it doesn't matter what resolution I choose, it doesn't matter if I choose windowed or fullscreen, the game always launches in "classic" mode, even if I choose the first option to launch the game "Diablo".

Any clue?

Have you tried just launching the game from the file itself? Also, I'm unsure if the game even generates one, but go into the game's file directory and maybe link a log file if you can spot one. Upload it to dropbox, or mediafire or something. I'm sure the devs could use the help.

It will LOOK like "classic" mode, mind you. Because they didn't remaster the graphics, or change the aspect ration from 4:3 to 16:9 or something else.

What they've done is added a SCALER feature which allows, for example, the old, svga screen to be stretched to fit whatever screen size you're using, including being antialiased (though the graphics remain pixelated, because they're 2D bitmaps, not 3D forms!).

You may not even be able to tell the difference if you can run "old" Diablo on your current setup, and if your monitor or graphics card is fully capable of hardware scaling. On the other hand, if your system is NOT capable of that, "classic" Diablo will appear as a tiny postage stamp on your screen, while "new" Diablo will fill the screen if set up properly.

It simply upscales the 640x480 to fill your screen without changing aspect, so you should just expect a slightly blurry version of the classic game running at 20fps. Is that not what you are seeing?

By the way, having watched a fascinating video of a speech given a couple of years back by David Brevik (who was the main guy behind Diablo), I understand better where the 20 frame per second "limitation" comes from.

Apparently, he wrote the game as a turn-based game, but with different "frame lengths (1x, 2x, 10x, etc) for various tasks in a "turn, and a limitation on how many "frames" per turn were permitted. Blizzard asked him to turn his turn-based game into a real-time game, and he did so by simply keeping the same mechanism but allowing a constant flow of 20 "frames" per second, happening simultaneously, instead of "one at a time." It allowed him to transform his turn-based role-playing game (in true Rogue form) into a real-time game over a single weekend!

So, the 20-frames-per-second limitation is actually so fundamental to the game's functioning that you could not bypass it without totally rewriting the game logic from scratch.

I had not known that. But I do now. :)

For DevilutionX we are currently working on separating the render and hopefully we can get it decoupled from the game logic so that it won't be locked to the same fps. It's not uncommon for games to have different tick rates for different parts of the game and indeed I believe this is already true for some parts of the system.