HACKER Q&A
📣 rwallace

Stories from Users of Minicomputers and Mainframes


I showed up too late to catch more than the tail end of the minicomputer era. I'd like to learn more about the computers that predated the 8-bit micros I fondly remember.

There is plenty of information on the technology of machines like the VAX, PDPs, 360 etc. Quite a bit also on how they were built. ('The Mythical Man-Month' and 'The Soul of a New Machine' are generally recognized as classics. A lesser-known favorite of mine: 'The Supermen'.) But not so much on how they were used, particularly at the 'coal face' level, so to speak.

This post was inspired by a comment on https://news.ycombinator.com/item?id=45818471

"... at the time I was working on an IBM 360, mostly doing Fortran for scientists running anemometer simulations. The center for this activity was the person in charge of the 360 who could dole out time on the computer.

The power dynamic was something I did not really notice, but in retrospect this was frustrating for the mathematicians/scientist trying to run simulations. They had to queue up and wait.

Then one day a mathematician brought in an Apple II running VisiCalc. His own personal computer. He ran his simulations on that."

One thing I found fascinating here: looking at the clock speeds of 1970s computers, by today's standards, you would expect 'the computer is too slow' to be a big limiting factor. But in the above account, it wasn't. The actual workload was such that an Apple II running VisiCalc was good enough to get the job done. The limiting factors were the actual availability of the 360 (presumably there genuinely was not enough machine time to go around), and the overhead of dealing with the politics of gaining access to it.

What were people's experiences with minicomputers and mainframes? What did they enable, that was more difficult or impossible than before? What were the actual bottlenecks and limiting factors? I'm interested in both direct firsthand accounts, and links to published material in any format.

  👤 PaulHoule Accepted Answer ✓
I took some classes in high school when I was elementary school student where I had access to a PDP-8 which was a pretty crappy machine but could support 1, 2 or 3 terminals with BASIC where you got more memory with fewer terminals. We also had the Colossal Cave ADVENTURE. You had very few registers and it had 12-bit words instead of being based on the 8-bit byte, the "very few registers" made it a leader for interrupt-driven applications because context switches were fast.

I really enjoying using a PDP-11 with RSTS/E, here each user got two 64k address spaces, one for data, one for code, so you got a BASIC experience that was a bit better than the Apple ][ or TRS-80, particularly you could save you work on a hard drive.

In the computer explorers I got to use an IBM 3090 in the later 1980s which was close to the pinnacle of bipolar mainframes running the 370 XA architecture. At this point in time the IBM Mainframes had

https://en.wikipedia.org/wiki/VM_(operating_system)

IBM really struggled to develop timesharing for the 360 and wound up applying virtual machine technology to give every software developer their own copy of a DOS-like operating system. In the early 1970 it was really a struggle to get time on a 360, by the late 1980s VM was everywhere and you could just log in with your 3270 terminal when you wanted to.

Circa 1980 it was easy for a microcomputer user to "look up" to the PDP-11 and think the future might look like that but actually the PDP-11 became obsolete rather quickly because the user had access to only 64k which was the same problem micros were about to have as memory got cheap and you could afford more than 64k of RAM. Digital came out with the VAX which was basically a modern computer, my high school got one by the late 1980s and it was a really capable on machine where you had good compilers for languages like PASCAL which was great for CS education.

👤 MrCoffee7
Mainframes in the early 1980s allowed people to access computer power via interactive terminals instead of sitting at a keypunch machine creating large decks of punched cards to take over to a card reader machine. This was before personal computers were widely available at an affordable price, so office workers did not have a personal PC sitting at their cubicle at that point. The computer monitors at that point were not full color displays - they just had bright green text against a black screen. The language used on the mainframe was COBOL. If your computer job required tapes, there would be a short wait for someone to manually mount the correct tapes needed. You would need to manually go to a room near the computer to sort through printouts to pick up your specific printout of your job yourself, where the job was printed on wide sheets of white and green paper in a font that looked like something from a high-speed dot matrix quality printer. Your printout stack might have been especially heavy and thick if your program terminated abnormally with an ABEND type of error and a sysdump printed out. For references, see the books "Mainframes, Computing on Big Iron" by Patrick Stakem and "IBM's 360 and Early 370 Systems" by Emerson W. Pugh.