I suppose, but it is hardly a new one. As I said, I and others have mentioned it here before, and the site has been around for at least five years that I know of (and the revision history says ten). A search on "x86 opcode" should find it quite easily - and by 'quite easily', I mean 'very first thing listed by both Google and DuckDuckGo', at least for the US versions.
In fact, just to make this point,
Let Me Google That For You. See?
http://ref.x86asm.net, right there. They even sell a printed version to go along with the free XML files.
I suspect that part of the problem here was that ARISTOS didn't know the best terms to search on. I am willing to give them the benefit of the doubt on this, actually, as the word 'opcode' isn't something even most programmers would think of.
But hiding it wasn't the point I was trying to make to ARISTOS.
Not doing that search, while not something that will endear ARISTOS to this group, isn't the real issue; the fact is that those listings, while extremely useful, are not a substitute for digging into the
Intel and
AMD manuals.
I will, however, add (shameless self-promotion alert) that
compilerdev page on
"Books and Papers" has a link to a complete
online book on developing assemblers and linkers, which, while dated, should be useful. Also, some of the compiler books cover the topic as well (most notably chapter 7 of
Modern Compiler Design).
Assuming, that is, that by 'create an assembler' the OP means they intend to develop a new assembler (or perhaps debugger), rather than (as has been suggested) wanting to write an assembly language program and looking for either an assembler or an assembly language tutorial. While I am pretty sure that ARISTOS is in fact planning on developing a new assembler, I could be wrong - a lot of people have come in here who had difficulty writing in English (sadly, many of the worst of these were native English speakers), or just didn't know what to ask.
It doesn't seem to be mentioned in the wiki, as I thought it was, so I'll
go correct that oversight right now.