Not quite as virtual as he thought

It's the 1980s, and this IT contractor pilot fish is working on a big project to install a utility's new accounting system in the days before ERP.

"The project wasn't going well, as the utility's accountants hated the consultants from the major consulting firm managing the project, and refused to provide them with information," says fish.

"Eventually the project was suspended and the consulting firm was fired. But I had been doing a good job, actually managing other coders, so the utility wanted to retain me for whenever the project was resumed."

That leaves fish without any pressing deadlines, but with an opportunity for the first time to explore an IBM mainframe virtual-machine operating system, VM/CMS, which gives each user a virtual computer all their own -- virtual CPU, virtual drives, "just like a real mainframe computer for your very own," fish says.

He soon decides to focus on the REXX scripting language, and after a few days he's getting the hang of programming, logic flow and I/O from a terminal screen.

And to test out his new grasp of the language, he writes a simple program to determine if a number is a prime, with no divisors except 1 and the entered number itself.

The program works fine for the first few numbers he tries -- 11, 37 and 113. Then fish enters a seven-digit number. Nothing happens for the first few minutes, but that's no surprise, since it's a scripting language and the program doesn't use a particularly efficient algorithm.

Besides, it's no big deal -- fish is the only user of this virtual computer, right?

Then the phone rings. It's an operator at the mainframe site. Is there any chance you have a program in an accidental endless loop? he asks fish.

"Apparently the mainframe was completely unresponsive -- for everyone, not just my virtual machine," says fish. "Unbelievably, there was no limit on the amount of real CPU that a REXX process could suck up, up to 100 percent!

"Well, yes, I said, it was certainly possible that I had an endless loop. He asked, 'So I can terminate this process?' Please do, I said. I thought it was better not to mention that it wasn't an endless loop, just a very, very, very long loop. Could have happened to anybody, right?

"Fortunately they came up with real work for me in just a few more days!"

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