TL;DR
Salvatore Sanfilippo, known as antirez, has developed a "clean room" emulator for the Z80 microprocessor and the ZX Spectrum, along with an emulator for CP/M. This approach emphasizes building the emulator from scratch based on high-level specifications rather than reverse engineering existing software. Antirez drafted a markdown document outlining the emulator's goals, such as executing complete instructions at once and accurately tracking clock cycles, which is particularly important for mimicking the ZX Spectrum's memory access behaviors.