Also note that IBM mainframes can run 64-bit Linux just fine. Indeed, IBM's been marketing its LinuxONE mainframe line as a z series machine that doesn't run z/OS at all.
We're talking Mainframe system designs and code from the 70s and 80s. No they aren't running 64-bit linux. I think you guys need to re-read my post. The legacy systems on Y2K had none of these features.