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