A VTAM® application program directly relates to an ACB for which an OPEN macroinstruction successfully issues. A program which runs under the control of a host operating system can open several ACBs and thus appear as multiple VTAM application programs.

On the other hand, a VTAM application program can be divided among several tasks controlled by the operating system. This is discussed further in Operating system facilities.

The operating systems under which VTAM runs offer several facilities (for example, multitasking and service request blocks (SRBs)) that allow you to structure your programs for more efficient operation.

By using the 31-bit addressing facility, you can use virtual storage more efficiently.

However, for most VTAM application programs, a less complicated operating system environment, called the normal environment, is sufficient. The following sections describe the normal environment. This book assumes that the environment is the normal environment, except where explicitly stated otherwise. Some of the ways an application program can use the special operating system facilities are described in Operating system facilities. That chapter also describes how the environment is changed from the normal environment by the use of the special facilities.