Re: ZX Spectrum, cartridge port?
Interface 2 had a literal ROM socket. It worked just like a console. You plugged in a ROM when the computer was off, turned it back on, and it instantly booted into a game. About 10-12 games were made on the format, but it saved the 5+ minute loading time.
It worked by exposing the memory bus directly to the expansion port, and the IF2 exposed some memory pins of that direct to a ROM chip in a cartridge. Literally when you executed the bootup / ROM routines, it was just using the chip on the cartridge instead of the normal Spectrum ROM. I believe size was limited to 16Kb because of the way the expansion port as designed.
I had one. It was great. Also had two Sinclair joysticks ports you could use with games (and all ROM games supported those joysticks too - I believe the Jetpac ROM was rewritten to support it rather than Kempston / Cursor interface), Instant booting, guaranteed loading, Nintendo-like cartridges, dual joystick ports, and you could still plug in things like the ZX printer etc. into the back of the IF2.
Sadly, never very many games but I did have (may still have somewhere) Jetpac, Planetoids and Hungry Horace on that format. The ROMs were a chip on a PCB covered in a black plastic case with a red rubber "skirt" to stop dirt getting on the exposed PCB edge.