SB - Starting a session with a BLDL identifier

The calling application identifies the program object by providing an open DCB and an entry returned by the MVS™ BLDL service.

Table 1. SB parameter list
Parameter	Usage	Format	Content
1	in	structure	'SB', X'0001'
2	in/out	binary word	mtoken Must contain zero when the service is called. The service will supply a value if the service is successful.
3	in	structure	DCB Note: The parameter list points directly to the DCB, not to a pointer variable
4	in	structure	Member name or BLDL directory entry; only the first 8 bytes are inspected. Note: The parameter list points directly to the directory entry Caution: Releases of z/OS® prior to V1R9 require a full BLDL entry here of at least 62 bytes, beginning with the member name.

This service is primarily designed for DCBs opened within the job step in the job step key. It can, however, also be used for DCBs opened by MVS, such as JOBLIB/STEPLIB, LINKLIB, or PGM=*.DD.

The service makes a number of tests to determine if the DCB passed by the caller can be used. These tests involve the access method and record format specified when the DCB is opened, the key in the DCB that is opened, and the current program authorization level. If the DCB cannot be used, the service attempts to open a DCB for the same DDname. If the DDname exists, but the OPEN fails, the request is rejected. Reason codes are expected from an SJ request rather than an SB request.

If the caller's DCB cannot be used, and the DDname does not exist for the current job step (as is the case for the LINKLIB DCB), a special authorized service is used to read the program object. In this case, the program object image is maintained in 31-bit storage within the address space rather than in 64-bit storage. This can change the storage requirements significantly.

Sample assembler code

         CALL  (15),(SBIL,MTOKEN,MYDCB,DE),VL

SBIL     DC    C'SB',X'0001'
MTOKEN   DC    F'0'          Replaced by the service
MYDCB    DCB   DDNAME=PDSE,DSORG=PO,RECFM=U,MACRF=R
BLDLIST  DC    H'1',H'62'    62 is the minimum for fast data
DE       DC    CL8'MEMBERNM'
         DS    CL50          Filled by BLDL