Exult Forum

I remember that RG used to always say that every Ultima was written from the ground up, and not a single line of code was kept from the previous game.
However I have also heard that Ultima VII was built on a heavily modified version of the U6/WoU engine, that Ultima VIII was built on heavily modified version of that, and that the original version of the Ultima Online client was made from a heavily modified version of the Ultima VIII engine (that last one I've seen confirmed several times.)

Now we should have members of the dev teams from Nuvie, Exult and Pentagram on here, and although you don't have access to the source code, you could probably make a good guess based on the binary decompiliations.
So, taking into account the Ship of Thesus (how much do we have to change the engine before it stops being the same engine?), in your opinion, is there any credibility to the legend that 6, 7 and 8 have the same core engine?

They don't IMO.
Of course there are things they did the same or named the same (gumps, or usecode (used in U7, U8 and U9), but it's not the same engine.

my opinion there is very little that could be in common between u8 and u7. the engines are very different and do most things very differently even if some engine concepts are the same. u7 and u8 use completely different memory mangement systems. U7 does things in an unusual way by putting the cpu into so called "unreal mode" also known as flat real mode while u8 uses a dos extender and runs in "protected mode" as such even doing things like reading a file from disk needs to be done differently in the engines.

I don't know much about u6, but it seems unlikely it has anything to do with u7's engine. u6 has a completely different interface and control system to u7 and u6 has a flat world made of 20x20 pixel tiles compared to u7 having a pseudo 3d world constructed over 8x8 pixel tiles. There doesn't even seem like there would have been much value in prototyping things for u7 in the u6 engine they are too different. prototyping u8 ideas in u7 makes some sense as both engines are pseudo 3d

As Dominus and Colourless Dragon stated, yeah, these games' engines are completely different. You can take RG's word on it.