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Old 24 September 2013, 20:09   #442
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Join Date: Jun 2012
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Originally Posted by dlfrsilver View Post
The tilemap system never have direct access to the tiles. The main processor is requesting the tiles display by the X/Y hexa coordinates in VRAM. This is why you can make graphic operations that an amiga just won't be able to do.

Look at Megadrive, CPS-1, CPS-2 games. The 68000 take care of the game logic, and is constantly pushing tiles coordinates (bytes or words) to the graphic chips, and the same happens for the sprites.

The tiles in themselves are stored in ROM, they take their weight as you said, but it was not my point.

the amiga is moving the tiles in RAM "really", when a console or a coin-op machine is only moving tile references in VRAM. CPS system and megadrive are helped by a much flexible and powerful hardware functions.

Tilemap system are not magic, they just remove a huge burden off the back of the main processor. And should i add that the CPS-1 for instance has the ability for example to perform hardware calculation (yes, capcom hardware engineers have actually moved this function inside the PPU chip.), this means that the board can process calculations that would make the 68000 sink if it had to do it itself.
You are right that it takes a burden off of the CPU. (Though it's not as huge as all that, really, but then, every little bit counts at <10 MHz.) But if the VDP didn't have access to the tile definitions as you claim, it would have no way of drawing the tiles. It has to fetch tile data from memory and work it into the picture just like any other kind of video display, because those pointers have to point to something.

And the Megadrive/Genesis VDP doesn't access tile definitions from ROM. The SNES doesn't either. (The NES usually did, though even then they were mapped into the PPU's memory, which allowed some carts to put RAM there instead for freely-editable tiles.) The Genesis, like the SNES, the SMS, the PCE, the Gameboy, and so on, gets all of its graphics data from the internal video RAM, including the tile definitions. They are stored in ROM on the cartridge (because, well, they have to be,) but the CPU has to copy them into VRAM first before the VDP can use them.
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