ACB      MACRF=(CNV,...),...

With NUB (no user buffering) and NCI (normal control interval access), specify in the MACRF parameter that the data set is to be opened for keyed and addressed access, and for control interval access. For example, MACRF=(CNV, KEY, SKP, DIR, SEQ, NUB, NCI, OUT) is a valid combination of subparameters.

   RPL      OPTCD=(CNV,...),...

Usually, control interval access with no user buffering has the same freedoms and limitations as keyed and addressed access have. Control interval access can be synchronous or asynchronous, can have the contents of a control interval moved to your work area (OPTCD=MVE) or left in VSAM's I/O buffer (OPTCD=LOC), and can be defined by a chain of request parameter lists (except with OPTCD=LOC specified). A sequential GET without user buffering adds additional, existing buffers for the RPL to allow for read-ahead processing.

Except for ERASE, all the request macros (GET, PUT, POINT, CHECK, and ENDREQ) can be used for normal control interval access. To update the contents of a control interval, you must (with no user buffering) previously have retrieved the contents for update. You cannot alter the contents of a control interval with OPTCD=LOC specified.

Both direct and sequential access can be used with control interval access, but skip sequential access may not. That is, specify OPTCD=(CNV,DIR) or (CNV,SEQ), but not OPTCD=(CNV,SKP).

With sequential access, VSAM takes an EODAD exit when you try to retrieve the control interval whose CIDF is filled with 0s or, if there is no such control interval, when you try to retrieve a control interval beyond the last one. A control interval with such a CIDF contains no data or unused space, and is used to represent the software end-of-file. However, VSAM control interval processing does not prevent you from using a direct GET or a POINT and a sequential GET to retrieve the software end-of-file. The search argument for a direct request with control interval access is the RBA of the control interval whose contents are desired.

The RPL (or GENCB) parameters AREA and AREALEN have the same use for control interval access related to OPTCD=MVE or LOC as they do for keyed and addressed access. With OPTCD=MVE, AREA gives the address of the area into which VSAM moves the contents of a control interval. With OPTCD=LOC, AREA gives the address of the area into which VSAM puts the address of the I/O buffer containing the contents of the control interval.

You can load an entry-sequenced data set with control interval access. If you open an empty entry-sequenced data set, VSAM lets you use only sequential storage. That is, issue only PUTs, with OPTCD=(CNV,SEQ,NUP). PUT with OPTCD=NUP stores information in the next available control interval (at the end of the data set).

You cannot load or extend a data set with improved control interval access. VSAM also prohibits you from extending a fixed-length or variable-length RRDS through normal control interval access.