Initialization process

The system initialization process prepares the system control program and its environment to do work for the installation. The process essentially consists of:

System and storage initialization, including the creation of system component address spaces.
Master scheduler initialization and subsystem initialization.

When the system is initialized and the job entry subsystem is active, the installation can submit jobs for processing by using the START, LOGON, or MOUNT command.

The initialization process begins when the system operator selects the LOAD function at the system console. MVS™ locates all of the usable central storage that is online and available to the system, and creates a virtual environment for the building of various system areas.

IPL includes the following major initialization functions:

Loads the DAT-off nucleus into central storage.
Loads the DAT-on nucleus into virtual storage so that it spans above and below 16 megabytes (except the prefixed storage area (PSA), which IPL loads at virtual zero).
Builds the nucleus map, NUCMAP, of the DAT-on nucleus. NUCMAP resides in virtual storage above the nucleus.
Allocates the system's minimum virtual storage for the system queue area (SQA) and the extended SQA.
Allocates virtual storage for the extended local system queue area (extended LSQA) for the master scheduler address space.

The system continues the initialization process, interpreting and acting on the system parameters that were specified. NIP carries out the following major initialization functions:

Expands the SQA and the extended SQA by the amounts specified on the SQA system parameter.
Creates the pageable link pack area (PLPA) and the extended PLPA for a cold start IPL; resets tables to match an existing PLPA and extended PLPA for a quick start or a warm start IPL. For more information about quick starts and warm starts, see z/OS MVS Initialization and Tuning Reference.
Loads modules into the fixed link pack area (FLPA) or the extended FLPA. Note that NIP carries out this function only if the FIX system parameter is specified.
Loads modules into the modified link pack area (MLPA) and the extended MLPA. Note that NIP carries out this function only if the MLPA system parameter is specified.
Allocates virtual storage for the common service area (CSA) and the extended CSA. The amount of storage allocated depends on the values specified on the CSA system parameter at IPL.
Page protects the: NUCMAP, PLPA and extended PLPA, MLPA and extended MLPA, FLPA and extended FLPA, and portions of the nucleus.
Note: An installation can override page protection of the MLPA and FLPA by specifying NOPROT on the MLPA and FIX system parameters.

See Figure 1 for the relative position of the system areas in virtual storage. Most of the system areas exist both below and above 16 megabytes, providing an environment that can support both 24-bit and 31-bit addressing. However, each area and its counterpart above 16 megabytes can be thought of as a single logical area in virtual storage.