For create and extend requests on a new volume, IGGPRE00 can return, in the IEXFMT1 field of the parameter list, the address of the data portion of a model format-1 DSCB, which starts with field PD1FMTID, also called DS1FMTID. Move the DSCB to the create or extend work area before you build the format-1 DSCB. The only fields that can be nonzero in the model DSCB are the DS1REFD field (the data-last-referenced field) and fields currently unused. Failure to zero out all fields, except for DS1REFD and all currently unused fields in the model format-1 DSCB, can abnormally terminate the task or cause unpredictable results. All other fields are initialized by allocate or extend.

IGGPRE00 might not provide a DSCB address in the IEXFMT1 field for a VIO allocate request (indicated by flag, IEXVIO, set to one), or if a partial DSCB has been provided to allocate instead of a JFCB (indicated by flag IEXMF1 being set to one). In the latter case, IEXFMT1 is passed to IGGPRE00, initialized to the address of the PD1FMTID field of the partial format-1 DSCB (supplied to allocate by its caller) in the create work area, and DS1REFD can be initialized by IGGPRE00. If extend is successful, IEXFMT1 is zeroed out before giving control to IGGPOST0.