Explanation of the A=INITIALIZE SYSTEM CONTROL PROGRAM, A3 Field

This field specifies the operator load function to IPL the MVS™ operating system.

Selecting the operator load function causes the hardware to read an IPL (initial program loader) program into storage from the system residence volume. For this reason, loading and initializing the system is often called the “IPL procedure” or just “IPL”. Likewise, “IPLing” the system means loading and initializing the system.

The IPL program is what actually loads the system software; if the IPL program does not get into storage or does not receive control properly, the entire load process stops and the processor pauses. If the IPL program does not finish properly, it puts the system into a disabled wait state with an error code in the low-order 12 bits of the program status word (PSW). To continue loading the system, display the PSW, note the error code, and follow the instructions for that code given in z/OS MVS System Codes. The processor operations documentation tells you how to display the PSW.