It is definitely plausible that they were rendering their spinning cube in real-time. A lot of people have written wolf3d-style raycasting engines for z80 (mostly z80-based TI graphing calculators, which are clocked between 6MHz and 15MHz. much faster than a TRS-80 to be fair). This guy even made a doom-style engine: http://benryves.com/journal/3739423 (see animated screenshot). The stuff the demoscene has done with z80 microcomputers can be even more impressive.
True, but I was concerned with it being a school lab, they were already demonstrating dual dacs, linked list data structure with semi-complicated data structure (well, OK, x and y values and maybe just a dumb lookup table) in assembly. Asking them to add a third aspect of trig might have been too much for one lab so maybe they canned the rotation and just had a very large data table animation rather than calculate on the fly.
My project for the demo day in 68hc11 class was the 68hc11 was a weirdo among microcontrollers for having an external memory bus, so I slapped a 32k sram onto it making it a 32K MC and made what amounts to a drum sampler, press this button to record a couple seconds off the onboard A/D and press this button to play it back using an offboard DAC. Now a days of course microcontrollers rarely expose their memory bus and you'd just buy a COTS MC with more memory on the chip. We only had one day to prep for demo day and asking the Z80 kids to do DACs, and a vector algo, and trig all in one day might have been asking too much.
Its been awhile, but I think the demo day theme was we all had to use at least one off chip DAC.
