Category: DOS kernel
INT 25 - DOS 1+ - ABSOLUTE DISK READ (except partitions > 32M)
AL = drive number (00h = A:, 01h = B:, etc)
CX = number of sectors to read (not FFFFh)
DX = starting logical sector number (0000h - highest sector on drive)
DS:BX -> buffer for data
Return: CF clear if successful
CF set on error
AH = status (see #02547)
AL = error code (same as passed to INT 24 in DI)
AX = 0207h if more than 64K sectors on drive -- use new-style call
may destroy all other registers except segment registers
Notes: original flags are left on stack, and must be popped by caller
this call bypasses the DOS filesystem
examination of CPWIN386.CPL indicates that if this call fails with
error 0408h on an old-style (<32M) call, one should retry the
call with the high bit of the drive number in AL set
Novell DOS 7 decides whether the old-style or new-style (>32M) version
of INT 25 must be used solely on the basis of the partition's size,
thus forcing use of the new-style call even for data in the first
32M of the partition
PC Tools MIRROR as shipped with MS-DOS 5.0+ checks several signatures
at the beginning of INT 25h and INT 26h before it starts to patch
these vectors. The signatures it looks for are 83h, F9h, FFh, 74h
(CMP CX,-01; JZ ????) at offset +1 from the INT 25h/26h entry points
and 2Eh, FFh, 2Eh (JMP DWORD PTR CS:[????]) at the location pointed
to by the JZ ????. If it finds these signatures it will use the
target address of the far jump for its sub-sequent checks, otherwise
it will just take the previous interrupt entry points when scanning
for FAh, 2Eh, 8Ch, 16h (CLI; MOV CS:????,SS) or FAh, 2Eh, 89h, 26h
(CLI; MOV CS:????,SP) right at the beginning. Hence, it seems the
first two checks are to trace through a specific INT 25h/26h
filter. However, the purpose of the whole patch is unknown.
A method to detect the actual assignments of logical drive numbers
to physical BIOS drive units (for example to detect the boot drive),
is to temporarily mount an INT 13h handler recording the used DL
drive unit for any INT 13/AH=02h read operations and discarding any
attempts to access actual floppy drives. Then call INT 25h for
all the appropriate DOS drives and watch the results recorded by
the INT 13h interceptor.
Although all registers except segment registers may be destroyed
some software depends on some of the registers being preserved.
For example some Flash disk drivers requires that DX is not trashed.
DR-DOS 7.03 takes care of this.
BUGS: DOS 3.1 through 3.3 set the word at ES:[BP+1Eh] to FFFFh if AL is an
invalid drive number
DR DOS 3.41 will return with a jump instead of RETF, leaving the
wrong number of bytes on the stack; use the huge-partition version
(INT 25/CX=FFFFh) for all partition sizes under DR DOS 3.41
DR DOS 6.0 original issues 05/1991 & 08/1991 reported wrong error
codes for "drive not ready" and "write protect". This was fixed
with the DR DOS BDOS patch "PAT321" (1992/02/19, XDIR /C: 947Bh),
and later "full" rebuilds (see INT 21/AX=4452h for details).
SeeAlso: INT 13/AH=02h,INT 25/CX=FFFFh,INT 26,INT 21/AX=7305h,INT 21/AH=90h"PTS"