The CDC 6000 series used a 10 way multithreaded
architecture
virtualized as 10 independent 12-bit processors for I/O. No
interrupts necessary there either. But then, back in 1964, I don't
know if the word "multithreaded" had been invented yet.
This also apparently appears in the Honeywell 1800. Need to dig out
info on that. I knew the TX-2 and Xerox Alto implemented multiple
hardware contexts. I'm pretty sure Mark Smotherman covers this on
his site, which Brad mentioned a day or two ago.