VMS (officially OpenVMS; I hated that marketing name when it was first
proposed and I hate it now) is still alive and supported by a company
called VMS Software, Inc. (VSI). Here is a pointer to their document
OpenVMS Programming Concepts, Volume II, which describes the CLE in
detail:
I think it's worth mentioning that the OpenVMS Hobbyist Program is still alive and well and recently began supplying x86_64 licenses to hobbyists, so if you have a reasonably modern amd64 system, you can run it under QEMU.