When a program requests a module, the system searches for the requested module in various system areas and libraries, in the following order:

1. Modules that were loaded under the current task (LLEs)
2. The job pack area (JPA)
3. Tasklib, steplib, joblib, or any libraries that were indicated by a DCB specified as an input parameter to the macro used to request the module (LINK, LINKX, LOAD, ATTACH, ATTACHX, XCTL or XCTLX).
4. Active link pack area (LPA), which contains the FLPA and MLPA
5. Pageable link pack area (PLPA)
6. SYS1.LINKLIB and libraries concatenated to it through the LNKLSTxx member of parmlib. (Placing modules in the system search order for programs explains the performance improvements that can be achieved by moving modules from the LNKLST concatenation to LPA.)

When searching TASKLIBs, STEPLIBs, JOBLIBs, a specified DCB, or the LNKLST concatenation for a module, the system searches each data set directory for the first directory entry that matches the name of the module. The directory is located on DASD with the data set, and is updated whenever the module is changed. The directory entry contains information about the module and where it is located in storage. (For more information, see the "Program Management" topic in the z/OS MVS Programming: Assembler Services Guide.)

As mentioned previously, LLA caches, in its address space, a copy of the directory entries for the libraries it manages. For modules that reside in LLA-managed libraries, the system can quickly search the directories in virtual storage instead of using I/O to search the directories on DASD.