The z/OS® system libraries contain customization data that is essential to the running of the z/OS system. Some of these data sets are related to IPL processing, while others are related to the search order of invoked programs or to system security, among other functions.

SYS1.PARMLIB

SYS1.PARMLIB is a required partitioned data set that contains control parameters for the whole system. PARMLIB is an important data set in a z/OS operating system, and can be thought of as performing a function similar to /etc on a UNIX® system. The purpose of PARMLIB is to provide many control parameters in a pre-specified form in a single data set, thus minimizing the need for the operator to enter parameters during the initialization (IPL) process.

SYS1.PARMLIB must reside on a direct access volume, which can be the system residence volume. Parameters are specified in both IBM-supplied and installation-created members. All parameters and members of the SYS1.PARMLIB data set are described in z/OS MVS™ Initialization and Tuning Reference, SA22-7592.

SYS1.LINKLIB

SYS1.LINKLIB contains many of the executable code, also known as modules, for z/OS components and utilities. By default, SYS1.LINKLIB is the first data set in the linklist, which is a collection of libraries containing system and user code.

One of the modules in SYS1.LINKLIB is MSTJCL00, which contains the initial job control language (JCL) statements that start the master scheduler subsystem. The master scheduler subsystem establishes communication between the operating system and the primary job entry subsystem, which is either JES2 or JES3.

System libraries for the link pack area (LPA)

The LPA is part of an address space's common area storage, and is divided into pageable, fixed, and modified sections:

• Libraries specified in SYS1.LPALIB, the LPALSTxx, or PROGxx parmlib members are loaded into pageable LPA (PLPA). These libraries contain modules for read-only system programs, along with any read-only reenterable user programs selected by an installation that can be shared among users of the system.
• IEAFIXxx members specify the modules loaded into fixed LPA (FLPA). This area should be used only for modules that significantly increase performance when they are fixed rather than pageable. The best candidates for the FLPA are modules that are infrequently used, but are needed for fast response. Modules placed in FLPA are always in central storage.
• IEALPAxx members specify the modules loaded into modified LPA (MLPA). The MLPA is used to contain reenterable routines from APF-authorized libraries that are to be part of the pageable extension to the link pack area during the current IPL. Note that the MLPA exists only for the duration of an IPL.

Link pack area (LPA) modules are loaded in common storage, and shared by all address spaces in the system. Because these modules are reentrant and are not self-modifying, each can be used by a number of tasks in any number of address spaces at the same time.

Modules placed anywhere in the LPA are always in virtual storage. To prevent their pages from being stolen, LPA modules must be referenced very often. When a page in LPA (other than in FLPA) is not continually referenced by multiple address spaces, it tends to be stolen.

SYS1.PROCLIB

SYS1.PROCLIB is a required partitioned data set that contains the IBM-supplied JCL procedures used to perform certain system functions. The JCL can be for system tasks or for processing program tasks invoked by the operator or the programmer.

One JCL procedure in SYS1.PROCLIB is the JES2 cataloged procedure, which defines job-related procedure libraries.

SYS1.NUCLEUS

SYS1.NUCLEUS contains the basic supervisor modules of the system.