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INT 25 - DOS - ABSOLUTE DISK READ (except DOS 4.0/COMPAQ DOS 3.31 >32M partitn)
        AL = drive number (0=A, 1=B, etc)
        DS:BX = Disk Transfer Address (buffer)
        CX = number of sectors to read
        DX = first relative sector to read
Return: CF set on error
            AL = error code issued to INT 24h in low half of DI
            AH = 80h if attachment failed to respond
                 40h if seek operation failed
                 20h if controller failed
                 10h if data error (bad CRC)
                 08h if DMA failure
                 04h if requested sector not found
                 03h if write-protected disk (INT 26 only)
                 02h if bad address mark
                 01h if bad command
        CF clear if successful
            AL = 00h
Notes:  ORIGINAL FLAGS ON STACK!  Must be popped or discarded by adjusting SP
        May destroy all registers except segment regs
BUG:    DOS 3.1 through 3.3 set the word at ES:[BP+1Eh] to FFFFh if AL is an
        invalid drive number
SeeAlso: INT 26, INT 13/AH=02h