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Re:State of the OpenVMS Port to x86_64 [pdf] by WallWextra on Wednesday October 12, 2016 @11:17AM (#12693624)
I do believe that MACRO in the slides refers to MACRO-11, a PDP assembler. Or maybe MACRO-32, for VAX. In either case, they have been compiling this "assembly" language for Alpha (and now I guess Itanium) since VMS has run on that architecture.
Re:State of the OpenVMS Port to x86_64 [pdf] by ch_123 on Wednesday October 12, 2016 @12:21PM (#12694220)
That would be MACRO-32, the VAX assembler/assembly language dialect. As a considerable part of the VMS kernel is written in VAX assembly, they treat it as a high level language and compile it to Alpha, IA64 and now x86-64.

Other parts of the kernel are written in C (newer code) and BLISS (older code). See https://en.wikipedia.org/wiki/BLISS