Starting z/OS:

How often do you start or reboot the operating system for your personal computer or notebook? Probably at least once a day, maybe more often, if you want to install upgrades or security patches. Depending on the work you are doing, you might reboot almost automatically, without doing more than a few save operations. Starting z/OS® is, in a very general sense, a similar process, but is done far less frequently and only with much careful planning beforehand.

z/OS initialization, or an initial program load (IPL), is the act of loading a copy of the operating system from disk into the processor's real storage and executing it. This process essentially consists of:

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

z/OS systems are designed to run continuously with many months between reloads, allowing important production workloads to be continuously available. Change is the usual reason for a reload, and the level of change on a system dictates the reload schedule. For example:

A test system may be IPLed daily or even more often.
A high-availability banking system may only be reloaded once a year, or even less frequently, to refresh the software levels.
Outside influences may often be the cause of IPLs, such as the need to test and maintain the power systems in the machine room.
Sometimes badly behaved software uses up system resources that can only be replenished by an IPL, but this sort of behavior is normally the subject of investigation and correction.

Many of the changes that required an IPL in the past can now be done dynamically. Examples of these tasks are:

Adding a library to the linklist for a subsystem such as CICS®
Adding modules to LPA

Shutting down z/OS happens as rarely as an IPL. To shut down the system, each task must be closed in turn, in the correct order. Today's z/OS installations use an automation package to control and execute this process. Shutting down the system usually requires a single command, which results in the removal of most tasks except for the automation task itself. The automation task is closed manually, followed by any commands needed to remove the system from a sysplex or serialization ring.