Very useful!Although the idea of a virtual card reader, in my opinion, was pretty stupid,
You could link a virtual card punch to another VM's virtual card reader to send files around! Of course the files had to be 80 bytes fixed record length ...
I worked as a systems programmer on IBM's VM/SP (running on an Amdahl mainframe). Every single program running on the machine was a VM.
Most users had a fairly simple single user OS (called CMS) running in their VM. I ran a multi-user UNIX system called UTS. You could, I think, run any IBM mainframe OS in a VM. In particular, you could run VM/SP itself in a VM which was handy for maintenance and testing.
All the peripherals, punched card machines, printers, hard disks etc were emulated in software.
It was all written in assembler and the source was available (the best quality hand written assembler I have ever seen). The beginners introduction to using CMS simply told you how to run the assembler.
Later the mainframes provided hardware support for VM's with domains etc.
It was very fast. It could support 100 users or so doing real work, on a machine less powerful than a Pi (but then it could do thinks like send a screen of formatted text in a single machine instruction).
Statistics: Posted by jahboater — Sun Mar 02, 2025 9:45 am