There are several control blocks related to $SCAN. First, the facility recognizes a set of “primitive” control blocks specified in the scan table entries. They are the HCT, HCCT, the current PCE, the current DTE, DCTs, the user control table (UCT), or any control block pointed to by a name/token pair. The UCT is not generated or specifically used by JES2, but rather is an optional user control block pointed to by the $UCT field of the HCT. Installations requiring a central control block for use in exit routines or user modifications needs to generate a UCT, or a their own control table pointed to by a name/token pair, and use it as their central main task control block rather than adding new fields to the HCT.

Additionally, scan table entries can indicate the control block from the previous “level” of scanning or a temporary control block should be used. A subsequent search for the actual required control block through control block chains and subscript indexing can also be indicated by the table entries.

Another important control block for the JES2 uses of $SCAN is the JES2 master control table (MCT). The MCT is pointed to by field $MCT in the HCT and it contains the addresses of many JES2 statically-defined tables and related routines. Importantly, the MCT contains the doublewords containing addresses of the scan table pairs. The MCT is assembled into module HASPTABS in load module HASJES20.

The scan table entries themselves form control blocks which are mapped by the $SCANTAB macro. Also, during a scan, a work area is used by $SCAN and passed to prescan and postscan exit routines. The scan work areas are allocated through $GETWORK and mapped by the $SCANWA macro.