I am doing a little testing of some stuff, and I can't enumerate the fullscreen display modes on my laptop before creating a display. If I call this code after I create a display it runs fine no problem.

   printf("Fullscreen modes :\n");
   for (int i = 0 ; i < al_get_num_display_modes() ; ++i) {
      ALLEGRO_DISPLAY_MODE mode;
      if (al_get_display_mode(i , &mode) == &mode) {
         printf("Mode #%i w,h = %d , %d , format = %d , refresh_rate = %d\n" ,
                i , mode.width , mode.height , mode.format , mode.refresh_rate);
      }
   }

Debugging session :

c:\ctwoplus\progcode\allegro5\test\Multisampling>gdb minfo.exe
Program received signal SIGSEGV, Segmentation fault.
0x6f128abe in _al_d3d_get_num_display_modes (format=format@entry=0, refresh_rate=refresh_rate@entry=0, flags=0) at C:\mingw\LIBS\A5GIT\allegro\src\win\d3d_disp.cpp:2896
2896          num_modes = _al_d3d->GetAdapterModeCount(adapter, (D3DFORMAT)d3d_formats[j]);
(gdb) p _al_d3d
$1 = (LPDIRECT3D9) 0x0
(gdb) bt
#0  0x6f128abe in _al_d3d_get_num_display_modes (format=format@entry=0, refresh_rate=refresh_rate@entry=0, flags=0) at C:\mingw\LIBS\A5GIT\allegro\src\win\d3d_disp.cpp:2896
#1  0x6f11de54 in win_get_num_display_modes () at C:\mingw\LIBS\A5GIT\allegro\src\win\wsystem.c:394
#2  0x0040392e in main ()
(gdb)

A display hasn't been created, so the d3d or opengl driver hasn't been setup yet, but it calls the _al_d3d driver while it is still null and crashes.

Hasn't this sort of been a lingering design issue? If the driver isn't set up before creating the display then any functions that rely on it will fail. Which driver to set up though? DX or OGL? That isn't determined until the display is created, currently.

Is D3D required then on Windows?

Here's another issue - take a look at what happens if I then create an OpenGL window :

c:\ctwoplus\progcode\allegro5\test\Multisampling>ms.exe
Monitor info :
Adapter #0 : x1 y1 x2 y2 = 0 0 1280 800
Fullscreen modes :
Mode #0 w,h = 640 , 480 , format = 23 , refresh_rate = 60
Mode #1 w,h = 800 , 600 , format = 23 , refresh_rate = 60
Mode #2 w,h = 1024 , 768 , format = 23 , refresh_rate = 60
Mode #3 w,h = 1280 , 800 , format = 23 , refresh_rate = 60
Mode #4 w,h = 640 , 480 , format = 13 , refresh_rate = 60
Mode #5 w,h = 800 , 600 , format = 13 , refresh_rate = 60
Mode #6 w,h = 1024 , 768 , format = 13 , refresh_rate = 60
Mode #7 w,h = 1280 , 800 , format = 13 , refresh_rate = 60
New window x,y = 2147483647,2147483647
INT_MAX = 2147483647
Fullscreen modes after setting OpenGL display :
Mode #0 w,h = 640 , 480 , format = 0 , refresh_rate = 60
Mode #1 w,h = 640 , 480 , format = 0 , refresh_rate = 60
Mode #2 w,h = 640 , 480 , format = 5 , refresh_rate = 60
Mode #3 w,h = 640 , 480 , format = 5 , refresh_rate = 60
Mode #4 w,h = 640 , 480 , format = 8 , refresh_rate = 60
Mode #5 w,h = 640 , 480 , format = 8 , refresh_rate = 60
Mode #6 w,h = 800 , 600 , format = 0 , refresh_rate = 60
Mode #7 w,h = 800 , 600 , format = 0 , refresh_rate = 60
Mode #8 w,h = 800 , 600 , format = 5 , refresh_rate = 60
Mode #9 w,h = 800 , 600 , format = 5 , refresh_rate = 60
Mode #10 w,h = 800 , 600 , format = 8 , refresh_rate = 60
Mode #11 w,h = 800 , 600 , format = 8 , refresh_rate = 60
Mode #12 w,h = 1024 , 768 , format = 0 , refresh_rate = 60
Mode #13 w,h = 1024 , 768 , format = 0 , refresh_rate = 60
Mode #14 w,h = 1024 , 768 , format = 5 , refresh_rate = 60
Mode #15 w,h = 1024 , 768 , format = 5 , refresh_rate = 60
Mode #16 w,h = 1024 , 768 , format = 8 , refresh_rate = 60
Mode #17 w,h = 1024 , 768 , format = 8 , refresh_rate = 60
Mode #18 w,h = 1280 , 800 , format = 0 , refresh_rate = 60
Mode #19 w,h = 1280 , 800 , format = 5 , refresh_rate = 60
Mode #20 w,h = 1280 , 800 , format = 8 , refresh_rate = 60

c:\ctwoplus\progcode\allegro5\test\Multisampling>

The modes returned by the driver change.

Edit
Oh yeah, and today I was messin around and I managed to come up with an invisible allegro window using ALLEGRO_FRAMELESS and al_create_display(0,0). It worked with D3D anyway, didn't try OpenGL, but it could be used to setup a window (and the driver) so you could enumerate the modes available for your intended driver before displaying anything.

Oh, okay. Wasn't thinking that al_set_new_display_flags could be used to alter the results of al_get_display_mode.

Edit
Yeah, that works.

How could I compile allegro without DX? Ie, say I wanted to trim down my libs and programs by removing it, how do I do that?

Sorry, miscommunication. All I meant to say is that the number and format of modes differs whether you use D3D or OpenGL to enumerate it. So that makes a difference as to which driver should be in use before a display is created. But as you said, enumerating modes for a specific backend is as simple as calling al_set_new_display_flags. That is not intuitive to me. It seems like more global mystery state.