I am using page flipping and whenever I load sprites into VRAM, so I can do fast VRAM->VRAM blits, the sprites appear scrambled. RAM->VRAM blits in these modes work fine.
The sprites just look like static on a TV screen, as if the bitmap pointer is pointing into the wilderness or something.
Under Windows, everything looks fine though, but when compiling for DOS (using DJGPP) it won't work. I am using Allegro 4.2.0 and am limited to this release, so it won't be able to upgrade for me.
I have written a function that allows me to upload sprites into video memory:
It is called with a pointer to a normal memory bitmap and is supposed to return with the pointer to the video bitmap.
I am using a video mode of 320x240 with 8-bit colours.
Under DOS, Allegro is using the Mode-X driver for this purpose (automatically set via GFX_AUTODETECT). It reports a virtual screen size of 320x819, regardless of which call I use of these:
if (set_gfx_mode(GFX_AUTODETECT, 320, 240, 0, 240 * num_pages) != 0)
if (set_gfx_mode(GFX_AUTODETECT, 320, 240, 0, 0) != 0)
When I try triple buffering under DOS, there seems to be an error when loading sprites into VRAM. Again, under Windows, everything works fine.
Note: I am using DOSBox for tests of my code. I tried exupdate.c, exflip.c and ex3buf.c to see if DOSBox was responsible for this and every one of them worked fine. But they don't do any VRAM->VRAM blits as far as I know.
When trying to execute the DOS DJGPP executable under XP, the same error occurs -> scrambled sprites.
Now I suspect anything to be wrong with my code that does not show on Windows.
Thanks for any advice.