... this stuff never ran on a '51. It started out with a 68k and moved to various other architectures from that.

Yes, I tried the C code for square root calculation on a '51 simulator just for fun and it still takes quite a while (~800 us @ 12 Mhz on a 1-clocker, but somehow I think the simulator is making that number up. 10000 cycles, come on ?), but that's mainly because there's some bit-shifting involved that would work much better when done in assembly (most compilers are horrible at tasks like "shift two MSBs of variable A into the two LSBs of variable B"), or on a processor that can natively shift 32 bit integers, or by doing it in C in a way that avoids shifting anything larger than 8 bit most of the time.

However, I think it will still be faster than the big chunk that I found.