Format unit (ps)

        Entry:
          AH = 1Ah
          AL = defect table count
          CL = format modifiers
              bit 4: generate periodic interrupt
              bit 3: perform surface analysis
              bit 2: update secondary defect map
              bit 1: ignore secondary defect map
              bit 0: ignore primary defect map
          DL = drive
          ES:BX -> defect table

        Return:
          CF set on error
          AH = status (see AH=01h)

        Note:   If periodic interrupt selected, INT 15h/AH=0Fh is called
                after each cylinder is formatted