How to animate the water as seen in Zelda DX
By Marc Gale
marc@sbirmc.ac.uk

The way I've done it uses the following:


1. Four seperate animation tiles which will cycle around happily.  In 
other words, displaying tile 1, 2, 3, 4, 1 will look good.
2. A byte in RAM somewhere for the frame counter
3. Use a specific single tile number for all water
4. A byte in RAM somewhere for the water animation delay
5. A byte in RAM somewhere for the water animation count


What I did was to ensure that the specified tile number was used 
for all water on the map.  Then, in the V-Blank interrupt, copy the 
appropriate frames' tile from ROM to the tile data area, and then 
update the frame counter.  Of course, checking to see if we're 
ready for the animation first.  A pseudo code for the interrupt 
follows:


VBlank_Int:
        grab the animation counter
        increase it by one
        put it back
        compare the counter to the animation delay value
        if we haven't reached the delay value, return


        reset the animation counter


        grab the frame number
        increase the frame number, resetting it if neccessary


        copy the appropriate tile data from rom into the specified 
                tile to be used for water
fin:


Or something like that.  When I have some replacement graphics 
for my project, I'll release the full source code (any volunteers?) as 
I can't release the project whilst it still uses Zelda's graphics.