ISAM files consist not only of data blocks but also, for example, of blocks which contain management information for ISAM processing. An NK-ISAM file consists of data blocks, index blocks, a control block, free blocks and, possibly, overflow blocks, while a K-ISAM file consists of data blocks, index blocks and free blocks. Since the control block and the overflow blocks are created and managed by ISAM, there is no difference, at the user interface, in the processing of NK-ISAM and K-ISAM files.
The following sections describe the structural elements of an ISAM file, starting with the user's records, which are stored in data blocks and, with NK-ISAM, in some cases extend into so-called “overflow blocks”. In NK-ISAM, the data blocks are managed with the aid of the index blocks and the control block, in K-ISAM only with the aid of the index blocks.