Building a chain of request parameter lists

When GENCB is used to build a chain of request parameter lists, the request parameter lists may be chained using only GENCB macros or using GENCB and MODCB macros together. When only GENCB is used, the request parameter lists are created in reverse order, as follows:

SECOND  GENCB  BLK=RPL
        LR     2,1
FIRST   GENCB  BLK=RPL,NXTRPL=(2)

SECOND GENCB creates the second request parameter list, which makes its address available for the first request parameter list. The address of the request parameter list is returned in register 1 and is loaded into register 2. FIRST GENCB creates the first request parameter list and supplies the address of the next request parameter list using register notation. GENCB and MODCB macros may be used together to create a chain of request parameter lists, as follows:

      GENCB    BLK=RPL,COPIES=2
      LR       2,0
      SRL      2,1
      LR       3,1
      LA       4,0(2,3)
      MODCB    RPL=(3),NXTRPL=(4)

The GENCB macro creates two request parameter lists. The length of the parameter lists is returned in register 0 and loaded into register 2. The address of the area in which the lists were created (and, therefore, the address of the first one) is returned in register 1 and loaded into register 3. The SRL statement divides the total length of the area (register 2) by 2. The LA statement loads the address of the second request parameter list into register 4. The MODCB macro modifies the first request parameter list (register 3) by supplying the address of the second request parameter list (register 4) in the NXTRPL parameter.

Each request parameter list in a chain should have the same OPTCD subparameters. Having different subparameters may cause logical errors. You cannot chain request parameter lists for updating or deleting records—only for retrieving records or storing new records. You cannot process records in the I/O buffer with chained request parameter lists. (OPTCD=UPD and LOC are invalid for chained request parameter lists.)