Reassigning LXs when the LX reuse facility is enabled

The limit on the number of LXs is 32768. Some of the LXs are reserved as system LXs; the rest are available as non-system LXs. You can use the NSYSLX parameter in the IEASYSxx member of SYS1.PARMLIB to set the number of system LXs available for the system's use. System and non-system LXs are subdivided into 12-bit LXs and 24-bit LXs. 12-bit LXs are in the range 0-2047, and 24-bit LXs are in the range 2048-32768 (the specific value of the LX depends upon the LXSIZE parameter of the LXRES macro).

Use the LXRES macro with SYSTEM=YES to obtain a system LX; with SYSTEM=NO to obtain a non-system LX. Use the LXRES macro with LXSIZE=12 to obtain a 12-bit LX; with LXSIZE=16|23|24 to obtain a 24-bit LX. 24-bit LXs are further subdivided into reusable and non-reusable LXs.

Guideline: Always request a 24-bit LX when running on z/OS® V1R6.

There are different rules for each category of LX.

Non-Reusable system LX: The system does not reassign the LX. The original requestor of the LX should reconnect to the LX if the address space terminates and then restarts.
Reusable system LX: The system reassigns the LX after the owning address space has terminated or after the owner has used LXFRE to free the LX.
Non-Reusable non-system LX: The system reassigns the LX after all entry tables are disconnected from the LX. This is the case when the owning address space terminates or when the owner uses ETDIS to disconnect the entry table from the LX and uses LXFRE to free the LX; and all address spaces that have used ETCON to connect the LX to that entry table have either terminated or used ETDIS to disconnect the entry table from the LX.
Reusable non-system LX: The system reassigns the LX after the owning address space has terminated or after the owner has used LXFRE to free the LX.

Guideline: IBM® suggests that when using non-system LXs you use reusable non-system LXs.