# File lib/file_systems/PascalDos.rb, line 39
def self.get_block(file_system_image,block_no)
                track=(block_no / 8).to_i
                first_sector=2*(block_no % 8)
                raise "illegal block no #{block_no}" if track>=file_system_image.track_count
                return file_system_image.get_sector(track,first_sector)+file_system_image.get_sector(track,first_sector+1)
        end