In the mainframe era, companies were automating financial operations, using ISAM and other flat-file databases by and large running largely on mainframes.
Never one to shy away from pushing into new areas and developing new techniques, his live ISAM show was truly breath taking and much like his audio output.
Out-dated and legacy systems that use file systems (text, spreadsheets, ISAM, flat files, etc.) for their consistency model lack any kind of data-integrity model.
When an ISAM file is created, index nodes are fixed, and their pointers do not change during inserts and deletes that occur later (only content of leaf nodes change afterwards).
The key improvement in ISAM is that the indexes are small and can be searched quickly, thereby allowing the database to access only the records it needs.