In the 1990 or so time frame, the IBM Palo Alto Research Center forked a version of mainframe Unix and made it multiprocessing. They referred to it as PAIX, tho

In the 1990 or so time frame, the IBM Palo Alto Research Center forked a version of mainframe Unix and made it multiprocessing. They referred to it as PAIX, though I think it sank into irrelevance and was never released as a product. I was adding async I/O capabilities, the better to match the latent bandwidth of the mainframe I/O channel processors. My PAIX instance was running in a VM partition, while the rest of the machine served hundreds of other users. While debugging my kernel additions in PAIX, I would once in a while crash the entire machine. So much for the vaunted rock solid reliability of mainframes.