Directory Header (Track 18 sector 0)

          $00       Track of first directory sector.
          $01       Sector of first directory sector.
          $02       $41, ASCII 'A' indicating 4040 format.
          $03       $2A, DOS version.
          $04-$8F   Block Availability Map,  BAM, 35 tracks, 4  bytes each.
                    First  byte  has  number of sectors free on that track.
                    The other three bytes are a bit stream. Bit zero of the
                    first  of the three bytes is sector 0.  If the  bit  is
                    set, then the block is free.
          $90-$9F   Disk name padded with shifted spaces ($A0).
          $A0-$A1   Two extra characters for disk name.
          $A2-$A3   Disk ID.
          $A4       Shifted space ($A0).
          $A5-$A6   ASCII '2A' for DOS version.
          $A7-$AA   Shifted spaces ($A0).
          $AB       Track for DESKTOP's buffer.
          $AC       Sector for DESKTOP's buffer.
          $AD-$BC   ASCII  'GEOS format VX.X',  only the first 11 bytes are
                    used to check for GEOS format.
          $BD-$FF   Filled with zeroes.