The index is stored in a file with the .ISM extension and the data is stored in file with the .ISD extension. See section 15.4 Using myisamchk for crash recovery . Most of the things for MyISAM tables are also true for ISAM tables. See section 8.1 MyISAM tables . Only support prefix compression on strings Smaller key limits.
Dynamic tables gets more fragmented.
8.3 HEAP tables 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. There is no extra space needed for free lists. HEAP tables use a fixed record length format. HEAP doesn't support an index on a NULL column.