Atlas - INCOMP.DOC

Atlas - INCOMP.DOC
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1
2Areas where 2.0 is not compatible with  previous  versions  of
3the 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
34The following subroutine will read the fat into  the  area  of
35memory specified by DS:BX.  DL contains the drive number, DL=0
36means read the fat from the default drive, DL=1 indicates read
37from drive A:, and so on.
38
39getfat:
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
55driveok:
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
[FILE END]
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