When you extend an indexed sequential data set with resume load, the disposition parameter of the DD statement must specify MOD. To ensure that the necessary control information is in the DSCB before attempting to add records, you should at least open and close the data set successfully on a system that includes resume load. This is necessary only if the data set was allocated on a previous version of the system. Records can be added to the data set by resume load until the space allocated for prime data in the first step has been filled.

During resume load on a data set with a partially filled track or a partially filled cylinder, the track index entry or the cylinder index entry is overlaid when the track or cylinder is filled. Resume load for variable-length records begins at the next sequential track of the prime data set. If resume load abnormally ends after these index entries have been overlaid, a subsequent resume load will result in a sequence check when it adds a key that is higher than the highest key at the last successful CLOSE but lower than the key in the overlaid index entry. When the SYNAD exit is taken for a sequence check, register 0 contains the address of the high key of the data set. However, if the SYNAD exit is taken during CLOSE, register 0 will contain the IOB address.