Both AS/400 and System/370 have a so called language environments.
And yes I call them mainframes, because it is as I always heard people referring to them during my Summer job back in the day, so the name stuck with me even it isn't correct.
Language environment in IBM parlance is closer to "ABI" than "VM", or "sandbox" in the rest of the world. It's a common set of idioms to allow two languages (generally ASM and a higher level language like C) to call eachother and interoperate.
AS/400 though does use hardware heavily in it's isolation model, going so far as to have a custom PowerPC variant currently that adds tagged memory.