IPL Phases in z/OS Mainframe System

Hardware IPL Phase

The volume where the OS is stored is refered to as the SYSRES volume or the IPLable disk.

IRIM Phase

IPL Resource Initialization Model. The IRIM reads the LoADPARM information. The LOADPARM consists of IODF volume number, LOADxx version, IPL prompt feature and the alternate nucleus id.

NIP Phase

Expands the SQA and the extended SQA creates the pageable link pack area and extended PLPA Loads modules into the fixed link pack area and extended FLPA loads modules into Modified LPA and extended MLPA.
Allocates virtual Storage for the Common service area and extended CSA.

MSI Pase - Master Scheduler Initialization

Master Scheduler initialization routines initialize system services such as the system log and communications task, and start the master scheduler itself.
They also cause creation of the system address space for the job entry subsystem.

Subsystem Initialization Phase - JES2 Phase

Subsystem initialization is the process of readying a subsystem for use in the system.