With a basic understanding of the PSW and the I/O process we can now explore the IPL process. The IPL is initiated by the operator. The device address of the IPL device is selected by the operator. When the IPL key is pressed the CPU initiates a read command to the specified IPL device.
The initial read is executed by the channel just as if a Start I/O command had been issued. An “implied” CCW is generated for this read command. The protect key value for the CAW is zero. The implied CCW is a read command with a data address of zero and a length of 24. The Command Chain and Suppress Incorrect Length bits are set. The implied CCW address is zero so when the CCW command successfully completes a new CCW will be fetched from location 8 (because the Command Chain flag is set).
When the device signals Device End for the last CCW in the chain and there are no exception conditions a new PSW is loaded from locations 0-7.
When the PSW that is loaded is in BC mode, the I/O address of the IPL device is stored in locations 2 and 3. When in EC mode the device address is stored in locations 186-187.
The information read into locations 8-15 and 16-23 by the IPL process should contain CCWs for reading additional information into memory.