This program calculates rings with equal radius.
The method is the following:

You can calculate the squares of x or y.
You can compare the sum to a squaretable, so 
you that you know if the radius must be increased.

Compiled with cc65-2.10.1 and should be compatible to
all (cbm)machines with conio support.

Change the #ifdef at the beginning of the program
to #ifndef, and you see a small difference in the listing.

V1.0

Greetings Carlos