System IPL: Sequence and key controls

The initialization process begins when the system programmer or operator selects the LOAD function at the Hardware Management Console (HMC).

To successfully IPL z/OS®, the system programmer needs to supply the following information:

The device address of the IPL volume
The LOADxx member that contains pointers to system parameters
The IODF data set that contains the configuration information
The device address of the IODF volume

z/OS locates all usable central storage that is online and available, and begins creating the various system areas.

Not all disks attached to a CPU have loadable code on them. A disk that does is generally referred to as an "IPLable" disk, and more specifically as the SYSRES volume.

IPLable disks contain a bootstrap module at cylinder 0 track 0. At IPL, this bootstrap is loaded into storage at real address zero and control is passed to it. The bootstrap then reads the IPL control program IEAIPL00 (also known as IPL text) and passes control to it. This in turn starts the more complex task of loading the operating system and executing it.

Figure 1. IPLing the machine

After the bootstrap is loaded and control is passed to IEAIPL00, IEAIPL00 prepares an environment suitable for starting the programs and modules that make up the operating system, as follows:

It clears central storage to zeros before defining storage areas for the master scheduler.
It locates the SYS1.NUCLEUS data set on the SYSRES volume and loads a series of programs from it known as IPL Resource Initialization Modules (IRIMs).
These IRIMs begin creating the normal operating system environment of control blocks and subsystems.

Some of the more significant tasks performed by the IRIMs are as follows:

Read the LOADPARM information entered on the hardware console at the time the IPL command was executed.
Search the volume specified in the LOADPARM member for the IODF data set. IRIM will first attempt to locate LOADxx in SYS0.IPLPARM. If this is unsuccessful, it will look for SYS1.IPLPARM, and so on, up to and including SYS9.IPLPARM. If at this point it still has not been located, the search continues in SYS1.PARMLIB. (If LOADxx cannot be located, the system loads a wait state.)
If a LOADxx member is found, open and read information including the nucleus suffix (unless overridden in LOADPARM), the master catalog name, and the suffix of the IEASYSxx member to be used.
Load the operating system's nucleus.
Initialize virtual storage in the master scheduler address space for the System Queue Area (SQA), the Extended SQA (ESQA), the Local SQA (LSQA), and the Prefixed Save Area (PSA). At the end of the IPL sequence, the PSA will replace IEAIPL00 at real storage location zero, where it will then stay.
Initialize real storage management, including the segment table for the master scheduler, segment table entries for common storage areas, and the page frame table.

The last of the IRIMs then loads the first part of the Nucleus Initialization Program (NIP), which invokes the Resource Initialization Modules (RIMs), one of the earliest of which starts up communications with the NIP console defined in the IODF.

During the NIP stage, the system might prompt the system programmer or operator to provide system parameters that control the operation of z/OS. The system also issues informational messages about the stages of the initialization process. IEASYSnn, a member of PARMLIB, contains parameters and pointers that control the direction that the IPL takes. The system programmer or operator may alter these parameters as necessary.

IEASYSnn, a member of PARMLIB, contains parameters and pointers that control the direction that the IPL takes. Figure 2 illustrates partial content of an IEASYSxx member.

Figure 2. Partial listing of IEASYS00 member

----------------------------------------------------------------------------
File  Edit  Edit_Settings  Menu  Utilities  Compilers  Test  Help            
-------------------------------------------------------------------------------
EDIT       SYS1.PARMLIB(IEASYS00) - 01.68                  Columns 00001 00072
Command ===>                                                  Scroll ===> CSR  
*********************************** Top of Data ******************************
000001 ALLOC=00,
000002 APG=07,
000003 CLOCK=00,
000004 CLPA,
000005 CMB=(UNITR,COMM,GRAPH,CHRDR),
000006 CMD=(&CMDLIST1.),
000007 CON=00,
000008 COUPLE=00,  WAS FK
000009 CSA=(2M,128M),
000010 DEVSUP=00,
000011 DIAG=00,
000012 DUMP=DASD,
000013 FIX=00,
000014 GRS=STAR,
000015 GRSCNF=ML,
000016 GRSRNL=02,
000017 IOS=00,
000018 LNKAUTH=LNKLST,
000019 LOGCLS=L,
000020 LOGLMT=999999,
000021 LOGREC=SYS1.&SYSNAME..LOGREC,
000022 LPA=(00,L),
000023 MAXUSER=1000,
000024 MSTRJCL=00,
000025 NSYSLX=250,
000026 OMVS=&OMVSPARM.,

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.
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. An installation can override page protection of the MLPA and FLPA by specifying NOPROT on the MLPA and FIX system parameters.

To see information on how your system was IPLed, you can issue the D IPLINFO command, as Figure 3 shows.

Figure 3. Output of the D IPLINFO command

D IPLINFO
IEE254I  11.11.35  IPLINFO DISPLAY 906
 SYSTEM IPLED AT 10.53.04 ON 08/15/2007
 RELEASE z/OS 01.07.00    LICENSE = z/OS
 USED LOADS8 IN SYS0.IPLPARM ON C730
 ARCHLVL = 2  MTLSHARE = N
 IEASYM LIST = XX
 IEASYS LIST = (R3,65) (OP)
 IODF DEVICE C730
 IPL DEVICE 8603 VOLUME Z17RC1