I would like to know how the collision detection was done in The Legend of Zelda: A Link To The Past.

The game is 16x16 tile based, so how did they do the tiles where only a quarter or half of the tile is occupied? Did they use a smaller grid for collision detection like 8x8 tiles, so four of them make one 16x16 tile of the texture grid?

But then, they also have true half tiles which are diagonally cut and the corners of the tiles seem to be round or something. If Link walks into tiles corner he can keep on walking and automatically moves around it's corner. How is that done?

I hope someone can help me out here.