The ADDRESSCHECK option is used to ensure records are not shifted during I/O operations. When the ADDRESSCHECK option is set in DASDL, an addresscheck word is allocated in each block. This addresscheck word is stored after the last word of data and after the checksum word if one is present. Each time the block is written, the disk address of the block is stored in the addresscheck word. When the block is read, the address is checked and an error results if the correct value is not present.

Neither addresscheck nor checksum words are put in nondata blocks. For example, block 0 (zero) for index sequential sets and the available space tables (DKTABLE) for standard data sets do not contain addresscheck or checksum words.