I happened to find a rather interesting discussion of Ultima 3's mechanics, based on a disassembly, elsewhere on the Internet.

https://gamefaqs.gamespot.com/boards/562659-ultima-iii-exodus/80667133

Some of the interesting things I learned:
* In the DOS version, Alchemists do not get the half-thief bonus.
* There's a bug in the DOS version making bomb traps far more common and both types of poison traps incredibly rare.
* The DOS version's RNG is actually pretty good, unlike that of the NES and Amiga versions. (Amiga version isn't discussed in that thread.) On the other hand, the use of modulus makes lower rolls more common.

Nice, thanks for sharing! I love to see discussions about these internal mechanics of games.

It's funny to see discussions of Ultima in so many different places. Also, the first time I've seen a disassembly in a word document, feels kinda wrong.

What is RNG?

Random Number Generator. It's how the game generates random numbers.

Thing is, computers don't use true random numbers (hardware TRNGs exist, but not on the hardware of the time, and they're mainly useful for cryptography), so they have to "fake" it with various algorithms. Sometimes, however, the algorithms used aren't as random as one would like, leading to predictable (and sometimes manipulatable) results.

For example, in Ultima 3 NES, given a dungeon encounter formation, it's sometimes possible to win a battle by consuming a few random numbers (for example, by healing), and then casting a mass death spell (normally about 75% success per enemy).

Or, in Ultima 3 Amiga, the RNG there is extremely streaky, so I've had mass death fail entirely on groups of 8, and chests tend to alternate in streaks where every one is trapped, and streaks where the Appar Unem spell repeatedly fails.

There's an AGDQ run of Dragon Warrior on the NES where the player manipulates the RNG to win at level 7. (Normally 17 is the lowest level at which there's even a remote chance of victory.)

Thanks. I'm familiar with random number generators (I've done a fair amount of low level programming in my career!) but for some reason, that acroynm just wasn't clicking.