# File lib/file_systems/AppleDos.rb, line 295
def self.find_catalog_slot(file_system_image,filename)
  vtoc_sector=file_system_image.get_sector(vtoc_track_no,vtoc_sector_no)
  catalog_filename=catalog_filename(filename.upcase) unless filename.nil?
  catalog_track_no=vtoc_sector[01]
  catalog_sector_no=vtoc_sector[02]
  
  while (catalog_track_no+catalog_sector_no>0) do
    catalog=file_system_image.get_sector(catalog_track_no,catalog_sector_no)    
    (0..7).each do |slot_no|
      slot_start=slot_no*0x23+0x0B
      if filename.nil? && (catalog[slot_start]==0x00)|| (catalog[slot_start]==0xFF) then        
        return [catalog_track_no,catalog_sector_no,slot_start]
      end
      if (!filename.nil?) && (catalog[slot_start+0x03..slot_start+0x20]==catalog_filename) then
        return [catalog_track_no,catalog_sector_no,slot_start]
      end
    end
    catalog_track_no=catalog[01]
    catalog_sector_no=catalog[02]    
  end
  nil
end