MS-DOS/v2.0/bin/INCOMP.DOC

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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