The DB was not as flexible as segment registers, and there was only one of them, but you could easily load them both and then get a 16 bit value into the accumulator. Index registers also crossed bank boundaries. In contrast to the 8086, where the segments were a bit of a jail.

Mind, that's a feature in that you could write a multiprocessing system without memory protection, and have code reasonably safe from accidentally stomping on other areas (for example, if your C compiler compiled to the Tiny model, a rogue C ptr shouldn't be able to break out, obviously assembly could do anything).

Regarding the 68K and larger address space, on the original Macintosh, you were unable to request a single block of memory larger than 32K. So, like the Genie, unlimited power -- itty bitty living space. Sure, pointers could access anything, but at a system level, you weren't going to get a large enough chunk of memory, all at once, to warrant a larger pointer, so you were working with almost "virtually" segmented memory anyway.

On the IIGS, based on the 816, they used a similar memory manager. The segment manager on IIGS is really pretty cool. It's 2.0 of what the Macintosh had.