The organization of a VTAM® application program affects how much storage it uses, how well it performs, how easy it is to write, and how easy it is to migrate the application program to another release of VTAM, if required. Before you begin writing a VTAM application program, it might be helpful to understand some of the ways in which an application program can be organized. This chapter discusses:

• General coding guidelines to consider when writing an application program
• Coding guidelines to facilitate migration of application programs
• Whether the application program should be single-thread or multithread
• Whether the application program operation should be synchronous or asynchronous and what posting mechanism should be used
• VTAM and user control block decisions to be made that affect application program organization