It was 50 years ago this month that IBM released the first modern mainframe, the System/360. The System/360 was notable for being the first series of systems built with interchangeable parts, rather than being custom-built. It’s also notable because its direct descendants are still in production: In the 1970s, it became the System/370, the System/390 in the 90s, and the z series today. The systems originally ranged from 1 MHz to 50 MHz in speed, and came with anywhere from 8 KB to 8 MB of RAM. To put that in perspective, the low-end model was comparable in power to an early Apple II desktop computer from 1977, and the high-end model was comparable in power to the 486 PCs we ran Windows 3.1 on in the 1993-94 timeframe. Or you could compare it to one of my souped-up Amigas, if you prefer (I do). But the same software that ran on the low-end model would run on the high-end model, and there’s a pretty good chance that software from the 1960s will run on a modern Z series mainframe today, with little to no modification.
Twenty years ago this architecture was supposed to be on its way out, but it never really went away. IBM keeps modernizing it, so I expect z/OS has a long life ahead of it. It’s entrenched, and when technology gets entrenched, there’s no getting rid of it.
There isn’t much new, young mainframe expertise in training these days, and it turns out there are certain jobs that mainframes do better than smaller PCs do. Most large companies have at least one mainframe, and it’s not going anywhere, but the people who can care for it and feed it are retiring fast. If you want some job security, you can do a lot worse than learning everything you can about IBM Z series mainframes in addition to the other things you know.