There is a flag in the MyISAM file that indicates whether or not the table was closed correctly. This will soon be used for automatic repair in the MySQL server. Dynamic size rows will now be much less fragmented when mixing deletes with updates and inserts. This is done by automatically combining adjacent deleted blocks and by extending blocks if the next block is deleted. MyISAM also supports the following things, which MySQL will be able to use in the near future.
All key segments have their own language definition.
This will enable MySQL to have different language definitions per column. HEAP HEAP tables use a hashed index and are stored in memory. This makes them very fast, but if MySQL crashes you will lose all data stored in them. HEAP tables uses a fixed record length format. HEAP doesn't support an index on a NULL column.