In this example, the access method control block for one data set is generated at execution; the other is generated at assembly.

        GENCB  BLK=ACB,        An access method control block.         x
               DDNAME=DATA
        LTR    15,15
 
        BNZ    ERROR
 
        LR     2,1             Address of the control block.

        OPEN   (BLOCK,,(2))    A label is used for the access method   x
                               control block generated by ACB;         x
                               register notation is used for the       x
                               one generated by GENCB.  The two commas x
                               indicate the omission of options.
 
BLOCK   ACB   ,                Another access method control block.