[retrac]

TSS and the other virtualized OSes on the PDP-8 are really quite an amazing technical achievement. Particularly the limited hardware required to virtualize the PDP-8. The PDP-8's memory is not paged, but in banks of 4 kW with banking handled by IO instructions. If you trap on I/O, everything can be lifted into software. OSes would virtualize things like the serial interface, so unmodified programs would run correctly in the virtual machine. All shockingly modern and clearly inspired by VM/370 only a few years after that had been invented. But early versions of IBM's VM required a machine about 10x as large as the PDP-8 just to boot and handle a handful of users! Something like a dozen users could be supported on a machine with 20 kilowords of RAM running around 100,000 instructions per second, each user presented with a full virtual PDP-8 with 32 kW of memory, a serial interface and a block storage interface, etc. And the entire operating system kernel fit in two banks of memory, 8 kW or 12 kilobytes.