Were there emulators running on 8-bit computers (preferably "mainstream" personal ones), available to general public (as opposed to in-house developers only)? I am mostly after emulators running fully in software (e.g. CPU emulation), not hardware add-on cards, and mostly after emulators running on well-know machines. It is also better if the emulators were written/used in the 8-bit era, but recent emulators are fine too, if they run on retro machines - thus an emulator running on Atmel AVR would not quite fit.
Obviously, running on 8-bit machines poses quite a significant set of challenges. First, emulated RAM must fit into the host RAM, and the emulated speed will be an order of magnitude below the host.
I know of CHIP-8 and its implementation on several 8-bit computers, but that's more of a virtual machine than an emulator.
I also know of these emulators (as you can see, I am a bit biased):
- ZX-81 emulator by Johan Koelman, running on ZX Spectrum from 1997, which is exactly what fits the question (though it came a bit late). Author says it managed to reach about 33% speed, mostly by translating ROM routines.
- SAM2_ZX81 (by the same author), emulating ZX81 on Sam Coupé (the CPU runs natively)
- Unusable ZX Spectrum emulator emulating Spectrum on 8-bit Atari, from 2007 - i.e. recent, author claims ~10% speed
- and there is of course this emulation of Windows 3.1 running on ZX Spectrum