On the IBM 360/20 there was a blue button on the CPU console labeled “IPL.” On the 370/158 you initiated an IPL using the integrated operator console (a 3270 device with a light pen). The end result was the same – the IPL function of the processor was invoked.
The device to be used for the IPL process is specified by the operator prior to invoking the IPL function. It might have been specified using the dials and switches or it may have been specified on the operator console.
When the IPL function is initiated the processor issues a read command to the IPL device. This read command reads 24 bytes of data. Yes! That’s all, 24 bytes. But this is enough to get the process started.
It can’t be just any 24 bytes of data. It has to be 24 bytes of very specific data. To understand the IPL process we have to understand two hardware data structures – the Program Status Word (PSW) and Channel Command Word (CCW). This is because the 24 bytes read by the IPL process must be an eight-byte PSW followed by two eight-bytes CCWs.
In the next section we will explore the PSW so we have a better understanding of how to get our initial program loader up and running.