Retro Virtual Machine is a ZX Spectrum emulator with a one cycle accuracy.
The emulation of the Z80 processor is almost (until someone discover something new) accurate, including features like "memptr" register and the "q" flag.
The Spectrum ULA emulation is cycle accurate also, including the "floating bus", the "snow effect" (not 100% accurate for now) and the contended memory timings.
The AY Sound chip and the UPD765 are emulated too.
The emulation core pass all the test we have found.