Selecting aligned or unaligned maps

CICS® maps that are used by basic mapping support (BMS) can be defined as aligned or unaligned. In aligned maps, the length field associated with a BMS data field in the BMS DSECT is always aligned on a halfword boundary. In unaligned maps, the length field follows on immediately from the preceding data field in the map DSECT. An aligned map is compiled with the AMAP option, and an unaligned one is compiled with the MAP option. A combination of aligned and unaligned maps can be used.

In unaligned maps, there is no guarantee that the length fields in the BMS DSECT are halfword-aligned. Some COBOL and PL/I Compilers, in this case, generate extra code in the program, copying the contents of any such length field to, or from, a halfword-aligned work area when its contents are referenced or changed.

Specifying map alignment increases the size of the BMS DSECT, at worst by one padding byte per map data field, and marginally increases the internal path length of BMS in processing the map. The best approach, therefore, is to use unaligned maps, except where the compiler being used would generate inefficient application program code.

In COBOL, an unaligned map generates an unsynchronized structure. In PL/I, an unaligned map generates a map DSECT definition as an unaligned structure. Correspondingly, aligned maps produce synchronized structures in COBOL and aligned structures in PL/I.

In CICS, BMS maps are always generated in groups (map sets). An entire map set must be defined as aligned or unaligned. Also, maps can be used by application programs written in various languages. In these cases, it is important to select the option that best suits the combination of programs and, if there is any requirement for both aligned and unaligned maps, select the ALIGNED option.

Avoid converting maps, for example, from aligned to unaligned, because changing the map DSECT also requires reassembly or recompilation of all application programs that reference it.

Map alignment is defined when maps are assembled. Aligned maps use the SYSPARM(A) option. The BMS=ALIGN/UNALIGN system initialization parameter defines which type of map is being used.

The map and map set alignment option can also be specified when maps and map sets are defined using the screen definition facility (SDF II) licensed program.

The importance of map alignment is demonstrated by inspecting programs that handle screens with many fields. Try recompiling the program when the BMS DSECT is generated first without, and then with, the map alignment option. If the program size, as indicated in the linkage edit map, drops significantly in the second case, use aligned maps where possible.