Installing or Replacing the DESERV Exit

Your program, while operating in task mode, can establish DESERV exits by issuing the DESERV macro with FUNC=EXIT and appropriate parameters.

Table 1. Installing or Replacing the DESERV Exit
If EXIT_OPTION =	then:	and these parameters...
REPLACE	The new exit definition replaces the most recently defined exit. The DST (DESERV Screen Table) address of the previously defined exit's DST is returned to the caller.	EXIT_DST and EXIT_PREV_DSTPTR must be specified.
NOREPLACE (with no exit currently existing)	The exit definition is activated. A value of zero is returned as the previous DST address.	See REPLACE.
NOREPLACE (with an exit currently existing)	The currently defined exit remains the active exit. The address of the currently defined DST is returned as the previous DST address.	See REPLACE.
Note: If EXIT_OPTION=REPLACE is specified the caller must expect that a previous DST address is returned. It is the responsibility of the caller to replace the old DST address when eventually disabling the new exit. Also the exit routine might need to be aware that there was a previous exit defined, and might choose to invoke the previously defined exit.

EXIT_DST defines the DST address. The DESERV FUNC=EXIT caller owns and manages storage for the DST. The DST can be encapsulated inside other application managed control blocks. This can help the exit routine determine the context of the application in which it was called. When the DST is used with a task level exit, the DST_FLAGS_PROP bit can be set on to indicate that this DST should be propagated to tasks that are attached by this task. The propagate function is not supported for the global exit. The format of the DST is shown in Table 2.

Table 2. DESERV Screen Table Structure
Offset	Length or Bit Pattern	Name	Description
0 (X'0')	20	DST	(structure)
0 (X'0')	16	DST_HEADER	(character)
0 (X'0')	8	DST_ID	Eyecatcher 'IGWDST' (character)
08 (X'08')	4	DST_LEN	Length of DST
08 (X'08')	X'14'	DST_LEN_IV	Constant to be used with DST_LEN
12 (X'0C')	1	DST_LEV	Control block level (unsigned)
12 (X'0C')	X'01'	DST_LEV_IV	Constant to be used with DST_LEV
13 (X'0D')	1	DST_FLAGS	DST flags (unsigned)
	xxxx xxx.	-	Reserved
	.... ...1	DST_FLAGS_PROP	Propagate this DST to lower level tasks
14 (X'0E')	2	DST_RES	Reserved
16 (X'10')	4	DST_EXIT	Address of exit routine screen table (address)

EXIT_PREV_DSTPTR returns the address of the DST that was defined prior to this DESERV FUNC=EXIT call.