60 lines
2.6 KiB
Plaintext
60 lines
2.6 KiB
Plaintext
|
||
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
|
||
|