Begin function (function code X'FE')

The begin function of the session management exit routine is required; all other functions are optional. This function is processed only once. It is processed before any of the other functions and is either initialized immediately following VTAM® initialization, or is activated by issuing a MODIFY EXIT,ID=exitname,OPTION=ACT command. See z/OS Communications Server: SNA Operation for more information about this command.

During begin function processing, the exit routine selects the other functions to process for all LU-LU sessions. If no other functions are selected at this time, the session management exit routine is not invoked for the other functions. Table 1 shows the begin function parameter list pointed to by register 1. The parameters are described in Parameter descriptions.

Table 1. Begin function parameter list
Dec (Hex) Offset	Size (Bytes)	Description
0 (0)	4	Address of environment vectors
4 (4)	4	Address of function code and related session information
8 (8)	4	Address of user data field
12 (C)	4	Address of exit options
16 (10)	4	Address of a parameter string if you specify a value for the PARMS operand on the MODIFY EXIT command. The parameter string is in the form of a 2-byte length field followed by the actual character data. If you do not specify a value, this pointer will be zero.
72 (48)	4	Address of VTAM Exit Services parameter list