Why the reset using A8? If you do this your wave can have only 256 bytes to draw your wave patterns.

To reset at 512 you'll have to use a 10 bit counter (at least). 9 bits to count 0-511, the 10th bit to reset the counter. A 4040 (cmos) or 74hct4040 (ttl) give you a 12 bit counter in a 16 pin package.

hth

orlando