v2.0/source/INCOMP.txt

   1
   2 Areas where 2.0 is not compatible with  previous  versions  of
   3 the DOS.
   4
   5 + Direct calls to the BIOS
   6         Any program which jumped directly to the BIOS  by  way
   7         of the jump table at 40:0 will no longer work.
   8
   9 + FAT pointer calls
  10         Programs which used system calls 27 and 28  to  get  a
  11         pointer  to  the FAT will no longer work.  Because the
  12         FAT is now cached with other disk resources, there  is
  13         no  fixed  location  in memory to pass the address of.
  14         The calls still exist,  however,  and  have  the  same
  15         format.  THEY CAN ONLY BE USED TO GET THE FAT ID BYTE.
  16         On return ES:BX points to a FAT ID BYTE for the Drive.
  17         Doing  anything  except  READing  this  ONE  byte will
  18         probably crash the system.  In order  to  get  at  the
  19         FAT,  programs  will  first call DSKRESET (call 13) to
  20         flush out any dirty buffers, and then  make  a  GETDPB
  21         call  (call  31 or 50) to find out which sector on the
  22         disk the FAT starts at, how big it is,  and  how  many
  23         copies of  it there are.  Then INT 25H and INT 26H can
  24         be used to transfer the FAT in and out of the programs
  25         memory space.
  26
  27 + INT 25H and INT 26H
  28         In order for the  above  to  work,  and  in  order  to
  29         maintain  some  order  in  the  world of multi-surface
  30         disks, it is required that INT 25H  and  26H  use  the
  31         MSDOS sector mapping rather than some rather arbitrary
  32         head-cylinder-sector mapping.
  33
  34 The following subroutine will read the fat into  the  area  of
  35 memory specified by DS:BX.  DL contains the drive number, DL=0
  36 means read the fat from the default drive, DL=1 indicates read
  37 from drive A:, and so on.
  38
  39 getfat:
  40         push    bx              ; save pointer to fat area
  41         push    ds
  42         mov     ah,50           ; request the dpb
  43         int     21h
  44         mov     cx,[bx+15]      ; get fat sector count
  45         mov     dx,[bx+6]       ; first sector of fat
  46         pop     ds              ; restore fat area pointer
  47         pop     bx
  48         mov     al,dl           ; is it the default drive?
  49         or      al,al
  50         jnz     driveok         ; if not, load fat
  51
  52         mov     ah,19h          ; ask for default drive
  53         int     21h             ; get the default drive
  54         inc     al              ; map a=0 to a=1
  55 driveok:
  56         dec     al              ; map a=1 to a=0
  57         int     25h             ; read the fat into DS:BX
  58         pop     ax              ; clean up the stack
  59         ret
  60