This is a 4-bit version of the thing. Each combination of 4-bit numbers (8-bit) gives you a different address in the ROM, last 4 digits give the answer. As you can see, if the first 8 digits are all 1, the answer is a fraction and is undefined.
I have the same problem as you(can't upgrade to 8-bit), but maybe a different mind can use this to create an 8-bit version of the thing.