AH = 3
        AL = number of sectors to write
        CH = track (for hard disk, bits 8,9 in high bits of CL)
        CL = sector
        DH = head
        DL = drive
        ES:BX = address of buffer
Return: CF = set if error occurred
        AH = status (see AH=1 above)
        AL = number of sectors written
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