# File lib/file_systems/AppleCPM.rb, line 39
  def self.set_block(file_system_image,block_no,contents)
    raise "invalid block #{block_no} - length was #{contents.length}" unless contents.length==BLOCK_SIZE
    track_no=(block_no/4)+3
    0.upto(3) do |i|
      sector_no=SECTORS_IN_BLOCK[block_no%4][i]
      file_system_image.set_sector(track_no,sector_no,contents[(i*256),256])
    end  
  end