOS
        Any program which jumped directly to the BIOS  by  way
        of the jump table at 40:0 will no longer work.

+ FAT pointer calls
        Programs which used system calls 27 and 28  to  get  a
        pointer  to  the FAT will no longer work.  Because the
        FAT is now cached with other disk resources, there  is
        no  fixed  location  in memory to pass the address of.
        The calls still exist,  however,  and  have  the  same
        format.  THEY CAN ONLY BE USED TO GET THE FAT ID BYTE.
        On return ES:BX points to a FAT ID BYTE for the Drive.
        Doing  anything  except  READing  this  ONE  byte will
        probably crash the system.  In order  to  get  at  the
        FAT,  programs  will  first call DSKRESET (call 13) to
        flush out any dirty buffers, and then  make  a  GETDPB
        call  (call  31 or 50) to find out which sector on the
        disk the FAT starts at, how big it is,  and  how  many
        copies of  it there are.  Then INT 25H and INT 26H can
        be used to transfer the FAT in and out of the programs
        memory space.

+ INT 25H and INT 26H
        In order for the  above  to  work,  and  in  order  to
        maintain  some  order  in  the  world of multi-surface
        disks, it is required that INT 25H  and  26H  use  the
        MSDOS sector mapping rather than some rather arbitrary
        head-cylinder-sector mapping.

The following subroutine will read the fat into  the  area  of
memory specified by DS:BX.  DL contains the drive number, DL=0
means read the fat from the default drive, DL=1 indicates read
from drive A:, and so on.

getfat:
        push    bx              ; save pointer to fat area
        push    ds
        mov     ah,50           ; request the dpb
        int     21h
        mov     cx,[bx+15]      ; get fat sector count
        mov     dx,[bx+6]       ; first sector of fat
        pop     ds              ; restore fat area pointer
        pop     bx
        mov     al,dl           ; is it the default drive?
        or      al,al
        jnz     driveok         ; if not, load fat

        mov     ah,19h          ; ask for default drive
        int     21h             ; get the default drive
        inc     al              ; map a=0 to a=1
driveok:
        dec     al              ; map a=1 to a=0
        int     25h             ; read the fat into DS:BX
        pop     ax              ; clean up the stack
        ret