After Compaq bought Tandem, they said they were going to port NonStop from MIPS to the Alpha[0], but then cancelled that for Itanium[1]. I'm not sure if NonStop was ported/expanded from 32bit to 64bit when it was running on MIPS.

[0] https://www.scmp.com/article/256948/compaq-port-himalaya-alp...

[1] https://www.informationweek.com/it-leadership/compaq-to-aban...