To use VTAM® macroinstructions, the application program must reserve and manipulate storage for the control information that VTAM needs to process the application program's request. These storage areas are called control blocks. (Some of the VTAM macroinstructions can be used to build the control blocks and initialize them to appropriate values. In addition, IBM® supplies assembler-language DSECTs with VTAM that help the application program manipulate control block storage fields.)

When an application program requests VTAM services, it must ensure that the appropriate control block fields are initialized. In many cases, the application program can do so with parameters on the VTAM macroinstruction. When VTAM finishes processing a macroinstruction request, it passes back completion information to the application program in these control blocks and in general-purpose registers.