Begin vector (X'00')

VTAM® calls the exit routine with the begin vector either during VTAM initialization or after you issue a MODIFY EXIT,ID=exitname,OPTION=ACT (or OPTION=REPL) command. See z/OS Communications Server: SNA Operation for more information about this command. The exit then performs its initial setup and returns the data in the begin vector to VTAM. VTAM uses the begin vector to determine which specific tasks the exit is to perform. The exit must return the address of the begin vector in register 1 of the configuration services XID exit routine output parameter list.

Table 1. Begin vector format
Dec (Hex) offset	Size (Bytes)	Description
0 (0)	1	Bit Setting B'1... ....' This dynamic build bit is an indication to VTAM to call the exit when VTAM receives an XID from a switched device that is not defined to VTAM. This bit must be on if the unknown device is to be defined dynamically. If this bit is on, receipt of all XIDs that are not boundary function (BF)-based APPN TGs causes VTAM to call the exit with an XID vector. In addition, receipt of BF-based APPN TG XIDs causes VTAM to invoke the exit if requested using bits 3, 4, and 5 below. B'.1.. ....' This connection status bit indicates whether the exit routine will be sent connection status information about all switched PUs. If this bit is on when the vector is returned from the exit, VTAM calls the exit with the connection status vector. B'..1. ....' This call-if-PU-defined bit is an indication to VTAM to call the exit routine with an XID vector when VTAM receives an XID for a PU that is already defined to VTAM. B'...1 ....' This APPN connection network bit is an indication to VTAM to call the exit routine with an XID vector when VTAM receives an XID for a connection from a destination node through a connection network (the XID was received on a dial-in link connected to a connection network). B'.... 1...' This APPN ACTPU suppression bit is an indication to VTAM to not call the exit routine with an XID vector when VTAM receives an XID requesting a BF-based APPN TG connection that indicates ACTPU suppression. B'.... .1..' This APPN ACTPU bit is an indication to VTAM to not call the exit routine with an XID vector when VTAM receives an XID requesting a BF-based APPN TG connection requesting ACTPU. B'.... ..1.' This REB extension indicator bit is an indication to VTAM that the Extended Resource Entry Block (REB) is to be used. B'.... ...1' Reserved.
1 (1)	1	B'1... ....' This bit, when set on, indicates to VTAM that the exit wants to be called for EE XIDs. B'.111 1111' Reserved.
2 (2)	17	This field contains the network qualified control point name of the host in which the exit resides.