Booting an IBM MP 3000 S/390 System

On Thu, 6 Aug 2015, geneb wrote: "Why CAN'T the operating system have full functionality during booting?" I had an interesting conversation almost 30 years ago with a published expert on operating systemes and C programming, when he was bothered by why IO.SYS/IBMBIO.COM and DOS.SYS/IBMDOS.COM had to be in specific places on the drive. "Booting" is of course short for "bootstrapping", which is a multi-hundred year old term for a obviously ridiculously impossible task: "pulling yourself up by your bootstraps". I had always thought that that derived from Baron Von Munchausen, but a little research turns up that the baron had lifted himself and his horse out of the swamp by his pigtail, not his bootstraps. It wasn't until early 1800s that "bootstrapping" became the iconic example. The reason that IPL is called "booting" is because it is such an obviously ridiculously impossible task. "You can't use the operating system to load the operating system." Obviously it is simplest if somebody (or machine) outside, loads the code into memory, and then triggers a GOTO. Which is cheaper, or more reliable, a "trained" operator, or a smaller external machine? The really clever way, though, was to toggle in, or have a little ROM, to load a TINY bit of stored code ("boot sector") into RAM, GOTO it, and it could contain enough code to load a bigger chunk, which could have plenty of code to load the rest. Why not just put the OS in ROM? That would require more ROM, would make bug-fixes more difficult, and would make it more difficult to modify the OS to add new features, such as security holes.