TTM refers to "translation table memory" (or mapping, I forget which).

I could explain it here, but I'd simply be paraphrasing the document
that I read, which is:

http://www.tungstengraphics.com/mm.pdf

EXA does not have an official expansion, AFAIK.  The XA signifies
"acceleration architecture" as it does with XAA.  The hermeneutics of
the E are, however, only known to a few practioners of esoteric black
arts.  I prefer to call it "Excellent" a la "Bill and Ted's Excellent
Adventure".

And never worry about asking an "obvious" question of me, as I'm
"guilty" of that fairly often myself. ;)

Regards,

--Blair