You need two zip files
	EMACS21_2.ZIP - the source tree as provided by FactSet
	EMACS21_2_IVMS.ZIP - the updates for OpenVMS/I64

You may want to look at two text files (from EMACS21_2_IVMS.ZIP)
	EMACS_ON_I64.TXT - this file
	BUILDING_EMACS_21_FOR_VMS.TXT - instructions as provided by FactSet

Unzip the files. Two sub-directories are created, EMACS.DIR and PLAY.DIR. I have
no idea what PLAY is good for.

Follow the BUILDING_EMACS_21_FOR_VMS instructions.

For example, a log of what I did:

$ unzip emacs21_2.zip
$ unzip emacs21_2_ivms.zip      ! and A to "overwrite": create a new version
$ define /tran=(conc,term) /job axpa $1$DGA3841:[hb.]
$ set def axpa:[emacs.build]
$ del/symbol/global cc		! it will not build with strict ansi setting 
				! also, don't use HP C V7.2-001
$ del/symbol/global ed		! maybe other global symbols need to go as well
$ set def [.tradcpp]		! the "do this once" stuff
$ mms clean
$ mms
$ set def [-]
$ @[-.emacs212_3]configure --with-tcpip=YES --with-x=YES --prefix=AXPA:[] --startupdir=AXPA:[STARTUP]
$ mms/ignore=warn clean		! not really necessary, here
$ mms/ignore=warn		! yes, there are %ILINK-W-MULDEFs and other warnings
$!  and the build aborts with "%MMS-F-ABORT, for target XMENU.OBJ...", just give it another try
$ mms/ignore=warn 
$ mms/ignore=warn install

Done.

For the terminal version
$ def TERM vt300		! my DECterm says so
$ mcr [.vms]temacs.exe -map [.vms]temacs.dump

or

$ set displ/cre/node=<ip-node-addr>/transp=tcpip
$ define display "<ip-node-addr>:0.0"
$ mcr [.vms]temacs.exe -map [.vms]temacs.dump

Emacs identifies itself with the alpha prefix: alpha-dec-vmsBBV. I didn't
bother to edit CONFIGURE.COM to get it right. That command procedure longs for a
rewrite, anyway.

In DECwindows mode I sometimes get an emacs message window

 Warning: past 85% of memory limit
 Killing some buffers may delay running out of memory.
 However, certainly by the time you receive the 95% warning,
 you should clean up, kill this Emacs, and start a new one.

I think emacs should not do its own memory management: it should not use its own
malloc and friends. There should be a macro or configure option for using the 
OS's memory routines. However, I never tried to disable that.