buddy90> The tiles were compressed because of space concerns, perhaps (why else would you compress stuff?). At any rate, it makes things difficult since if a replacement sprite has more information at maximum compression (than the original), then it can't write over the original since it would also write over other data. It needs to be placed at a different location and pointed to using the pointer in the Pokémon's data chunk, and this is a bit of a hassle. There are tools for it however.
Ryumaru> I'm just talking about "ROM" (i.e. the file. All the data of the game, graphics and code and music is stored there) and "RAM" (specifically, the memory for tiles).
Gameboy games litter the gfx all over the place in the "ROM", but the game program collects the stuff into one place when it is to be showed on the screen. However, the hardware can move gfx data into the video memory so fast that the effect can be used for animation, and maybe extra intermediate (flicker) colors. I think this is done between screen updates though.
At a minimum, it seems you have 256 tiles of space in video memory. As I understand it, sometime during the screen update the hardware can move the tile table pointer 128 tiles forward, giving you 128 old tiles plus 128 new ones.
Let's try to imagine how many tiles we need for the battle screen:
7*7 for opponent.
4*4 for back/self. (Edit, looks like 8*8 or baked in 2X, and stored twice!)
8 for HP bar animation +2 for ends
About 45ish for CAPS font with nums and symbols, and 26 more for small letters (apparently usable at top of screen in names too, not just in the dialog box at the bottom). Edit: Also some apostrophes and accents. 96 total with some unused, and maybe 32 blank chars excluded.
About 10ish for GUI trinkets.
Spawn and attack effects, no clue. Perhaps not all kept in ram at once.
6 for GUI box (which occasionally is used higher up on the screen so it's probably not benefiting from the +128 trick either).
I get <200 for that that stuff (Edit: True if not counting the weirdness going on), so they probably don't do a lot of swapping out unused stuff (Edit: e.g attack splash effects). Anyways, looking at the tile table in an emu would perhaps show the layout and economy.